What is DNA?

DNA — deoxyribonucleic acid — is a double-helix molecule made of nucleotide pairs: adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G). Human DNA contains approximately 3.2 billion base pairs organized into 23 chromosome pairs. If unwound, the DNA from a single cell would stretch about 2 meters, and the total DNA in your body placed end-to-end would span roughly 2× the diameter of the Solar System. Despite this complexity, 99.9% of DNA sequences are identical between any two humans — the 0.1% accounts for all our genetic diversity. Only about 1.5% of DNA codes for proteins; the rest was once called "junk DNA" but is now known to have regulatory roles.