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8/6/2007

GEZEGENLER

 

 

Güneş sistemimizin sekiz gezegeni. (Büyüklükler ve uzaklıklar ölçeklenmemiştir. Bir uydu olmasına rağmen ay resimde görülmektedir)
Güneş sistemimizin sekiz gezegeni. (Büyüklükler ve uzaklıklar ölçeklenmemiştir. Bir uydu olmasına rağmen ay resimde görülmektedir)
Güneş Sistemi'ndeki gezegenlerin yörüngeleri
Güneş Sistemi'ndeki gezegenlerin yörüngeleri

 

 

The terrestrial planets: Mercury, Venus, Earth, Mars (Sizes to scale)
The terrestrial planets: Mercury, Venus, Earth, Mars (Sizes to scale)
The four gas giants against the Sun: Jupiter, Saturn, Uranus, Neptune. (Sizes to scale.)
The four gas giants against the Sun: Jupiter, Saturn, Uranus, Neptune. (Sizes to scale.)
 
 
The gods of Olympus, after whom the Solar System's planets are named
The gods of Olympus, after whom the Solar System's planets are named

 

The eight planets and three dwarf planets of the Solar System. (Sizes to scale, distance from the sun is compressed)
 

Bir yıldızın etrafında dolanan ve kendisi yıldız olmayan doğal gök cisimlerine gezegen adı verilir. Dar anlamıyla, Güneş Sistemi içinde, Güneş'in doğrudan uydusu olan ve Uluslararası Gökbilim Birliği (IAU) tarafından bu tanıma uygun bulunmuş 8 gök cismini belirlemede kullanılır. Güneş Sistemi'nde, resmi olarak kabul edilen 'sekiz gezegen'den başka, bu cisimlerle boyut, yörünge ve fiziksel özellikler açısından aynı gruba konabilecek yeni gök cisimlerinin keşfedilmesi, bir yandan da başka yıldızların etrafında da Güneş Sistemi gezegenlerine benzer gök cisimlerinin dolandığının saptanması, 'gezegen' tanımının sınırlarının bulanıklaşmasına neden olmuştur.

Uluslararası Gökbilim Birliği'nin (IAU), 1919 yılından bu yana kabul ettiği Güneş Sistemi'nin 8 gezegeni, güneşe yakınlık sıralarına göre şunlardır:

1. Merkür
2. Venüs
3. Dünya
4. Mars
5. Jüpiter
6. Satürn
7. Uranüs
8. Neptün
9.
10.
11.
12. 
    12  tane bulunacak

Bu 8 gezegenin dışında daha önce gezegen olarak tanımlanan Plüton IAU'nun yeni tanımlamasına göre Cüce Gezegen olarak kabul edilmektedir.

Güneş Sistemi dışındaki gezegenler

Bu gezegenlerin en büyüğü Subutay'dır. 1995 yılında Michel Mayor ve Didier Queloz tarafından 51 Pegasi adlı yıldızın çevresinde dönen bir gök cismi keşfedildiğinde, bu cismin 'gezegen' olarak tanımlanması uygun görüldü. 1995-2005 yılları arasında yapılan gözlemlerle, 100'ü aşkın değişik yıldız çevresinde dolanan 150'den fazla gezegen bulundu. Güneş Sistemi gezegenleri ile karıştırılmaması için bu cisimlere 'Güneş dışı gezegenler' veya Güneş Sistemi dışı gezegenler adı verilmektedir. Yine karışıklığı önlemek amacıyla, bu tür gezegenlerin yıldızları ile birlikte oluşturdukları sistemlere genel olarak gezegen sistemi ya da 'yıldız sistemi' adı verilmektedir. 'Güneş Sistemi' adı ise, yalnızca özel ad olarak Güneş ve uydularının oluşturduğu gezegen sistemini tanımlamada kullanılır. ek olarak 1996 yılında amerikalı uzay bilimcisi Arthur Frank Elbourn 'un yapmış olduğu bir takım araştırmalar uzay hakkında daha da fazla bilgi almamızı sağlamıştır. Arthur Frank Elbourn un yapmış olduğu çalışmlarda 10 olan gezegen sayısı aslında 12 gezegen vardi. Goono ve Afelbourn ismi verdiği iki gezegen daha keşfetti. NASA tarafından doğrulanan bu gezegenler fazla medyaya duyurulmadı.

Tarih boyunca gezegen kavramı

Elimize ulaşan tarihsel kayıtlar incelendiğinde, Türkçe'nin genç sözcüklerinden olan 'gezegen'in diğer dillerde uzun süredir var olan karşılıklarının, gökyüzünde yıldızların alışılmış hareketlerinden farklı davranışları ile dikkati çeken 'aykırı' yıldızlar için kullanıldığı görülür. Batı dillerinde gezegen kavramı Eski Yunan'da 'başıboş dolaşan' anlamında kullanılan planitis (πλανήτης) sözcüğünden türetilmiş sözcüklerle ifade edilmektedir. Yakın tarihe kadar Türkçe'de kullanılan Arapça kökenli seyyare sözcüğü de benzer anlam taşımaktadır. Türkçe gezegen sözcüğü de, bu yıldızların gökyüzünde diğer sabit yıldızların arasında 'gezinmelerinden' esinlenilerek türetilmiştir.

17.ci yüzyıla dek bilinen beş gezegen (Merkür, Venüs Mar, Jüpiter ve Satürn), insan kültürü ile tarih boyunca içiçe olmuş, çeşitli kültürlerde tanrılarla bağdaştırılarak mitolojinin, klasik elementlerle bağdaştırılarak felsefenin ve astrolojinin önemli bir parçasını oluşturmuşlardır. 17.ci yüzyılda Kopernik'in o güne dek yaygın olan yermerkezli görüşü sarsan kozmolojik devrimi ile güneşmerkezli evren anlayışının ağırlık kazanması sonucunda dünyanın da bir gezegen olduğu kabul edilmiş, böylece gezegen kavramı 'gökte başıboş dolaşan yıldız'dan günümüzdeki gökbilimsel anlamına oturmuştur.

18.ci yüzyılda keşfedilen Uranüs gezegenler listesine yedinci sırayla kolaylıkla eklenirken, 1801 ve 1802'de Güneş Sistemi'nin Ceres ve Pallas adlarını alan iki yeni üyesi bulunduğunda, küçüklükleri nedeniyle gezegen sayılmayarak Sir William Herschel'in verdiği asteroit tanımı içine alındılar. İzleyen yıllarda keşfedilen benzer niteliklerde yeni küçük gök cisimleri de bu kategoriye eklendiler. Böylece Titius-Bode yasn öngördüğü şekilde Mars ile Jüpiter yörüngeleri arasında bir başka gezegen bulunması gerektiği sorunu çözümlenmiş oldu. Ancak bu kez Uranüs yörüngesindeki tedirginliklerden sorumlu yeni bir gezegen arayışı başladı. Bu sorunun yanıtını da 1846 yılında bulunan ve sekizinci gezegen olarak benimsenen Neptün getirdi. Güneş Sistemi içinde gözlenen tüm tedirginliklerin henüz keşfedilmemiş bir 'bilinmeyen gezegen' ile açıklanabileceği yaklaşımının bu şekilde meyvasını vermesi, 'gezegen avcılarını' cesaretlendirerek dokuzuncu gezegenin aranmasına başlandı. Ancak, giderek daha güçlü teleskopların yapılması, gökyüzünü inceleyen insan ve kuruluş sayısının artması, 19.yüzyıl sonunda astrofotografitekniğinin ortaya çıkması gibi gelişmeler sayesinde önemsiz sayılacak gökcisimlerinin saptanabilir hale gelmesine ve yeni bulunan asteroit sayısının bini aşmasına karşın, 1930'da Plüton bulunduğunda neredeyse yüz yıl geçmişti. Bu uzun bekleyiş, Plüton'a dokuzuncu gezegen olma onurunu kazandırırken, açıklamasını da birlikte getiriyordu: yeni gezegen o ana dek bilinen en küçük gezegen Merkür'ün yarısından daha küçük çapta ve otuzda biri kütlesinde, aralarında Ay'ın da bulunduğu birçok gezegen uydusundan daha küçük, üstelik alışılmadık bir yörüngede idi. Bütün bunlara karşın, en büyük asteroit Ceres'ten daha büyük olan ve Güneş çevresinde dönen dokuzuncu büyük gök cismi olan Plüton'un dokuzuncu gezegen sıfatı 20. yüzyıl sonlarına kadar tartışma konusu olmadı.

Hollandalı gökbilimci Kuiper tarafından kuramsal olarak ortaya atılan ve bugün Kuiper kuşağıolarak bilinen bölge, Güneş'ten 30-50 A.Ü (astronomi ünitesi-gökbilim birimi) yani yaklaşık 4,5-7,5 milyar km. uzaklıktaki alanı kaplar ve Güneş çevresinde dönen çok sayıda küçük gök cisminin bu aralıkta yer aldıklarına 1950'lerden bu yana inanılmaktadır. 1992 yılında, o ana dek Kuiper kuşağının bilinen tek üyesi Plüton gezegeni iken, (15760) 1992 QB1 geçici adıyla tanınan 'ilk Kuiper kuşağı cismi'nin bulunması ve bunu kısa sürede çok sayıda yenilerinin izlemesi ile bu yeni gök cisimi sınıfı bir kavram olarak netleşmeye başladı. Plüton'un bilimsel anlamda bu sınıfın bir üyesi olduğu gökbilim çevreleri tarafından kabul edilirken, hala bir gezegen olarak kabul edilip edilmeyeceği konusu popüler bir tartışma biçimini aldı. Uluslararası Gökbilim Birliği (IAU) 1999 yılında Plüton'un resmi olarak Güneş sistemi'nin dokuzuncu gezegeni kabul edildiğini ve bunun değiştirilmesinin düşünülmediğini açıklayan bir bildiri yayınlamak zorunda kaldı.

2002 yılında Plüton'un yarısı çapındaki 50000 Quaoar'ın, 2004'te ise neredeyse Plüton büyüklüğünde 90377 Sedna'nın keşfi, Plüton'un diğer Kuiper kuşağı cisimlerinden (Kuiper Belt Objects-KBO) fazla ayrıcalıklı olmadığını göstermesi bakımından önemli görüldü. 29 Temmuz 2005'de üç yeni Kuiper kuşağı cisimi daha bulunduğu açıklandı. Bunlardan 2003 U313 adlı olanı, Plüton'dan daha büyük olması nedeni ile bazılarınca 10.cu gezegen ilan edilirken bir yandan da Plüton'un gezegen sıfatının gözden geçirilmesi tartışmaları yeniden alevlendi. amerika da yapılan araştırmalar sonucunda aslında 12 gezegen dışında dört gezegen daha keşfedilmiş. bunlar pluton dan daha büyük ve yapılan araştırmalarda bu dört gezegenin bir tanesinde yaşamsal bir belirti olabileceği söylenmektedir. yalnız dunyaya çok uzak olan bu dört gezegen nasa nın yapmış olduğu gizlia raştırmalar sonucunda ortaya çıkarılmış, ve medyadan bugune kadar saklanmıştır. medyaya nasıl sızdığı bilinmemekte olup araştırmaların devam ettiği söylenmektedir.

 

  • Plüton'un statüsü hakkında 3 Şubat 1999 Uluslararası Gökbilim Birliği (IAU) bildirisi

    The International Astronomical Union (IAU) was founded in 1919.  Its mission is to promote and safeguard the science of astronomy in all its aspects through international cooperation.  Its individual members are professional astronomers all over the world, at the Ph.D. level and beyond, and active in professional research and education in astronomy. Besides, the IAU maintains friendly relations with organizations that include amateur astronomers in their membership. As of September 2006, the IAU has 9,783 Individual Members in 87 countries worldwide. Of those 64 are National Members.

