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Stretched end to end, the DNA inside a single human cell would be about two metres long, yet it fits into a nucleus about six thousandths of a millimetre across. That is like packing more than 20 kilometres of fine thread into a tennis ball.

Biology

Science

The story behind the card

This is possible because the DNA is wound up with great care. The double helix wraps around tiny spools of proteins called histones, each spool taking about 150 “letters” of DNA, and the spools sit along the strand like beads on a string. The beads coil and fold again into a fibre called chromatin, and before a cell divides, chromatin is packed tighter still into the 46 chromosomes visible under a microscope. The German biochemist Albrecht Kossel discovered histones in 1884; the beads were first seen with an electron microscope in 1974. The packing is not only storage: when the cell loosens a stretch, it can read the genes there, and when it keeps a stretch tight, those genes stay switched off. Each nucleus holds its own two-metre copy, about three billion letters inherited from each parent.

Türkçe oku
Tek bir insan hücresindeki DNA uç uca eklense yaklaşık iki metre tutar. Oysa bu DNA, çapı milimetrenin binde altısı kadar olan hücre çekirdeğine sığar. Bu, 20 kilometreden uzun ince bir ipliği bir tenis topunun içine yerleştirmeye benzer.

Bu, DNA'nın çok düzenli biçimde sarılmasıyla mümkün olur. Çift sarmal, histon denen küçük protein makaralarına sarılır. Her makaraya DNA'nın yaklaşık 150 “harfi” dolanır ve makaralar ipe dizilmiş boncuklar gibi sıralanır. Bu boncuk dizisi kendi üzerine yeniden kıvrılarak kromatin denen lifi oluşturur. Hücre bölünmeden önce kromatin daha da sıkı paketlenir ve mikroskopta görülen 46 kromozom ortaya çıkar. Histonları 1884'te Alman biyokimyacı Albrecht Kossel keşfetti; boncuklar ise ilk kez 1974'te elektron mikroskobuyla görüldü. Bu paketleme yalnızca yer kazanmak için değildir: Hücre bir bölgeyi gevşettiğinde oradaki genleri okuyabilir, sıkı tuttuğunda ise o genler kapalı kalır. Her hücre çekirdeği iki metrelik kendi kopyasını taşır: Biri anneden, biri babadan gelen, her biri yaklaşık üç milyar harflik iki dizi.

Sources and further reading

Background reading: DNA
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