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Almost every cell in the human body that has a nucleus carries essentially the same DNA. A skin cell, a liver cell and a nerve cell differ because each switches on its own set of genes and keeps the rest switched off.

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Human DNA holds about 20,000 genes that are recipes for proteins. Switching a gene on so that the cell reads it is called gene expression. As an embryo develops, proteins called transcription factors and chemical tags on the DNA lock each cell into its role; the study of such tags, which change how genes are used without changing the DNA itself, is called epigenetics. In 1962 the British biologist John Gurdon showed that specialised cells keep every instruction: he put the nucleus of a tadpole's gut cell into a frog egg whose nucleus had been destroyed, and it grew into a normal tadpole. In 2006 Shinya Yamanaka turned mature mouse cells back into stem cells, which can become any type of cell, using four genes. The two shared the 2012 Nobel Prize in Physiology or Medicine. Mature red blood cells, which have no nucleus, are an exception.

Türkçe oku
İnsan vücudunda çekirdeği olan hücrelerin hemen hepsi özünde aynı DNA'yı taşır. Deri, karaciğer ve sinir hücrelerini birbirinden ayıran, her birinin kendine özgü bir grup geni çalıştırıp gerisini kapalı tutmasıdır.

İnsan DNA'sında protein tarifi taşıyan yaklaşık 20.000 gen bulunur. Bir genin açılmasına, yani hücrenin onu okuyup proteinini üretmesine gen ifadesi denir. Embriyo gelişirken transkripsiyon faktörü denen proteinler ve DNA'ya eklenen kimyasal işaretler her hücreyi kendi görevine kilitler. Bu işaretler DNA dizisini değiştirmez, yalnızca genlerin nasıl kullanılacağını etkiler; onları inceleyen bilim dalına epigenetik denir. Uzmanlaşmış hücrelerin talimatların tamamını hâlâ taşıdığını 1962'de İngiliz biyolog John Gurdon gösterdi: Bir iribaşın bağırsak hücresinden aldığı çekirdeği, kendi çekirdeği yok edilmiş bir kurbağa yumurtasına yerleştirdi ve bu yumurtadan normal bir iribaş gelişti. 2006'da Shinya Yamanaka, olgun fare hücrelerini dört gen yardımıyla, her tür hücreye dönüşebilen kök hücrelere geri çevirdi. İkisi 2012 Nobel Fizyoloji veya Tıp Ödülü'nü paylaştı. Olgun alyuvarlar ise bir istisnadır, çünkü çekirdekleri yoktur.

Sources and further reading

Background reading: Cellular differentiation
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