     

    The scientific and educational activities of the IAU are organized by its 12 Scientific Divisions and, through them, its 40 specialized Commissions covering the full spectrum of astronomy, along with its 76 Working and Program Groups. The long-term policy of the IAU is defined by the General Assembly and implemented by the Executive Committee, while day-to-day operations are directed by the IAU Officers. The focal point of its activities is the IAU Secretariat, hosted by the Institut d'Astrophysique de Paris, France.

     

    The key activity of the IAU is the organization of scientific meetings is. Every year the IAU sponsors nine international IAU Symposia. The IAU Symposium Proceedings series is the flagship of the IAU publications. Every three years the IAU has its General Assembly, which offers six IAU Symposia and some 25 Joint Discussions and Special Sessions. The proceedings of the last two are published in the Highlights of Astronomy series. The reports of the GA business meetings are published in the Transactions of the IAU - B series.

     

    Among the other tasks of the IAU are the definition of fundamental astronomical and physical constants; unambiguous astronomical nomenclature; promotion of educational activities in astronomy; and early informal discussions on the possibilities for future international large-scale facilities. Furthermore, the IAU serves as the internationally recognized authority for assigning designations to celestial bodies and surface features on them.

     

    The IAU works to promote astronomical education and research in developing countries through its Program Groups on International Schools for Young Astronomers (ISYA), on Teaching for Astronomy Development (TAD), and on World Wide Development of Astronomy (WWDA), as well as through joint educational activities with COSPAR and UNESCO.

     

    This web site provides on-line information on the Union's activities and links to the web sites of the IAU Divisions, Commissions, Working Groups, and Program Groups. Further contact with the IAU membership is maintained through the IAU Information Bulletin, published twice per year, and downloadable from this web site.

     

    Contact address:
    IAU-UAI Secretariat
    98bis bd Arago, F-75014 Paris, France
    Tel: +33 1 43 258 358 -- Fax: +33 1 43 252 616
    E-mail:     iau(@)iap.fr -- URL: http://www.iau.org/

     

    Uluslararası Gökbilim Birliği (IAU) tarafından 'gezegen' tanımı-Şubat 2003

    Rather than try to construct a detailed definition of a planet which is designed to cover all future possibilities, the WGESP has agreed to restrict itself to developing a working definition applicable to the cases where there already are claimed detections, e.g., the radial velocity surveys of companions to (mostly) solar-type stars, and the imaging surveys for free-floating objects in young star clusters. As new claims are made in the future, the WGESP will weigh their individual merits and circumstances, and will try to fit the new objects into the WGESP definition of a "planet", revising this definition as necessary. This is a gradualist approach with an evolving definition, guided by the observations that will decide all in the end.

    Emphasizing again that this is only a working definition, subject to change as we learn more about the census of low-mass companions, the WGESP has agreed to the following statements:

    1) Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.

    2) Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed nor where they are located.

    3) Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).

    These statements are a compromise between definitions based purely on the deuterium-burning mass or on the formation mechanism, and as such do not fully satisfy anyone on the WGESP. However, the WGESP agrees that these statements constitute the basis for a reasonable working definition of a "planet" at this time. We can expect this definition to evolve as our knowledge improves.

     

     

    A planet, as defined by the International Astronomical Union (IAU), is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, not massive enough to cause thermonuclear fusion in its core, and has cleared its neighbouring region of planetesimals.[1][2]

    After stars and stellar remnants, planets are some of the most massive objects known to man. They play an important part in the structure of planetary systems, and are also considered, along with large moons, the most feasible environment for life.[3] Thus planetary science is essential not only to comprehend the structure of the universe, but also to better understand the development of life, and to aid the search for extraterrestrial intelligence. Additionally, the planets visible from Earth have played a vital role in the shaping of human culture, religion and philosophy in numerous civilisations. Even today, many people continue to believe the movement of the planets affects their lives, although such a causation is rejected by the scientific community.

    In the absence of a formal scientific definition of "planet", the number of objects described as such has varied throughout history. This changed in 2006, when the IAU officially adopted a resolution defining planets within the Solar System. This definition has been both praised and criticised, and remains disputed by some scientists.

    Under IAU definitions, there are eight planets in the Solar System (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) and also at least three dwarf planets (Ceres, Pluto, and Eris). Many of these planets are orbited by one or more moons, which can be larger than small planets. There have also been more than two hundred planets discovered orbiting other stars.[4] Planets are generally divided into two main types: large, low-density gas giants and smaller, rocky terrestrials. Dwarf planets, a separate category, can either be terrestrials or frozen ice dwarfs.

    In ancient times, astronomers noted how certain lights moved across the sky in relation to the other stars. To explain their motions, the ancient Greeks adopted a geocentric model.[5] These objects were believed to orbit the Earth, which was considered to be stationary. The lights were first called "πλανήτης" (planētēs), meaning "wanderer", by the ancient Greeks, and it is from this that the word "planet" was derived.

    In near-universal practice in the Western world, the planets in the Solar system are named after Graeco-Roman gods, as, in Europe, it was the Greeks who first named them. However, the practice of naming planets after gods originated in the Western world with the Sumerians, who lived in modern-day Iraq in about 3000 BCE. Successive Mesopotamian civilizations, such as the Babylonians, retained the Sumerian naming convention but adapted it to their own very different pantheons. The Greeks borrowed much of their astronomy, including constellations and the zodiac, from the Babylonians, and by 600 BCE had already begun using Babylonian concepts in their calculations.[6] The Greeks grafted the names of their own gods onto the Babylonian planet list, although there was some confusion in translation. For instance, the Babylonian Nergal was a god of war, and the Greeks, seeing this aspect of Nergal's persona, identified him with Ares, their god of war. However, Nergal, unlike Ares, was also a god of the dead and a god of pestilence.[7]

    Because of the influence of the Roman Empire and, later, the Catholic Church, in most countries in the West the planets are known by their Roman (or Latin) names rather than the Greek. The Romans, who, like the Greeks, were Indo-Europeans, shared with them a common pantheon under different names but lacked the rich narrative traditions that Greek poetic culture had given their gods. During the later period of the Roman Republic, Roman writers borrowed much of the Greek narratives and applied them to their own pantheon, to the point where they became virtually indistinguishable. When the Romans studied Greek astronomy, they gave the planets their own gods' names.

    To the Greeks and Romans, there were five known planets; each presumed to be circling the Earth according to the complex laws laid out by Claudius Ptolemy in the 2nd century. They were, in increasing order from Earth: Mercury (called Hermes by the Greeks), Venus (Aphrodite), Mars (Ares), Jupiter (Zeus), and Saturn (Kronos). Although strictly the term "planetes" referred only to those five objects, the term was often expanded to include the Sun and the Moon.[8] When subsequent planets were discovered in the 18th and 19th centuries, the naming practice was retained: Uranus (Ouranos) and Neptune (Poseidon). The Greeks still use their original names for the planets.

    Some Romans, following a belief imported from Mesopotamia into Hellenistic Egypt,[9] believed that the seven gods after whom the planets were named took hourly shifts in looking after affairs on Earth. The order of shifts began with Jupiter and worked inwards; as a result, a list of which god had charge of the first hour in each day became Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn, i.e. the usual weekday name order.[10] Sunday, Monday, and Saturday are straightforward translations of these Roman names. In English the other days were renamed after Tiw, Wóden, Thunor, and Fríge, Anglo-Saxon gods considered similar or equivalent to Mars, Mercury, Jupiter, and Venus respectively.

    Since Earth was only generally accepted as a planet in the 17th century, there is no tradition of naming it after a god. Many of the Romance languages (including French, Italian, Spanish and Portuguese), which are descended from Latin, retain the old Roman name of Terra or some variation thereof. However, the non-Romance languages use their own respective native words. Again, the Greeks retain their original name, Γή (Ge or Yi); the Germanic languages, including English, use a variation of an ancient Germanic word ertho, "ground," as can be seen in the English Earth, the German Erde, the Dutch Aarde, and the Scandinavian Jorde. The same is true for the Sun and the Moon, though they are no longer considered planets.

    Some non-European cultures use their own planetary naming systems. India uses a naming system based on the Navagraha, which incorporates the seven traditional planets (Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn) and the ascending and descending lunar nodes Rahu and Ketu. China, and the countries of eastern Asia subject to Chinese cultural influence, such as Japan, Korea and Vietnam, use a naming system based on the five Chinese elements.[10]

    History

    As scientific knowledge progressed, understanding of the term "planet" changed from something that moved across the sky (in relation to the starfield), to a body that orbited the Earth (or that were believed to do so at the time). When the heliocentric model gained sway in the 16th century, it became accepted that a planet was actually something that directly orbited the Sun. Thus the Earth was itself a planet,[11] while the Sun and Moon were not. Until the mid-19th century, any newly discovered object orbiting the Sun was listed with the planets by the scientific community, and the number of "planets" swelled rapidly towards the end of that period.

    During the 1800s, astronomers began to realize most recent discoveries were unlike the traditional planets. They shared the same region of space, between Mars and Jupiter, and had a far smaller mass. Bodies such as Ceres, Pallas, and Vesta, which had been classed as planets for almost half a century, became classified with the new designation "asteroid." From this point, a "planet" came to be understood, in the absence of any formal definition, as any "large" body that orbited the Sun. There was no apparent need to create a set limit, as there was a dramatic size gap between the asteroids and the planets, and the spate of new discoveries seemed to have ended after the discovery of Neptune in 1846.[12]

    However, in the 20th century, Pluto was discovered. After initial observations led to the belief it was larger than Earth, the recently-created IAU accepted the object as a planet. Further monitoring found the body was actually much smaller, but, as it was still larger than all known asteroids and seemingly did not exist within a larger population, it kept its status for some seventy years.[13]

    In the 1990s and early 2000s, there was a flood of discoveries of similar objects in the same region of the Solar System. Like Ceres and the asteroids before it, Pluto was found to be just one small body in a population of thousands. A growing number of astronomers argued for it to be declassified as a planet, since many similar objects approaching its size were found. The discovery of Eris, a more massive object widely publicised as the tenth planet, brought things to a head. The IAU set about creating the definition of planet, and eventually produced one in 2006. The number of planets dropped to the eight significantly larger bodies that had cleared their orbit (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus & Neptune), and a new class of dwarf planets was created, initially containing three objects (Ceres, Pluto and Eris

    It is not known with certainty how planets are formed. The prevailing theory is that they are formed during the collapse of a nebula into a thin disk of gas and dust. A protostar forms at the core, surrounded by a rotating protoplanetary disk. Through accretion—a process of sticky collision—dust particles in the disk steadily accumulate mass to form ever-larger bodies. Local concentrations of mass known as planetesimals form, and these accelerate the accretion process by drawing in additional material by their gravitational attraction. These concentrations become ever more dense until they collapse inward under gravity to form protoplanets.[20] After a planet reaches a diameter larger than the Earth's moon, it begins to accumulate an extended atmosphere, greatly increasing the capture rate of the planetesimals by means of atmospheric drag.[21]

    When the protostar has grown such that it ignites to form a star, the surviving disk is removed from the inside outward by photoevaporation, the solar wind, Poynting-Robertson drag and other effects.[22][23] Thereafter there still may be many protoplanets orbiting the star or each other, but over time many will collide, either to form a single larger planet or release material for other larger protoplanets or planets to absorb.[24][25] Those objects that have become massive enough will capture most matter in their orbital neighbourhoods to become planets. Meanwhile, protoplanets that have avoided collisions may become natural satellites of planets through a process of gravitational capture, or remain in belts of other objects to become either dwarf planets or small solar system bodies.

