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DNA is a set of instructions, written in a chemical code, that every living thing carries in almost every cell. The alphabet has only four letters — A, T, G and C — but there are about three billion of them in a human, arranged in a specific order. That order is what makes you a human being rather than an oak tree, and what makes you different from every other human being.
Deoxyribonucleic acid is a double-stranded polymer of nucleotides, each comprising a deoxyribose sugar, a phosphate group and one of four nitrogenous bases: adenine, thymine, guanine and cytosine. The two antiparallel strands are held together by hydrogen bonding between complementary base pairs — A with T, G with C — and wound into a double helix. The human genome contains approximately 3.1 billion base pairs.
The four letters
DNA's alphabet has four letters: adenine, thymine, guanine and cytosine, always written A, T, G and C. Their order along the chain is the information. Three billion letters is roughly a thousand books' worth of text, packed into a space too small to see.
Why it comes in pairs
DNA is two strands wound together, and the pairing is strict: A always pairs with T, G always pairs with C. That has an important consequence — each strand contains all the information, because you can always work out one from the other. It is how cells copy DNA before dividing, how they repair damage, and how gene editing finds its target in the first place.
How it is packed
Stretched out, the DNA in one human cell would be about two metres long. It fits inside a nucleus a few millionths of a metre across by winding tightly around spool-like proteins called histones and coiling those spools into chromosomes. How tightly a region is packed affects whether it can be read at all — which is the basis of epigenetics.
Common questions
What does DNA stand for?
Deoxyribonucleic acid — 'deoxyribo' for the sugar in its backbone, 'nucleic' because it was first found in cell nuclei, and 'acid' for its chemistry.
Is all my DNA genes?
No. Only about 1 to 2 per cent of human DNA codes for proteins. Much of the rest regulates when and how strongly genes are switched on, and some has no known function. 'Junk DNA' was a premature label.
Sources
- National Human Genome Research Institute
Talking Glossary of Genomic Terms ↗ - MedlinePlus Genetics, U.S. National Library of Medicine
Help Me Understand Genetics ↗