What is DNA?

DNA, deoxyribonucleic acid, is the molecule that stores the genetic instructions for building and operating an organism. Its structure is the famous double helix: two strands twisted around each other, with rungs made from four chemical bases, adenine, thymine, guanine, and cytosine, that pair in fixed combinations, A with T and G with C. The sequence of those letters is the information. Genes are stretches of that sequence that code for proteins, the working machinery of cells, and the human genome runs to roughly three billion base pairs, packaged into 46 chromosomes and copied into nearly every cell in your body, about two meters of DNA coiled into a space too small to see.

The base-pairing rule is what makes heredity possible: because A always pairs with T and G with C, either strand can serve as a template to rebuild the other, so DNA copies itself before cells divide, and instructions pass from parent to offspring. Structure was worked out in 1953 by Watson and Crick, drawing crucially on Rosalind Franklin's X-ray images, a contribution long underrecognized. Modern genetics has complicated the simple gene-equals-trait picture considerably: most traits involve many genes plus environment, large portions of the genome regulate rather than code, and epigenetics governs which genes are switched on where. Reading and editing DNA has become routine enough that sequencing a genome costs a tiny fraction of what the first one did, and tools like CRISPR allow targeted edits, which is why the ethical questions have become as pressing as the technical ones.