    The energetic impacts of the smaller planetesimals (as well as radioactive decay) will heat up the growing planet, causing it to at least partially melt. The interior of the planet begins to differentiate by mass, developing a denser core. Smaller terrestrial planets lose most of their atmospheres because of this accretion, but the lost gases can be replaced by outgassing from the mantle and from the subsequent impact of comets.[26] (Smaller planets will lose any atmosphere they gain through various

    • Kategori: GEZEGENLER | Yorum (1) Yorum yaz! Kalıcı Bağlantı

    4/3/2005

    MARS

     

    HUBBLE TELESKOBUNUN MARS GÖRÜNTÜLERİ
    Hubble's Closest View of Mars -- August 27,

    Hubble's Closest View of Mars -- August 27, 2003

     
    The Two Faces of Mars Image Astronomical

    The Two Faces of Mars

    PRC2003-22a
    Hubble's Close Encounter with Mars -- August 26, 2003 Image Type: Astronomical

    Hubble's Close Encounter with Mars -- August 26, 2003

    PRC2003-22c
    Mars oppositions Solar System diagram Image Type: Illustration

    Mars oppositions Solar System diagram

    Mars oppositions Solar System diagram without images Image Type: Illustration

    Mars oppositions Solar System diagram without images

    Hubble's Sharpest View of Mars -- August 24, 2003 Image Type: Astronomical

    Hubble's Sharpest View of Mars -- August 24, 2003

    Annotated Mars Image Type: Astronomical/Illustration

    Annotated Mars

    Annotated Mars: Next View Image Type: Astronomical/Illustration

    Annotated Mars: Next View

    Sharpest Ever Color View of Mars Image Type: Astronomical

    Sharpest Ever Color View of Mars


    Mars (eski adıyla Merih), Güneş Sistemi'nin dördüncü gezegenidir. İsmi Eski Roma'daki savaş tanrısı Mars'tan gelmektedir (Bu Tanrı Eski Yunan Mitolojisinde Ares'e karşılık gelir). Gece temiz bir havada basit bir teleskopla kırmızılığı görülebilir.

    Mars'ın 1877 yılında Amerikan astronom Asaph Hall tarafından keşfedilen Phobos ve Deimos adında iki uydusu vardır. Bu uyduların nasıl oluştukları bilinmemekle beraber, Mars'ın kütle çekim alanına kapılmış asteroitler oldukları düşünülmektedir. Bu uyduların isimleri Eski Yunan Mitolojisinde Ares'in Afrodit'ten olma iki oğlu Phobos ve Deimos'tan gelmektedir.

    Gel-git etkileri yüzünden, tıpkı Dünya ve Ay gibi her iki uydunun da yalnız bir yüzü Mars'a dönüktür. Phobos Mars'ın çevresinde Mars'ın kendi ekseni etrafında döndüğünden daha hızlı döndüğü için yörüngesi giderek küçülmektedir. Bu nedenle ileriki bir tarihte Phobos Mars'a çarpacaktır. Buna karşın, Deimos Mars'tan yeterince uzakta olduğu için, yörüngesi giderek büyümektedir.

     

     

    NASA için araştırmayı yapan bilim adamları, 25 saatlik bir Mars gününe astronotların hazırlanmaları amacıyla denekleri akşamları iki kez 45 dakika çok parlak ışığa maruz bıraktılar.

    ABD’nin seçkin üniversitelerinden Harvard Tıp Fakültesi ile Brigham ve Kadın Hastanesi tarafından yapılan ve sonuçları Proceedings of the National Academy of Sciences dergisinde yayınlanan araştırmada, 22 ila 33 yaşındaki 9 erkek ile 3 kadın 65 gün süreyle akşamları 90 dakika parlak ışıkta yaşamak zorunda bırakıldılar.

    İnsanların biyolojik saatinin 23 saat 47 dakikadan 24 saat 48 dakikaya çıkabileceğini ortaya koyan araştırma, ışığın biyolojik saat için önemini gösterdi.

    Araştırmanın, deneğin akşam iki kez 45 dakika canlı ışığa maruz bırakılmasıyla uyku döngüsünün uzatılabileceğini ortaya koyduğunu belirten bilim adamları, ışıkla tedavinin, saat farkı veya gece çalışma gibi uyku düzeninde bozulmaya neden olan unsurlardan ötürü uykusuzluk rahatsızlığı çekenler için yardımcı olabileceğini kaydettiler.

    Araştırmaya katılan tüm denekler, Dünya’dakinden bir saat fazla olan Mars gününe, uyku düzenlerini adapte edebildiler.

    Hayvanlar üzerinde daha önce yapılan deneylerde, uyku düzeninin uzunluğundaki doğal değişimin, günün uzunluğuna bağlı olduğu belirlenmişti.

     

     

     

    Mars (IPA: /ˈmɑrs/) is the fourth planet from the Sun in the Solar System. The planet is named after Mars, the Roman god of war. It is also referred to as the "Red Planet" because of its reddish appearance as seen from Earth.

    A terrestrial planet, Mars has a thin atmosphere and surface features reminiscent both of the impact craters of the Moon and the volcanoes, valleys, deserts and polar ice caps of Earth. It is the site of Olympus Mons, the highest known mountain in the solar system, and of Valles Marineris, the largest canyon. In addition to its geographical features, Mars’ rotational period and seasonal cycles are likewise similar to those of the Earth.

    Until the first flyby of Mars by Mariner 4 in 1965, it was speculated that there might be liquid water on the planet. This was based on observations of periodic variations in light and dark patches, particularly in the polar latitudes, which looked like seas and continents, while long, dark striations were interpreted by some observers as irrigation channels for liquid water. These straight line features were later proven not to exist and were instead explained as optical illusions. Still, of all the planets in our solar system, Mars is the most likely, other than Earth, to harbor liquid water, and perhaps life.

    Mars is currently host to three ********al orbiting spacecraft: Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter. This is more than any planet except Earth. The surface is also home to the two Mars Exploration Rovers (Spirit and Opportunity). Geological evidence gathered by these and preceding missions suggests that Mars previously had large-scale water coverage, while observations also indicate that small geyser-like water flows have occurred in recent years.[3] Observations by NASA's Mars Global Surveyor show evidence that parts of the southern polar ice cap have been receding.[4]

    Mars has two moons, Phobos and Deimos, which are small and irregularly shaped. These may be captured asteroids, similar to 5261 Eureka, a Martian Trojan asteroid. Mars can be seen from Earth with the naked eye. Its apparent magnitude reaches −2.9, a brightness surpassed only by Venus, the Moon, and the Sun, though for much of the year Jupiter may appear brighter to the naked eye than Mars.

     

    Liquid water cannot exist on the surface of Mars with its present low atmospheric pressure, but water ice is in no short supply, with two polar ice caps made largely of ice. In March 2007, NASA announced that the volume of water ice in the south polar ice cap, if melted, would be sufficient to cover the entire planetary surface to a depth of 11 metres (36 ft).[10] Additionally, an ice permafrost mantle stretches down from the pole to latitudes of about 60°.[11] Much larger quantities of water are thought to be trapped underneath Mars' thick cryosphere, only to be released when the crust is cracked through volcanic action. The largest such release of liquid water is thought to have occurred when the Valles Marineris formed early in Mars' history, enough water being released to form river valleys across the planet. A smaller but more recent event of the same kind occurred when the Cerberus Fossae chasm opened about 5 million years ago, leaving a sea of frozen ice still visible today on the Elysium Planitia.[12]

    More recently the high resolution Mars Orbiter Camera on the Mars Global Surveyor has taken pictures which give much more detail about the history of liquid water on the surface Mars. Despite the many giant flood channels and associated tree-like network of tributaries found on Mars there are no smaller scale structures that would indicate the origin of the flood waters. It has been suggested that weathering processes have denuded these, indicating the river valleys are old features. Higher resolution observations from spacecraft like Mars Global Surveyor also revealed at least a few hundred features along crater and canyon walls that appear similar to terrestrial seepage gullies. The gullies tend to be located in the highlands of the southern hemisphere and to face the Equator; all are poleward of 30° latitude.[13] The researchers found no partially degraded (i.e. weathered) gullies and no superimposed impact craters, indicating that these are very young features.

     

    Today, features on Mars are named from a number of sources. Large albedo features retain many of the older names, but are often updated to reflect new knowledge of the nature of the features. For example, Nix Olympica (the snows of Olympus) has become Olympus Mons (Mount Olympus).[19]

    Mars’ equator is defined by its rotation, but the location of its Prime Meridian was specified, as was Earth's (at Greenwich), by choice of an arbitrary point; Mädler and Beer selected a line in 1830 for their first maps of Mars. After the spacecraft Mariner 9 provided extensive imagery of Mars in 1972, a small crater (later called Airy-0), located in the Sinus Meridiani ("Middle Bay" or "Meridian Bay"), was chosen for the definition of 0.0° longitude to coincide with the original selection.

    Since Mars has no oceans and hence no 'sea level', a zero-elevation surface or mean gravity surface also had to be selected. Zero altitude is defined by the height at which there is 610.5 Pa (6.105 mbar) of atmospheric pressure. This pressure corresponds to the triple point of water, and is approximately 0.6% of the sea level surface pressure on Earth.[20]

    The dichotomy of Martian topography is striking: northern plains flattened by lava flows contrast with the southern highlands, pitted and cratered by ancient impacts. The surface of Mars as seen from Earth is thus divided into two kinds of areas, with differing albedo. The paler plains covered with dust and sand rich in reddish iron oxides were once thought of as Martian 'continents' and given names like Arabia Terra (land of Arabia) or Amazonis Planitia (Amazonian plain). The dark features were thought to be seas, hence their names Mare Erythraeum, Mare Sirenum and Aurorae Sinus. The largest dark feature seen from Earth is Syrtis Major.[21]

     

    Mars lost its magnetosphere 4 billion years ago, so the solar wind interacts directly with the Martian ionosphere, keeping the atmosphere thinner than it would otherwise be by stripping away atoms from the outer layer. Both Mars Global Surveyor and Mars Express have detected these ionised atmospheric particles trailing off into space behind Mars.[29][30] As such the atmosphere of Mars is relatively thin; atmospheric pressure on the surface varies from around 30 Pa (0.03 kPa) on Olympus Mons to over 1155 Pa (1.155 kPa) in the depths of Hellas Planitia, with a mean surface level pressure of 600 Pa (0.6 kPa). This is less than 1% of the surface pressure on Earth (101.3 kPa). The equivalent pressure of Mars' atmosphere can be found at a height of 35 km above the Earth's surface. The scale height of the atmosphere is about 11 km; higher than Earth's 6 km due to the lower gravity.

    The atmosphere on Mars consists of 95% carbon dioxide, 3% nitrogen, 1.6% argon, and contains traces of oxygen and water.[5] The atmosphere is quite dusty, containing dust particulates of about 1.5 µm across which give the Martian sky a tawny color when seen from the surface.[31]

    Several researchers claim to have detected methane in the Martian atmosphere with a concentration of about 10 ppb by volume.[32][33] Since methane is an unstable gas that is broken down by ultraviolet radiation, typically lasting in the atmosphere for about 340 years,[34] its presence on Mars could indicate that there is (or has been within the last few hundred years) a source of the gas on the planet. Volcanic activity, comet impacts, and the existence of life in the form of microorganisms such as methanogens are among possible sources. It was recently shown that methane could also be produced by a non-biological process involving water, carbon dioxide, and the mineral olivine, which is known to be common on Mars.[35]

    For its winter, each pole lies in continuous darkness, while the surface gets so cold that as much as 25–30% of the atmosphere condenses out into thick slabs of CO2 ice (dry ice).[36] When the poles are again exposed to sunlight, the CO2 ice sublimes, creating enormous winds that sweep off the poles as fast as 400 km/h (250 mph). These seasonal actions transport large amounts of dust and water vapor, giving rise to Earth-like frost and large cirrus clouds. Clouds of water-ice were photographed

    The current understanding of planetary habitability—the ability of a world to develop and sustain life—favors planets that have liquid water on their surface. This requires that the orbit of a planet lie within a habitable zone, which for the Sun is currently occupied by Earth. Mars orbits half an astronomical unit beyond this zone and so water is frozen on its surface. The past flow of liquid water, however, demonstrates the planet's potential for habitability.

