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← 🌱 Biology · usually met in Std 8-12

DNA

The molecule that carries the instructions for building and running a living thing, written in a four-letter alphabet and copied every time a cell divides.

A double helix Two metres of it is folded into every one of your cells. The pairing rule A T G C T A C G A always pairs with T, G always with C. That fixed rule is WHY it can be copied: split the two strands and each one carries everything needed to rebuild its partner exactly.
Drawn for Star Student.
A model of the DNA double helix
Ude (public domain) · Public domain · where it came from ↗

DNA is a double helix: two strands twisted around each other, held together by pairs of bases. There are four bases — A, T, G and C — and they pair in a fixed way, A with T and G with C. That rule is why the molecule can be copied: separate the strands and each one carries all the information needed to rebuild its partner.

A gene is a stretch of DNA that specifies a protein, and proteins do nearly all the work of a cell. The whole set of an organism's DNA is its genome. Mutations — changes in the sequence — happen constantly; most do nothing, some cause disease, and a very few turn out to be useful, which is the raw material natural selection works on.

Worth knowing

  • The DNA in one human cell is about two metres long, folded into a nucleus a few thousandths of a millimetre across.
  • Humans and chimpanzees share roughly 98-99% of their DNA sequence.
  • The structure was published in 1953 by Watson and Crick, using X-ray data produced by Rosalind Franklin.

Want to see it moving? Search YouTube for “DNA” ↗ — we send you to the search rather than to one video, because a link to one video eventually rots.

See also

🌱 The cell🌱 Genetics and inheritance🌱 Evolution by natural selection

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Last gone through: 1 October 2026 · How these entries are written

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