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Identical twins start life with almost the same DNA, but they grow more different with age. Diet, smoking, illness, stress and chance leave chemical tags on their genes, called epigenetic marks, which turn some genes up and others down.

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Epigenetics means changes in how genes are used that do not alter the DNA sequence itself. One common mark is a small chemical group, called a methyl group, attached to DNA, which tends to quieten the gene beneath it. In 2005, a team led by Manel Esteller in Madrid compared 40 pairs of identical twins aged 3 to 74. The youngest twins had almost identical marks, but older pairs differed much more, especially those who had spent more of their lives apart or lived differently. NASA's Twins Study offered a rare case: Scott Kelly spent 340 days on the International Space Station while his identical twin, Mark, stayed on Earth. Many changes in Scott's gene activity appeared in space, and most faded after he came home.

Türkçe oku
Tek yumurta ikizleri hayata neredeyse aynı DNA ile başlar, ama yaşları ilerledikçe birbirlerinden farklılaşırlar. Beslenme, sigara, hastalıklar, stres ve tesadüf, genlerin üzerinde epigenetik işaret denen kimyasal etiketler bırakır; bu etiketler bazı genleri daha çok, bazılarını daha az çalıştırır.

Epigenetik, DNA dizisinin kendisi değişmeden genlerin kullanılış biçiminde ortaya çıkan değişiklikler demektir. En yaygın işaretlerden biri metil grubu denen küçük bir kimyasal gruptur: DNA'ya bağlanır ve bağlandığı geni çoğu zaman susturur. Manel Esteller'in yönettiği bir ekip 2005'te Madrid'de, 3 ile 74 yaş arasındaki 40 çift tek yumurta ikizini karşılaştırdı. En küçük ikizlerin işaretleri neredeyse aynıydı; yaşça büyük çiftler ise birbirinden çok daha farklıydı. Aradaki açık en çok, hayatlarının daha büyük bölümünü ayrı geçirmiş ya da farklı yaşamış ikizlerde görüldü. NASA'nın İkizler Çalışması da ender bir örnek sundu: Scott Kelly Uluslararası Uzay İstasyonu'nda 340 gün geçirirken tek yumurta ikizi Mark Dünya'da kaldı. Scott'ın gen etkinliğinde uzaydayken pek çok değişiklik görüldü; bunların çoğu Dünya'ya döndükten sonra kayboldu.

Sources and further reading

Background reading: Epigenetics
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