    The lack of a magnetosphere and extremely thin atmosphere of Mars are a greater challenge: the planet has little heat transfer across its surface, poor insulation against bombardment and the solar wind, and insufficient atmospheric pressure to retain water in a liquid form. (Water instead sublimates to a gaseous state.) Mars is also nearly, or perhaps totally, geologically dead; the end of volcanic activity has stopped the recycling of chemicals and minerals between the surface and interior of the planet.[53]

    Evidence suggests that the planet was once significantly more habitable than it is today, but whether living organisms ever existed there is still unclear. The Viking probes of the mid-1970s carried experiments designed to detect microorganisms in Martian soil at their respective landing sites, and had some apparently positive results, including a temporary increase of CO2 production on exposure to water and nutrients. However this sign of life was later disputed by many scientists, resulting in a continuing debate, with NASA scientist Gilbert Levin asserting that Viking may have found life. A re-analysis of the now 30-year-old Viking data, in light of modern knowledge of extremophile forms of life, has suggested that the Viking tests were also not sophisticated enough to detect these forms of life. The tests may even have killed a (hypothetical) life form.[54]

    At the Johnson space center lab organic compounds have been found in the meteorite ALH84001, which is supposed to have come from Mars. They concluded that these were deposited by primitive life forms extant on Mars before the meteorite was blasted into space by a meteor strike and sent on a 15 million-year voyage to Earth. Meanwhile small quantities of methane, and formaldehyde recently detected by Mars orbiters are both claimed to be hints for life, as these particles would quickly break down in the Martian atmosphere.[55][56] It is possible that these compounds may be replenished by volcanic or geological means such as serpentinization.[35]

     

    To the naked-eye, Mars usually appears a distinct yellow, orange, or reddish color, and varies in brightness more than any other planet as seen from Earth over the course of its orbit. When farthest away from the Earth, it is more than seven times as far from the latter as when it is closest (when least favourably positioned, it can be lost in the Sun's glare for months at a time). At its most favourable times—which occur twice every 32 years, alternately at 15 and 17-year intervals, and always between late July and late September—Mars shows a wealth of surface detail to a telescope. Especially noticeable, even at low magnification, are the polar ice caps.[71]

    The point of Mars’ closest approach to the Earth is known as opposition. The length of time between successive oppositions, or the Synodic period, is 780 days. Because of the eccentricities of the orbits, the times of opposition and minimum distance can differ by up to 8.5 days. The minimum distance varies between about 55 and 100 million km due to the planets' elliptical orbits.[5] The next Mars opposition will occur on December 24, 2007.

    On August 27, 2003, at 9:51:13 UT, Mars made its closest approach to Earth in nearly 60,000 years: 55,758,006 km (approximately 35 million miles). This occurred when Mars was one day from opposition and about three days from its perihelion, making Mars particularly easy to see from Earth. The last time it came so close is estimated to have been on September 12, 57,617 BC., the next time being in 2287. However, this record approach was only very slightly closer than other recent close approaches. For instance, the minimum distance on August 22, 1924 was 0.37284 AU, compared to 0.37271 AU on August 27, 2003, and the minimum distance on August 24, 2208 will be 0.37278 AU.[72] The orbital changes of Earth and Mars are making the approaches nearer: the 2003 record will be bettered 22 times by the year 4000.

    As Mars approaches opposition it begins a period of retrograde motion, which means it will appear to moved backwards in a looping motion with respect to the background stars. The next retrograde period will begin on November 16, 2007 and last through to January 31, 2008 with Mars appearing to move backwards through the sky from the constellation Gemini into Taurus.[73]

    The only occultation of Mars by Venus observed was that of October 3, 1590, seen by M. Möstlin at Heidelberg.[74]

     

    Der Mars ist, von der Sonne aus gesehen, der vierte Planet in unserem Sonnensystem und der äußere Nachbar unserer Erde. Er zählt zu den erdähnlichen (terrestrischen) Planeten. Aufgrund seiner orange- bis blutroten Farbe wurde er nach dem römischen Kriegsgott Mars benannt und wird oft auch als der Rote Planet bezeichnet. Die Färbung selbst verdankt der Planet Eisenoxid-Staub, der sich auf der Oberfläche und in der Atmosphäre verteilt hat. Somit ist der Mars ein „rostiger“ Planet.

    Das astronomische Zeichen des Mars soll einen Schild mit einem dahinter stehenden Speer darstellen und steht für den römischen Kriegsgott; in der AstrologieKategori: MARS | Yorum (yok) Yorum yaz! Kalıcı Bağlantı

    23/1/2005

    İnsan Genom Projesi

     

    İnsan Genom Projesi “tüm çağların en özel günü” ifadesi ile 26 Haziran 2000 tarihinde ABD Başkanı Bill Clinton, İngiltere Başbakanı Tony Blair ve özel şirketleri temsilen Celera Genomics yetkilileri, projenin ilk ayağını tamamladıklarını dünyaya ilân etiler. Proje sonuçları 2001 yılında açıklanmış olsa da eksikler ancak 2003 yılında bitirilebildi. Ancak Celera ve ABD, İngiltere, Japonya gibi devletlerin oluşturmuş olduğu uluslararası konsorsiyum tarafından açıklanan sonuçların %30 çelişkili çıkması üzerine her iki grup da birlikte çalışarak bu farklılığı ortadan kaldırmak için çalışmalarına devam ediyorlar. Bu farklılığın sebebinin dizilemedeki yaklaşım olduğunu belirten Dr. Venter, Celera şirketinin yaklaşımının doğru olduğunu savunuyor. Gen haritasının çıkarılmasında kullanılan gen, James Watson'a aittir. Bu proje sayesinde ilaç ve kimya sanayii uzmanlarına, Alzheimer’den vereme, kalp hastalıklarından astıma kadar her türlü hastalığı tedavi olanağı sağlayacak. Proje sayesinde tıp biliminin ciddi biçimde değişikliğe uğrayacağı, ayrıca uluslararası iş dünyasının bundan önemli kazanç sağlayacağı belirtiliyor. Proje, kanserden depresyona ve hatta yaşlılığa kadar tüm hastalıkların teşhis ve tedavisinde devrim yaratacak.

    Projenin sunacağı yenilikler, etik tartışmaları da beraberinde getirdi. Çünkü bu projenin, öjenik çalışmaların önünü açağı ve bu amaç doğrultusunda yapılacak deneysel girişimlere de hız kazandıracağı belirtiliyor. Projeye karşı çıkanlar bu projenin, doğanın doğal düzenini tehlikeye atacağını ve insanın, istihdamdan sigortaya kadar günlük yaşamın her alanında “genetik ayırımcılığa” yol açacağını ileri sürmekte. Muhalifler özellikle öjenizm faktörünün altını çizerken, bu projeyle insanların, diğer canlı türlerinin genleriyle beraber yapılacak deneysel çalışmaların sınırlarını büyük ölçüde genişleteceğini, bu yüzden de sonu belirsiz bir biyolojik ve ekolojik felakete götüreceğini öne sürüyorlar.

    TARİHÇE

    • 1954 DNA'nın keşfi
    • 1977 DNA dizilenmesi yöntemleri (Sanger ve Maxam-Gilbert)
    • 1976 İlk genetik şirketi
    • 1981-82 İlk transgenik fare Gen bankası detaları oluşturuldu.
    • 1983 İlk genetik hastalık haritalandı.(Huntington hastalığı)
    • 1985 Polimer zincir reaksiyonu'nun keşfi (Saiki, Mullis)
    • 1986 Pozisyonel klonlama. İlk insan genetik haritası (RFLP)
    • 1990 İnsan genom projesi başladı.ELSI oluşturuldu, Etik, Legal, Sosyal program BAC’ların keşfi (Bacteral artificial chromosome)
    • 1991 Gen fragmanlar expressed sequence tag (EST) Kistik fibroz geninin klonlanması
    • 1992 İnsan genomunun 2’nci nesil haritası Dataların serbest bırakılması
    • 1994 İlk genetik olarak değiştirilmiş besin domates Ayrıntılı insan gen haritası
    • 1995 İş yerlerinde genetik ayrımcılığın yasaklanması İnsan genomunun fiziksel haritası tamamlandı
    • 1996 Fare genetik haritası 280.000 EST İnsan DNA dizilenmesi başladı
    • 1997 E.coli genomu dizilendi
    • 1998 Celera genomics firması 3 yıl içinde projenin tamamlanacağını duyurdu.Mycobacterium tuberculosis dizilendi
    • 1999 22.kromozom dizilenmesi tamamlandı
    • 2000 Genomik bilgiye serbest ulaşım (Bill Clinton, Tony Blair) Meyve sineği genomu dizilendi

    2l. kromozom Dizi sonuçları açıklandı

    • 2001 Uluslararası sonuçlarını aynı anda açıkladı.
    • 2003 Eksiklikler giderilerek her iki grup da taslaklarını tamamladı.
    • 2005 İnsan Genom Projesi için her iki grubun da çalışmaları devam ediyor.

    In biology the genome of an organism is its whole hereditary information and is encoded in the DNA (or, for some viruses, RNA). This includes both the genes and the non-coding sequences of the DNA. The term was coined in 1920 by Hans Winkler, Professor of Botany at the University of Hamburg, Germany, as a portmanteau of the words gene and chromosome.[1]

    More precisely, the genome of an organism is a complete DNA sequence of one set of chromosomes; for example, one of the two sets that a diploid individual carries in every somatic cell. The term genome can be applied specifically to mean the complete set of nuclear DNA (i.e., the "nuclear genome") but can also be applied to organelles that contain their own DNA, as with the mitochondrial genome or the chloroplast genome. When people say that the genome of a sexually reproducing species has been "sequenced," typically they are referring to a determination of the sequences of one set of autosomes and one of each type of sex chromosome, which together represent both of the possible sexes. Even in species that exist in only one sex, what is described as "a genome sequence" may be a composite from the chromosomes of various individuals. In general use, the phrase "genetic makeup" is sometimes used conversationally to mean the genome of a particular individual or organism. The study of the global properties of genomes of related organisms is usually referred to as genomics, which distinguishes it from genetics which generally studies the properties of single genes or groups of genes.

     

    Most biological entities more complex than a virus sometimes or always carry additional genetic material besides that which resides in their chromosomes. In some contexts, such as sequencing the genome of a pathogenic microbe, "genome" is meant to include this auxiliary material, which is carried in plasmids. In such circumstances then, "genome" describes all of the genes and non-coding DNA that have the potential to be present.

    In vertebrates such as sheep and other various animals however, "genome" carries the typical connotation of only chromosomal DNA. So although human mitochondria contain genes, these genes are not considered part of the genome. In fact, mitochondria are sometimes said to have their own genome, often referred to as the "mitochondrial genome".

     

    Note that a genome does not capture the genetic diversity or the genetic polymorphism of a species. For example, the human genome sequence in principle could be determined from just half the DNA of one cell from one individual. To learn what variations in DNA underlie particular traits or diseases requires comparisons across individuals. This point explains the common usage of "genome" (which parallels a common usage of "gene") to refer not to any particular DNA sequence, but to a whole family of sequences that share a biological context.

    Although this concept may seem counter intuitive, it is the same concept that says there is no particular shape that is the shape of a cheetah. Cheetahs vary, and so do the sequences of their genomes. Yet both the individual animals and their sequences share commonalities, so one can learn something about cheetahs and "cheetah-ness" from a single example of either.

