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The human genome, the complete set of DNA instructions for building a person, is about 3.1 billion base pairs long. Each pair is a rung on the twisted DNA ladder, made of two chemical 'letters' from a four-letter alphabet: A, C, G and T.

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Printed in ordinary type, those 3.1 billion letters would fill thousands of books. Most cells carry two copies, one inherited from each parent; red blood cells, which lose their nucleus, are the main exception. Stretched out, the DNA in a single cell would be about 2 metres long, yet it is wound tightly around proteins so that it fits inside a nucleus a few thousandths of a millimetre across. Only about 1 to 2% of the genome spells out recipes for proteins, in roughly 20,000 genes; much of the rest helps control when and where genes are switched on, and the role of some stretches is still debated. The Human Genome Project published an essentially complete sequence in 2003, and the last gaps were filled in 2022, when the Telomere-to-Telomere consortium read one human genome from end to end.

Türkçe oku
İnsan genomu, bir insanın yapılışını belirleyen DNA talimatlarının tümüdür ve yaklaşık 3,1 milyar baz çiftinden oluşur. Her baz çifti, burgulu DNA merdiveninin bir basamağıdır; bu basamak, dört harfli bir alfabeden (A, C, G ve T) iki kimyasal “harf”in eşleşmesiyle oluşur.

Sıradan bir yazı karakteriyle basılsa bu 3,1 milyar harf binlerce kitabı doldururdu. Hücrelerin çoğunda genomun iki kopyası bulunur; biri anneden, biri babadan gelir. Çekirdeğini kaybeden alyuvarlar bunun başlıca istisnasıdır. Tek bir hücredeki DNA uç uca eklense yaklaşık 2 metre uzunluğunda olurdu; ama proteinlere sıkıca sarılı olduğu için, çapı milimetrenin yalnızca binde birkaçı olan çekirdeğe sığar. Genomun yalnızca yüzde 1-2 kadarı, yaklaşık 20.000 gende proteinlerin tarifini taşır; geri kalanın büyük bölümü genlerin ne zaman ve nerede açılıp kapanacağını düzenler, bazı bölümlerin işlevi ise hâlâ tartışmalıdır. İnsan Genomu Projesi neredeyse eksiksiz diziyi 2003'te yayımladı; son boşluklar da 2022'de, Telomere-to-Telomere adlı uluslararası ekip bir insan genomunu uçtan uca okuyunca dolduruldu.

Sources and further reading

Background reading: Human genome
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