Definition
DNA — deoxyribonucleic acid — is the molecule that stores an organism's genetic information. It is a long double-stranded chain, twisted into a helix, whose sequence is written in just four chemical letters: the bases A, C, G, and T. The order of those letters is the information.
Sapolsky is careful to demote DNA from its popular status as destiny. DNA is a molecule, not a mastermind. It does nothing on its own; it is read, regulated, and acted upon by the rest of the cell. A gene is simply a stretch of this sequence that codes for a product.
Why it matters
How it works
The two strands of DNA are complementary: A pairs with T, C pairs with G. That pairing lets the molecule be copied faithfully when a cell divides — each strand serves as a template for a new partner. When a gene is needed, the strands locally separate so the sequence can be transcribed into RNA. The chemical marks layered onto DNA, and the proteins it wraps around, govern which stretches are accessible at any moment.