    The Human Genome Project was organized to map and to sequence the human genome. Other genome projects include mouse, rice, the plant Arabidopsis thaliana, the puffer fish, bacteria like E. coli, etc. In 1976, Walter Fiers at the University of Ghent (Belgium) was the first to establish the complete nucleotide sequence of a viral RNA-genome (bacteriophage MS2). The first DNA-genome project to be completed was the Phage Φ-X174, with only 5368 base pairs, which was sequenced by Fred Sanger in 1977 . The first bacterial genome to be completed was that of Haemophilus influenzae, completed by a team at The Institute for Genomic Research in 1995.

    In May 2007, the New York Times announced that the full genome of DNA pioneer James D. Watson had been recorded.[2] The article noted that some scientists believe this to be the gateway to upcoming personalized genomic medicine.

    Many genomes have been sequenced by various genome projects. The cost of sequencing continues to drop.

    Genomes are more than the sum of an organism's genes and have traits that may be measured and studied without reference to the details of any particular genes and their products. Researchers compare traits such as chromosome number (karyotype), genome size, gene order, codon usage bias, and GC-content to determine what mechanisms could have produced the great variety of genomes that exist today (for recent overviews, see Brown 2002; Saccone and Pesole 2003; Benfey and Protopapas 2004; Gibson and Muse 2004; Reese 2004; Gregory 2005).

    Duplications play a major role in shaping the genome. Duplications may range from extension of short tandem repeats, to duplication of a cluster of genes, and all the way to duplications of entire chromosomes or even entire genomes. Such duplications are probably fundamental to the creation of genetic novelty.

    Horizontal gene transfer is invoked to explain how there is often extreme similarity between small portions of the genomes of two organisms that are otherwise very distantly related. Horizontal gene transfer seems to be common among many microbes. Also, eukaryotic cells seem to have experienced a transfer of some genetic material from their chloroplast and mitochondrial genomes to their nuclear chromosomes.

    Als Genom oder auch Erbgut eines Lebewesens wird die Gesamtheit der vererbbaren Informationen einer Zelle bezeichnet, die als Desoxyribonukleinsäure (DNA) vorliegt. Einige Viren nutzen statt DNA RNA als Speichermedium. Das Genom enthält die Informationen, die zur Entwicklung (Ontogenese) und zur Ausprägung der spezifischen Eigenschaften des Lebewesens oder Virus notwendig sind. Diese Informationen sind in der Basensequenz der DNA enthalten.

    Der Begriff wurde 1920 von Hans Winkler geprägt. Die Erforschung des Genoms und die Wechselwirkung der darin enthaltenen Gene wird als Genomik bezeichnet (englisch „Genomics“).

    Die für die Vererbung von Eigenschaften und Merkmalen erforderlichen und auf der Ebene der Zellen und der Individuen weitergegebenen Informationen sind in den Desoxyribonukleinsäuren (DNA, von englisch „desoxyribonucleic acids“) enthalten, und zwar in Form von Sequenzen der DNA-Basen Adenin, Guanin, Cytosin und Thymin. Die DNA-Moleküle können in Abschnitte mit kodierenden und nicht-kodierenden Sequenzen eingeteilt werden. Die kodierenden Abschnitte (Gene) enthalten die Erbinformationen für bestimmte Proteine. Daneben gibt es DNA-Abschnitte, die der Genregulation dienen. Pseudogene sind durch Mutationen funktionslos gewordene und vom Organismus nicht mehr abgelesene Gene. Bei Eukaryoten findet durch das alternative Splicing eine Datenkompression statt, so dass die Genomgröße (in Basenpaaren gemessen) kleiner sein kann als die Anzahl der durch das Genom kodierten Merkmale.

    Bei allen Organismen, die komplexer als Viren sind, gibt es außerhalb der chromosomalen DNA (bei Eukaryoten „Karyom“ genannter Teil des Genoms) weitere Genombestandteile in anderen Zellteilen. So finden sich bei Bakterien und Archaebakterien essentielle Plasmide, bei Eukaryoten (Pflanzen, Tiere, Pilze) gibt es selbstständig vererbte DNA-Sequenzen in den Mitochondrien („Mitochondriom“) und Plastiden („Plastidom“), die aber zum Gesamtgenom der Zellen gehören.

    Die Angabe der Genomgröße eines Organismus bezieht sich auf die vorhandene Menge an DNA pro haploiden Zellkern, wobei entweder die Zahl der jeweils vorhandenen Basenpaare (bp) angegeben wird oder das Gewicht (die Masse) der DNA in der Einheit pg (Picogramm). 1 pg doppelsträngiger DNA besteht aus ca. 0,978·109 bp. Nach neueren Untersuchungen (Vinogradov, A.E., 2005: Genome size and chromatin condensation in vertebrates. Chromosoma 113: pp. 362-369) besitzt der Südamerikanische Lungenfisch (Lepidosiren paradoxa) mit 80 pg (7,84 x 1010 bp) das größte bisher bekannte tierische Genom. Ältere, aber wohl ungenauere Untersuchungen zeigen mit ca. 133 pg noch größere Genome, die ebenfalls bei Lungenfischen, allerdings bei der afrikanischen Art Äthiopischer Lungenfisch (Protopterus aethiopicus) gefunden wurden (aus: T.R.Gregory: Animal Genome Size Database. 2005). Mit 0,04 pg (weniger als 50 Millionen Basenpaare) besitzt das zum primitiven Tierstamm Placozoa gehörende, auf Algen lebende, etwa 2 mm große, wenig differenzierte Trichoplax adhaerens das kleinste bisher bekannte tierische Genom (T.R.Gregory 2005). Die Zahl der Basenpaare des Darmbakteriums Escherichia coli ist nur um einen Faktor 10 kleiner. Rekordhalter für das kleinste bakterielle Genom ist derzeit der Blattfloh-Endosymbiont Carsonella ruddii: er begnügt sich mit einem Ring aus nur knapp 160.000 Paaren von Nukleotiden, auf der er sämtliche Informationen speichert, die er zum Leben braucht[1].

    Bemerkungen [Bearbeiten]Bei Eukaryoten beziehen sich die Zahlenangaben auf den haploiden Chromosomensatz. Die meisten dieser Lebewesen sind Modellorganismen. Da die Anzahl der Gene je nach Organismus unterschiedlich genau bestimmt und Gegenstand aktueller Forschungen ist, können Angaben aus unterschiedlichen Quellen um wenige tausend Gene voneinander abweichen (Beispiel: Die Anzahl der Gene beim Homo sapiens sapiens liegt etwa im Bereich 20 000-30 000). Angaben zur Gendichte sind demnach auch nur als Richtwerte zu verstehen.


    Die DNA einer einzelnen menschlichen Zelle ist ca. 1,80 m lang. Eine Base auf einem DNA-Strang hat einen Informationsgehalt von 2 bit, da sie 22 = 4 Zustände (A / T / G / C) annehmen kann. Ausgehend von 3x109 Basenpaaren hat das Genom des Menschen einen Informationsgehalt von ca. 750 MB.

    Ein Vergleich der Genom-Größe mit der Komplexität und des Organisationsgrades des Organismus ergibt einen direkten Zusammenhang: Je größer das Genom, um so komplexer ist der Organismus.

    Ausnahmen bilden hierbei weniger komplexe Organismen mit hoher DNA-Menge (als „C-Wert-Paradoxon“ bezeichnet): einige Samenpflanzen, die Salamander und urtümliche Fische wie Stör, Hornhecht und Quastenflosser.

    Die höchste DNA-Menge weisen einfache Eukaryoten wie einige Amöben und die Urfarne (Psilopsida) mit rund einer Billion Basenpaare auf.

    Diese Arten enthalten einzelne Gene als tausendfache Kopien, und lange, nicht-Protein-kodierende Abschnitte. Auch im menschlichen Genom kommt ein etwa 300 Basenpaare langes DNA-Stück, die alu-Sequenz in ungefähr 300,000 Kopien vor und macht damit 3 % der gesamten DNA aus.

    Wird dagegen der Anteil der DNA, der nicht Proteine kodiert, betrachtet, ergibt sich ein direkter Zusammenhang zur Komplexität des Organisationsgrades. (Vergleiche dazu die Angaben zur Gendichte in der Tabelle oben rechts)

    Dieser Zusammenhang könnte darin begründet sein, dass diese Sequenzen zahlreiche regulatorische Aufgaben erfüllen. Zur Zeit (März 2005) wird die Möglichkeit diskutiert, dass die Komplexität eines Organismus in Zusammenhang mit der Menge an DNA steht, die zwar keine Proteine kodiert, aber dennoch transkribiert, also in RNA übertragen wird. Dabei werden Introns nicht als Reste alter Gene aufgefasst, sondern als Abkömmlinge beweglicher DNA-Abschnitte, vergleichbar mit den heutigen Gruppe-II-Introns. Diese und weitere RNA-Moleküle, die durch Transkription entstehen, und die weder m-, t- oder rRNAs sind, können Teil eines Regulationssystems sein, das neben den Proteinen die Entwicklung eines Organismus steuert. Zum Beispiel sind RNA-Signale an der Markierung des Chromatins beteiligt, wodurch die Genexpression gesteuert wird.

    Das RNA-Transkript enthält Basensequenzen, welche die Aminosäuresequenz von Proteinen kodieren. Die RNA wird dann als mRNA bezeichnet. Bei den Eukaryoten ist sie aus Exons und Introns zusammengesetzt und wird in diesem Zustand als prä-mRNA oder hnRNA bezeichnet. Sie wird noch vor der Translation bearbeitet (prozessiert), in dem die nichtkodierenden Introns herausgeschnitten werden. Die mRNA der Prokaryoten weist nie Introns auf.

    Die Aminosäuresequenzen-kodierende DNA ist beim Menschen 90 Mb groß, das sind 3 % des Genoms. Das entspricht 25000 Genen, die ungefähr 500.000 Proteine kodieren.

    Einige Basensequenzen stammen zwar von Genen ab, das Transkript wird aber nicht in eine Aminosäuresequenz übersetzt. Diese nichtkodierende DNA ist 810 Mb groß.Pseudogene sind veränderte Kopien funktionell aktiver Gene, die deren Expression steuern können. Introns werden noch im Zellkern der Eukaryoten aus der prä-mRNA herausgeschnitten. Ihre Funktion ist nicht vollständig geklärt. Einige enthalten Erkennungssequenzen für Replikationsfaktoren, die die Aktivität der RNA-Polymerase beeinflussen. Eine häufig geäußerte Vermutung ist, dass durch die zwischen die kodierenden Exons eingestreuten Introns die Mutationshäufigkeit in den kodierenden Sequenzen herabgesetzt ist. Dagegen spricht aber, dass auch in den Introns hochkonservierte consense-Sequenzen (siehe unten) zu finden sind. Eine andere Vermutung besteht darin, dass durch gelegentlich ungenaues Spleißen der prä-mRNA Eiweiße entstehen, die sich an bestimmten Stellen in nur wenigen Aminosäuren unterscheiden und somit etwas veränderte Eigenschaften aufweisen. Auf die Weise könnte bewerkstelligt werden, dass von einem Enzym-Typ stetes mehrere Versionen bereitgestellt werden, die etwas unterschiedliche Eigenschaften (zum Beispiel im pH- oder Temperatur-Optimum) aufweisen. Genfragmente entstehen dann, wenn es von einem Gen mehrere Kopien im Genom gibt und eine dieser Kopien durch Mutationen unbrauchbar wird.

    Das RNA-Transkript enthält Basensequenzen, welche die Basensequenz von RNAs kodieren. Diese Moleküle werden auch als ncRNAs (nc von engl. non coding = nicht kodierend) bezeichnet und erfüllen zahlreiche Aufgaben bei der Proteinbiosynthese. Einige davon sind erst vor kurzem bekannt geworden und noch nicht genauer erforscht. Es wird vermutet, dass die ncRNAs molekulare Fossilien aus der RNA-Welt sind (siehe chemische Evolution) und damit von Bedeutung für das Verständnis der Evolution der Lebewesen sind.tRNAs transportieren Aminosäuren zu den Ribosomen. rRNAs sind Bestandteile der Ribosomen und erfüllen dort strukturelle und katalytische Aufgaben. ssRNA (ssuRNA, small subunit RNA) ist die RNA für die kleine, lsRNA (lsuRNA, large subunit RNA) die für die große Untereinheit der Ribosomen. snRNAs sind Bestandteile der Spliceosomen, welche aus der prä-mRNA die Introns herausschneiden. Ebenfalls ein junges Forschungsgebiet ist die RNA-Interferenz (RNAi), eine weitere Möglichkeit der Regulation der Proteinbiosynthese, wobei kleinere RNA-Moleküle mit Teilen der mRNA reagieren und dadurch in der Regel die Translation verhindern. Solche RNA-Moleküle sind siRNAs (si von engl. short interfering), microRNAs, von welchen das menschlichen Genom mehrere Hundert aufweist. Es gibt auch Interaktionen von RNAs mit der DNA, mit Proteinen und mit niedermolekularen Substanzen.

    Mikro-RNA: Manche Introns enthalten zueinander komplementäre Abschnitte, so dass die prä-RNA nach der Transkription Haarnadelschleifen bilden kann. Diese werden durch spezielle Proteine des „Zensursystems“ (ursprünglich ein Abwehr-System gegen virale Doppelstrang-RNA) erkannt und so abgebaut, dass einsträngige RNA-Abschnitte entstehen, die an andere mRNAs binden und somit spezifisch (zielgenau) mRNA zerstören können (RNA-Interferenz) oder ihre Translation unterdrücken. Für einzelne Moleküle ist ihre Funktion bekannt: Sie sorgen dafür, dass Stammzellen sich nicht differenzieren, und steuern Zellvermehrung und Apoptose (programmierter Zellselbstmord) beim Umbau embryonaler Gewebe. Antisense-RNA: Die mRNA entsteht am codogenen (Matrizen-) Strang der DNA. Wird auch der komplementäre Strang abgelesen, entsteht eine zur mRNA komplementäre RNA. Verbinden sich mRNA und Antisense-RNA zu einem Doppelstrang, kann kein Protein mehr bei den Ribosomen gebildet werden. Auch dies stellt eine Möglichkeit der Regulation der Proteinbiosynthese dar. Beim Menschen gibt es mindestens 1600 antisense-Gene. 7SL-RNA ist Bestandteil der signal recognition particles, das sind Protein-RNA-Komplexe, welche den zielgerichteten Transport von Proteinen in der Zelle gewährleisten.

    Nichtkodierende Sequenzen

    Der übrige Teil des Genoms besteht aus Sequenzen, die nicht transkribiert werden. Er wird als extragenische DNA bezeichnet und weist ein Länge von insgesamt 2100 Mb auf.

    Davon besteht der größte Teil (1.680 Mb) aus einzelnen, individuellen oder nur selten wiederholten Basensequenzen. Dies sind in der Regel Sequenzen, an welche bestimmte Enzyme binden und dadurch die Replikation und Transkription steuern:An die Promoter-Sequenzen (TATA-Box) bindet die RNA-Polymerase Initiations- und Terminations-Sequenzen, markieren Beginn und Ende eines Gens Consense-Sequenzen sind hochkonservierte Sequenzen, die die Grenzen zwischen Exons und Introns markieren An Operator-Sequenzen oberhalb (engl. upstream) und unterhalb (engl. downstream) von Genen, an welche Regulatorproteine binden, um die Transkription zu beschleunigen oder zu verzögern und damit ihre Feinregulation übernehmen. Palindrome sind Erkennungssequenzen für Restriktionsendonukleasen. Bei den Abstandshaltern kommt es nicht auf die Sequenz, sondern die Zahl der Basen an. Deshalb können hier die Mutationsraten ohne Auswirkungen sehr hoch sein, solange es nicht zu Baseneinschub, oder Basenverlust kommt. Diese DNA-Abschnitte sorgen dafür, dass die Operator-Sequenzen im Falle der Transkription bei der Schleifenbildung in die richtige Position zu den Promotern gebracht werden, und so die RNA-Polymerase beeinflussen können. Untersuchungen an Cryptomonaden (einzelligen, Photosynthese betreibenden Eukaryonten) haben gezeigt, dass die Menge an so genannter nichtkodierenden DNA proportional zur Größe des Zellkerns ist und vermutlich eine wesentliche Rolle für die Strukturierung des Zellkerns hat.

    Der Rest der DNA von 420 Mb besteht aus hoch repetitiven Sequenzen.

    disseminierte (verstreute) genomweite Wiederholungen LTR-Elemente (LTR-Retrotransposons und Retroviren) (8,5 % des Gesamtgenoms). Sie gehen zum Teil auf Genom-Überreste von integrierten Retroviren zurück und können die gewebespezifische Aktivität von Wirtsgenen steuern. Zur Zeit (2005) sind 20 Gene des Menschen bekannt, die durch virale LTRs kontrolliert werden. Insgesamt konnten mindestens 600.000 retrovirale LTRs im menschlichen Genom gefunden werden. DNA-Transposone (3 % des Gesamtgenoms) LINE-Sequenzen (LINE 1, LINE 2) (long interspersed nuclear element) (21 % des Gesamtgenoms) SINE-Sequenzen (short interspersed nuclear element) (13 % des Gesamtgenoms) (z. B. Alu-Sequenz, die nur bei Primaten zu finden ist) ermöglichen eine Verlagerung einer Sequenz an eine andere Stelle des Genoms. Sie sind 70 bis ca. 500 Basen lange Retroposons, d.h. Elemente, deren Ortswechsel über eine transkribierte RNA-Sequenz erfolgt, deren cDNA-Produkt an anderer Stelle ins Genom integriert wird. In Genomen von Eukaryoten findet man bis zu 104 Kopien. Das Transkript der Alu-Sequenz wird durch das sogenannte „A-zu-I-Editing“ verändert: Das Nukleosid Adenosin zum Nukleosid Inosin umgewandelt. Dies findet vor allem im Gehirn statt. Es wird ein Zusammenhang zwischen Fehlern in diesem Prozess und Epilepsie und Depression vermutet.

    Tandemwiederholungen

    Die Anzahl der Wiederholungen variiert von Individuum zu Individuum, die Abweichungen sind vom Verwandtschaftsgrad abhängig. Deshalb sind sie für den genetischen Fingerabdruck geeignet. Die von der Norm abweichende Zahl an Wiederholungen kann Krankheiten auslösen.Mikrosatelliten-DNA, z. B. (CA)n, mit einer repetitiven Einheit von 2 bis 7 Basenpaaren. Sie sind im ganzen Genom verteilt, und werden auch zur genetischen Kartierung verwendet. Mikrosatelliten weisen eine hohe Mutat

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    30/4/2004

    GENETIK ENGLISH

    DNA, the molecular basis for inheritance.
    DNA, the molecular basis for inheritance.
    For the scientific journal see Genetics (journal)
     
    For the scientific journal see Genetics (journal)

    Genetics is the science of heredity and variation in living organisms.Knowledge that desired characteristics were inherited has been implicitly used since prehistoric times for improving crop plants and animals through selective breeding. However, the modern science of genetics, which seeks to understand the mechanisms of inheritance, only began with the work of Gregor Mendel in the mid-1800s.[4]

    Mendel observed that inheritance is fundamentally a discrete process with specific traits that are inherited in an independant manner. These basic units of inheritance is now known as "genes". In the cells of organisms, genes exist physically in the structure of the molecule DNA and the information genes contain is used to create and control the components of cells. Although genetics plays a large role in determining the appearance and behavior of organisms, it is the interaction of genetics with the environment an organism experiences that determines the ultimate outcome. For example, while genes play a role in determining a person's height, the nutrition and health that person experiences in childhood also have a large effect.

     

    Features of inheritance

    Discrete inheritance and Mendel's laws

    Main article: Mendelian inheritance
    A Punnett square depicting a cross between two pea plants heterozygous for purple (B) and white (b) blossoms.
    A Punnett square depicting a cross between two pea plants heterozygous for purple (B) and white (b) blossoms.

    At its most fundamental level, inheritance in organisms occurs by means of discrete traits, called "genes".[5] This property was first observed by Gregor Mendel, who studied the segregation of heritable traits in pea plants.[6][7] In his experiments studying the trait for flower color, Mendel observed that the flowers of each pea plant were either purple or white — and never an intermediate between the two colors. These different, discrete versions of the same gene are called "alleles".

    In the case of pea plants, each organism has two alleles of each gene, and the plants inherit one allele from each parent.[8] Many organisms, including humans, are diploid, with two alleles for each gene. Organisms with two copies of the same allele are called "homozygous", while organisms with two different alleles are "heterozygous".

    The set of alleles for a given organism is called its genotype, while the visible trait the organism has is called its "phenotype". When organisms are heterozygous, often one allele is called "dominant" as its qualities "dominate" the phenotype of the organism, while the other allele is called "recessive" as its qualities "recede" and are not observed. Dominant alleles are often abbreviated with a capital letter, while recessive alleles are given a lowercase version of the same letter.[9] Some alleles do not have complete dominance and instead have incomplete dominance by expressing an intermediate phenotype, or codominance by expressing both alleles at once.[10]

    When parents breed to produce children, their children randomly inherit one of the two alleles from each parent. The outcome of these crosses can be visualized by use of a Punnett square. These observations of discrete inheritance and the segregation of alleles are collectively known as "Mendel's first law" or the "Law of Segregation".

    Assortment and interactions of multiple genes

    Human height is a complex genetic trait. Francis Galton's data from 1889 shows the relationship between offsping height as a ******** of mean parent height. While correlated, the remaining variation in offspring heights indicates environment is also an important factor in this trait.
    Human height is a complex genetic trait. Francis Galton's data from 1889 shows the relationship between offsping height as a ******** of mean parent height. While correlated, the remaining variation in offspring heights indicates environment is also an important factor in this trait.

    Organisms have thousands of genes, and in diploid organism assortment of these genes are generally independent of each other. This means that the inheritance of an allele for yellow or green pea color is unrelated to the inheritance of alleles for white or purple flowers. This phenomenon, known as "Mendel's second law" or the "Law of independent assortment", means that the alleles of different genes get shuffled between parents to form children with many different combinations. (Some genes do not assort independently, demonstrating genetic linkage, a topic discussed later in this article.)

    Often different genes can interact in a way that influences the same trait. In the blue-eyed Mary, for example, there exists a gene with alleles that determine the color of flowers: blue or magenta. Another gene, however, controls whether the flowers have color at all: color or white. When a plant has two copies of this white allele, its flowers are white — regardless of whether the first gene has blue or magenta alleles. This interaction between genes is called "epistasis", with the second gene epistatic to the first[11]

    Many traits are not discrete features (eg. purple or white flowers) but are instead continuous features (eg. human height and skin color). These "complex traits" are the product of interactions of many genes.[12] The influence of these genes is mediated, to varying degrees, by the environment an organism has experienced. The degree to which an organism's genes contribute to a complex trait is called "heritability".[13] Measurement of the heritability of a trait is relative, though — in a more variable environment, the environment has a bigger influence on the total variation of the trait. For example, human height is a complex trait with a heritability of 89% in the United States. In Nigeria, however, where people experience a more variable access to good nutrition and health care, height has a heritability of only 62%.[14]

    The molecular basis for inheritance

    The molecular structure of DNA. Bases pair through the arrangement of hydrogen bonding between the strands.
    The molecular structure of DNA. Bases pair through the arrangement of hydrogen bondingbetween the strands.

    DNA and the genetic code

    Main articles: DNA and Genetic code

    The molecular basis for genes is deoxyribonucleic acid (DNA). DNA is composed of a chain of nucleotides, of which there are four types: adenine (A), cytosine (C), guanine (G), and thymine (T). Genetic information exists in the sequence of these nucleotides, and genes exist as stretches of sequence along the DNA chain.[15] Viruses are a small exception — the similar molecule RNA instead of DNA is often the genetic material of a virus.[16] In all non-virus organisms, which are composed of cells, each cell contains a full copy of that organism's DNA, called its genome.[17]

    In the cell, DNA exists as a double-stranded molecule, coiled into the shape of a double-helix. Each nucleotide in DNA preferentially pairs with its partner nucleotide on the opposite strand: A pairs with T, and C pairs with G. Thus, in its two-stranded form, each strand effectively contains all necessary information, redundant with its partner strand. This structure of DNA is the physical basis for inheritance: DNA replication duplicates the genetic information by splitting the strands and using each strand as a template for a partner strand.[18]

    The genetic code: DNA, through a messenger RNA intermediate, codes for protein with a triplet code.
    The genetic code: DNA, through a messenger RNA intermediate, codes for protein with a triplet code.

    Genes express their ********al effect through the production of proteins, which are complex molecules responsible for most ********s in the cell. Proteins are chains of amino acids, and the DNA sequence of a gene (through an RNA intermediate) is used to produce a specific protein sequence. Each group of three nucleotides in the sequence, called a codon, corresponds to one of the twenty possibly amino acids in protein — this correspondence is called the genetic code.[19] The specific sequence of amino acids results in a unique three-dimensional structure for that protein, thereby determining its behavior and ********.

    Chromosomes, recombination, and linkage

    Main articles: Chromosome, Chromosomal crossover, and Genetic linkage
    Thomas Hunt Morgan's illustration of a double crossover between chromosomes.
    Thomas Hunt Morgan's illustration of a double crossover between chromosomes.

    Genes are arranged linearly along the long chains of DNA sequence, called chromosomes. In bacteria, each cell has a single circular chromosome, while eukaryotic organisms (which includes plants and animals) have their DNA arranged in multiple linear chromosomes. These DNA strands are often extremely long; the largest human chromosome, for example, is about 140 million base pairs in length.[20]The DNA of a chromosome is associated with structural proteins which organize, compact, and control access to the DNA, forming a material called chromatin; in eukaryotes chromatin is usually composed of nucleosomes, repeating units of DNA wound around a core of histone proteins.[21]

    While haploid organisms have only one copy of each chromosome, most animals and many plants are diploid, containing two copies of each chromosome and thus two copies of every gene. An exception exists in the sex chromosomes, specialized chromosomes many animals have evolved that play a role in determining the sex of the organism. On the other chromosomes (called autosomes), the two alleles for a gene are located on identical loci of sister chromatids.

    The diploid nature of chromosomes allows for genes on different chromosomes to assort independently, thereby recombining to form new combinations of genes. Genes on the same chromosome would theoretically never recombine, however, were it not for the process of chromosomal crossover. During crossover, chromosomes exchange stretches of DNA, effectively shuffling the gene alleles between the chromosomes. This process of chromosomal crossover generally occurs during meiosis, a series of cell divisions that creates haploid germ cells which later combine with other germ cells to form child organisms.

    The probability of chromosomal crossover occurring between two given points on the chromosome is related to the distance between them. For an arbitrarily long distance, the probability of crossover is high enough that the inheritance of the genes is effectively uncorrelated. For genes that are closer together, however, the lower probability of crossover means that the genes demonstrate geneti linkage. The linkages between a series of genes can be combined to form a linear linkage map that roughly describes the arrangement of the genes along the chromosome.

    Epigenetic inheritance

    Although DNA is the genetic material of life, there are some aspects of cells that are not encoded in DNA sequence (eg. chromatin and DNA modifications) that are inherited by daughter cells when cells divide. These features are called "epigenetic" — the prefix "epi-" means "on top of" or "in addition to". Epigenetic features create differences between cells sharing the same genome, allowing cells to differentiate into different tissues in multicellular organisms. Although epigenetic features are generally dynamic over the development of organisms, some, like the phenomenon of paramutation, have multigenerational inheritance.

    Genetic research and technology

    The common fruit fly (Drosophila melanogaster) is a popular model organism in genetics research.
    The common fruit fly (Drosophila melanogaster) is a popular model organism in genetics research.

    Model organisms and genetics

    Although geneticists originally studied inheritance in a wide range of organisms, researchers began to specialize in studying the genetics of a particular subset of organisms. The fact that significant research already existed for a given organism would encourage new researchers to choose it for further study, and so eventually a few "model organisms" became the basis for most genetics research. Common research topics in model organism genetics include the study of gene regulation and the involvement of genes in development and cancer.

    Organisms were chosen, in part, for convenience — short generation times and facile genetic manipulation made some organisms popular genetics research tools. Widely used model organisms include the gut bacterium Escherichia coli, the plant Arabidopsis thaliana, baker's yeast (Saccharomyces cerevisiae), the nematode Caenorhabditis elegans, the common fruit fly (Drosophila melanogaster), and the common house mouse (Mus musculus).

    Medical genetics research

    Medical genetics research seeks to find and study the genetic causes of human diseases. When searching for an unknown gene that may be involved in a disease, researchers commonly use genetics linkage and genetic pedigree charts to find the location on the genome associated with the disease. At the population level, researchers take advantage of Mendelian randomization to look for locations in the genome that are associated with diseases, a technique especially useful for multigenic traits not clearly defined by a single gene. Once a candidate gene is found, further research is often done on the same gene (called an orthologous gene) in model organisms.

    Genetic technologies

    E coli colonies on a plate of agar, an example of cellular cloning and often used in molecular cloning.
    E coli colonies on a plate of agar, an example of cellular cloning and often used in molecular cloning.

    A variety of techniques exist for manipulating DNA in the laboratory. Restriction enzymes are a commonly used enzyme that cuts DNA at specific sequences, producing predictable fragments of DNA. The use of ligation enzymes allows these fragments to be stitched back together, and by ligating fragments of DNA together from different sources, researchers can create recombinant DNA. Often associated with genetically modified organisms, recombinant DNA is commonly used in the context of plasmids — short circular DNA fragments with a few genes on them. By inserting plasmids into bacteria and growing those bacteria on plates of agar (to isolate clones of bacteria cells), researchers can clonally amplify the inserted fragment of DNA (a process known as molecular cloning). (Cloning can also refer to the creation of clonal organisms, through various techniques.)

    DNA can also be amplified using a procedure called the polymerase chain reaction (PCR). By using specific short sequences of DNA, PCR can exponentially amplify a targeted region of DNA. Because it can amplify from extremely small amounts of DNA, PCR is often used to detect the presence of specific DNA sequences.

    DNA sequencing & genomics

    One of the most fundamental technologies developed to study genetics, DNA sequencing allows researchers to determine the sequence of nucleotides in DNA fragments. Developed in 1978 by Frederick Sanger and coworkers, chain-termination sequencing is now routinely used to sequence DNA fragments. With this technology, researchers have been able to study the molecular sequences associated with many human diseases. As sequencing has become less expensive and with the aid of computational tools, researchers have sequenced the genomes of many organisms by stitching together the sequences of many different fragments (a process called "genome assembly"). The Human genome project was completed in 2005, sequencing the genome to 92% coverage.

    The large amount of sequences available has created the field of "genomics", research which uses computational tools to search for and analyze patterns in the full genomes of organisms. Genomics can also be considered a subfield of bioinformatics, which uses computational approaches to analyze of large sets of biological data.

    Mutation and evolution

    Mutations

    Main article: Mutation
    Gene duplication allows diversification by providing redundancy: one gene can mutate and lose its original ******** without harming the organism.
    Gene duplication allows diversification by providing redundancy: one gene can mutate and lose its original ******** without harming the organism.

    During the process of DNA replication, errors occasionally occur in the polymerization of the second strand (these error rates are generally extremely low, 1 error in every 10-100 million bases).[22] These errors, called mutations, can have an impact on the phenotype of an organism, especially if they occur within the protein coding sequence of a gene. Processes which increase the rate of changes in DNA are called "mutagenic": mutagenic chemicals promote errors in DNA replication, often by interfering with the structure of base-pairing, while UV radiation induces mutations by causing damage to the DNA structure. Chemical damage to DNA occurs naturally as well, and cells use DNA repair mechanisms to repair mismatches and breaks in DNA -- nevertheless, the repair sometimes fails to return the DNA to its original sequence.

    In organisms which use chromosomal crossover to exchange DNA and shuffle genes, errors in alignment during meiosis can also cause mutations. Errors in crossover are especially likely when similar sequences cause partner chromosomes to adopt a mistaken alignment, which makes some regions in genomes more prone to mutating in this way. These errors create large structural changes in DNA sequence -- duplications, inversions or deletios of entire regions, or the accidental exchanging of whole parts between different chromosomes (called "translocation").

    Selection and evolution

    Main article: Evolution

    Mutations produce organisms with different genotypes, and those differences can result in different phenotypes. Many genetic mutations, called "neutral mutations", have a negligible effect on an organism's phenotype, health, and reproductive fitness. Mutations which do have an effect are often deleterious, but occasionally mutations arise which are beneficial in the current environmental context of the organism.

    Population genetics research studies the distributions of these genetic differences within populations and how the distributions change over time. Changes in the frequency of an allele in a population can be influenced by natural selection, where a given allele's higher rate of survival and reproduction causes it to become more frequent in the population over time. Genetic drift can also occur, where chance events lead to random changes in allele frequency.

    A genetic tree of life on earth, constructed by comparison of several orthologous gene sequences
    A genetic tree of life on earth, constructed by comparison of several orthologous gene sequences

    Over many generations, the genomes of organisms can change, resulting in the phenomenon of evolution. Mutations and the selection for beneficial mutations can cause a species to evolve into forms that better survive their environment, a process called adaptation. New species are formed through the process of speciation, a process often caused by geographical separations that allow different populations to genetically diverge.

    As sequences diverge and change during the process of evolution, these differences between sequences can be used as a molecular clock to calculate the evolutionary distance between them. Genetic comparisons are generally considered the most accurate method of characterizing the relatedness between species, an improvement over the sometimes deceptive comparison of phenotypic characteristics. The evolutionary distances between species can be combined to form evolutionary trees. These trees are commonly considered the most accurate representation of relatedness, although the transfer of genetic material between unrelated species (known as "horizontal gene transfer" and most common in bacteria) cannot be represented by the tree.

    History of genetics

    Main article: History of genetics
    Morgan's observation of sex-linked inheritance of a mutation causing white eyes in Drosophila led him to the hypothesis that genes are located upon chromosomes.
    Morgan's observation of sex-linked inheritance of a mutation causing white eyes in Drosophila led him to the hypothesis that genes are located upon chromosomes.

    Gregor Johann Mendl, a German-Czech Augustinian monk and scientist, is often called the "father of modern genetics", a title given to him due to his early work on the heredity of plants. In his paper "Versuche über Pflanzenhybriden" ("Experiments on Plant Hybridization"), presented in 1865 to the Brunn Natural History Society, Gregor Mendel traced the inheritance patterns of certain traits in pea plants and showed that they could be described mathematically.[6] Although not all features show these patterns of Mendelian inheritance, his work suggested the utility of the application of statistics to the study of inheritance.

    The significance of Mendel's observations was not understood until early in the twentieth century, after his death, when his research was re-discovered by other scientists working on similar problems. The word "genetics" itself was coined by William Bateson, a significant proponent of Mendel's work, in a letter to Adam Sedgwick, dated April 18, 1905.[23] Bateson promoted the term "genetics" publicly in his inaugural address to the Third International Conference on Plant Hybridization (London, England) in 1906.[24]

    In the decades following rediscovery and popularization of Mendel's work, numerous experiments sought to elucidate the molecular basis of DNA. In 1910 Thomas Hunt Morgan argued that genes reside on chromosomes, based observations of a sex-linked white eye mutation in fruit flies. In 1913 his student Alfred Sturtevant used the phenomenon of genetic linkage and the associated recombination rates to demonstrate and map the linear arrangement of genes upon the chromosome.

    Francis Crick's first sketch of a DNA double helix.
    Francis Crick's first sketch of a DNA double helix.

    Although chromosomes were known to contain genes, chromosomes were composed of both protein and DNA — it was unknown which was critical for heredity or how the process occurred. In 1928, Frederick Griffith published his discovery of the phenomenon of transformation (see Griffith's experiment); sixteen years later, in 1944, Oswald Theodore Avery, Colin McLeod and Maclyn McCarty used this phenomenon to isolate and identify the molecule responsible for transformation as DNA.[25] The Hershey-Chase experiment in 1952 identified DNA (rather than protein) as the genetic material of viruses, further evidence that DNA was the molecule responsible for inheritance.

    James D. Watson and Francis Crick resolved the structure of DNA in 1953, using the X-ray crystallographywork of Rosalind Franklin that indicated the molecule had a helical structure. Their double-helix model paired a sequence of nucleotides with a "complement" on the other strand. This structure not only provided a physical explanation for information contained within the order of the nucleotides, but also a physical mechanism for duplication through separation of strands and the reconstruction of a partner strand based on the nucleotide pairings. They famously observed this in their paper, stating: "It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.";

    In the following decades, an explosion of research based on this understanding of the molecular nature of DNA became possible. The development of chain-termination DNA sequencing in 1977 enabled the determination of nucleotide sequences on DNA,[26] and the PCR method developed by Kary Banks Mullis in 1983 allowed the isolation and amplification of arbitrary segments of DNA.[27] These and other techniques, through the pooled efforts of the Human Genome Project and parallel private effort by Celera Genomics, culminated in the sequencing of the human genome in 2001.

     

     

    Kategori: GENETIK ENGLISH | Yorum (1) Yorum yaz! Kalıcı Bağlantı

    27/4/2004

    GENLER VE ALELER

     

    GENLER VE ALELER
     

    Kalıtım olayı doğrudan kromozomların mitoz ve mayoz bölünmeler ve döllenmedeki davranışlarına bağlıdır. Her bir kromozomda sayısız kalıtım birimleri, genler bulunur ve bunların herbiri birbirinden farklı olup ayrı ayrı bir veya birkaç karekteri birden kontrol ederler. Genler kromozomlarda yer aldıklarına ve bir hücrede her bir kromozomdan iki tane bulunduğundan her bir hücrede genler bir çift olarak bulunur. Mayozda kromozomlar birbirlerinden ayrılıp döllenmede tekrar biraraya geldiklerine göre genler de aynı davranışı gösterirler. Her bir kromozom genetiksel olarak linear düzende gen dizilerine sahipmiş gibi davranır ve bu gen düzenlenişleri homolog kromozomlarda birbirinin aynısıdır.

    Bir karekteri kontrol eden gen, belli bir kromozomun belirli bir noktasında bulunur ve bu noktaya gen lokusu denir. Kromozomlar mayoz bölünme sırasında sinapsis gösterdiklerinde, homolog kromozomlar nokta nokta yani gen ile gen şeklinde aynı lokuslar karşılıklı gelecek şekilde birleşme gösterirler. Bir karakter kalıtımı ancak birbirlerine zıt iki durum olduğu zaman incelenebilir. Mendel'in çalışmalarında bezelyelerde ele aldığı şekil morfolojisinde düzgün ve buruşuk tohum özellikleri gibidir. Canlı birey böyle zıt durumlardan sadece birini gösterilebilir. Bu nedenle allel genlerden söz edilir.

    Homolog kromozomların aynı lokuslarında yer alan, iki veya bazen daha çok sayıda alternatif karekterlerin genlerine allel genler denir. Düzgün ve buruşuk tohum morfolojilerini belirleyen genler gibi. Gen çiftlerini benzer alleller şeklinde taşıyan bireyler homozigot genotipli, gen çiftlerini farklı alleller şeklinde taşıyan bireyler ise heterozigot genotipli olarak ayrılırlar. Buna göre "DD" ve "dd" genotipli bezelye bitkileri homozigot domaninat ve resesif genotipli, "Dd" genotipli bezelyeler ise heterozigotturlar. Bütün genlerin tek tek toplamı ve bütün kromozom seti o canlının genomu şeklinde tarif edilir.

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

    –         Dolly has been clone taken from her mother’s udder.

    –         Unfertilized egg cell with nucleus removed were fused with the cell from the udder

    –         Fused cell was stimulated to divide and implanted into a surrogate mother.

    –         Polly

    •          Single diploid cell originating from adult sheep was altered (human gene coding for blood cloting factor was introduced) and the cell was fused with denucleated egg cell. The fused cell was stimulated to divide and implated into surrogate mother.

    •          Lamb contains the human gene in every cell of her body.

               

    •          Gene Therapy for severe combined immunodeficiency (SCID)

     

    •          Gene therapy succeeded for 15 French baby boys who suffered from SCID, called X-linked SCID, because it is caused by a mutated gene encoding a subunit — called γc (gamma-c) — of the receptor for several interleukins, including interleukin-7 (IL-7).

     

    •          IL-7 is essential for converting blood stem cells into the precursors of T cells. Boys with X-linked SCID do not develop ********al immune response system.

     

    •          Doctors isolated blood stem cells from the bone marrow of each boy;

         treated the cells with a vector containing the normal gene for the γc interleukin receptor subunit; returned the treated cells to each donor.

     

    •          The results: Now after as long as three and one-half years, these boys  are able to live normal lives at home instead of inside a sterile "bubble"; have normal (with two exceptions*) numbers of T cells .


    Bacteriophage

     

    Bacteriophage

     

    Bacteriophages are bacteria viruses.

     When they infect their host they use host machinery to replicate their DNA.

     

     In bacteriophage vectors part of the phage DNA is replaced by the gene of interest (max. size is 15-25 kbp)

     

     When this new recombinant phage DNA infects a host the gene of interest will be replicated.

     

    commonly used phage vectors include M13 and λ.

    Infection of bacterial cell:

     

    Tail sticks to the cell wall

     

    DNA from head is squired into bacteria

     

    Phage genes are transcribed by bacterial RNA polymerase

     

    Corresponding mRNA is translated using bacterial machinery

     

    Newly replicated phage DNA and head and tail protein ensemble spontaneously

     

    Lyses of host bacteria results in formation of plagues.

     

    •          l phage – most common vector for construct of genomic DNA

    •          Cosmid vector – can accumulate up to 45 kb

    –         a cross between a plasmid and l vector

    –          contains antibiotic resistance genes and Ori for replication in bacteria

    –         Contains cos sites to be packed in phage particles

    –         Inside of bacteria, the cosmid DNA circularizes and replicates as a large plasmid

     

    •          YAG yeast artificial chromosome ,  up to 200 to 500 kb

    –         Large DNA fragments are inserted into yeast DNA sequence and introduced into yeast cell by transformation          


    T CELL

     


    Genes

     

    Genes are encoded in the sequence of chemical base pairs called nucleotides that make up the intertwining strands of the DNA double helix.  Each nucleotide consists of a 5-carbon sugar (deoxyribose for DNA or ribose for RNA), a phosphate group, and a nitrogen-containing base attached to the sugar.  The nitrogenous base determines the type of nucleotide; thymine (T), cytosine (C), adenine (A), and guanine (G) are all found in DNA, while C, A, G, and uracil (U) are found in RNA.  The bases on one strand pair up with bases on the complementary strand via hydrogen bonds to form the double-stranded helix of DNA.  T always pairs up with A, and C always pairs up with G 

    The order of the nucleotide sequence on the strand codes for gene products, but the end result depends heavily on the starting point.  The genetic code relates the nucleotide sequence (A, T, C, G) to the amino acid sequence of proteins, a process called translation that we will address in more detail at a later point.  For now, you should know that the nucleotide sequence is read in terms of codons, which are triplets of nucleotides (ATG, GAA, GTA, etc).  Each codon codes for an amino acid; for example, GCT codes for the amino acid alanine.  Changing the nucleotide sequence will also change the amino acid product.  Regions of the DNA sequence that can be translated into protein products are termed coding regions.  However, many genes, especially in higher organisms, have pieces of non-coding DNA (called introns) inserted within the coding regions (called exons)

    A genome is the sum total of the genes of an organism.  The structure of DNA differs in prokaryotes, such as bacteria, as compared to eukaryotes, such as protists, fungi, plants, and animals.  Compared to eukaryotes, prokaryotes have smaller, simpler genomes.  On average, prokaryotes have only about one-thousandth as much DNA as a eukaryote cell.  In most prokaryotes, the DNA is concentrated in the nucleoid region and consists of one double-stranded DNA molecule in the form of a ring.  In contrast, the genome of eukaryotes consists of linear DNA molecules packaged along with proteins into multiple chromosomes, which are found in the nucleus.  In the case of the prokaryote Escherichia coli (E. coli), about 4,400 genes have been identified in the 4.6-million nucleotide pairs of DNA sequenced in one circular chromosome.  In comparison to the genetic material of bacteria, the genome of humans has 3.3 X 109 base pairs and about 34,000 genes in a total of 46 chromosomes.

     

    DNA sequencing of genes revolutionized the study of genetic material and launched the era of genomics, the genetic mapping, physical mapping, and sequencing of entire genomes.  Databases were designed to store, search, analyze and display all of the information encoded in the genomes.  Located at the National Center for Biotechnology Information (NCBI) website, Entrez provides access to several sequence databases, as well as genomic information.  The Nucleotide database contains sequence data from GenBank, European Molecular Biology Laboratory (EMBL), and DNA Data Bank of Japan (DDBJ), the members of the tripartite, international collaboration of sequence databases.  The Genome database provides views for a variety of genomes, complete chromosomes, contiged sequence maps, and integrated genetic and physical maps.  In this lab, we will explore both of these databases to find the similarities and differences between prokaryotic and eukaryotic genes and genomes.

     


    DNA replication

     

    DNA replication. The double helix is unwound by a helicase and topoisomerase. Next, one DNA polymerase produces the leading strand copy. Another DNA polymerase binds to the lagging strand. This enzyme makes discontinuous segments (called Okazaki fragments) before DNA ligase joins them together.


    T7 RNA polymerase

     

     

    T7 RNA polymerase producing a mRNA (green) from a DNA template (red and blue). The enzyme is shown as a purple ribbon.


     telomere repeats

     

     

    Structure of a DNA quadruplex formed by telomere repeats.

    At the ends of the linear chromosomes are specialized regions of DNA called telomeres. The main ******** of these regions is to allow the cell to replicate chromosome ends using the enzyme telomerase, as the enzymes that normally replicate DNA cannot copy the extreme 3' ends of chromosomes. As a result, if a chromosome lacked telomeres it would become shorter each time it was replicated. These specialized chromosome caps also help protect the DNA ends from exonucleases and stop the DNA repair systems in the cell from treating them as damage to be corrected.In human cells, telomeres are usually lengths of single-stranded DNA containing several thousand repeats of a simple TTAGGG sequence.

     

    Kategori: GENLER VE ALELER | Yorum (yok) Yorum yaz! Kalıcı Bağlantı

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