Definition
Transcription is the first step of gene expression: the cell copies a gene's DNA sequence into a matching strand of RNA. The DNA stays in place as the master copy; the RNA is the working transcript the rest of the cell uses.
Sapolsky highlights transcription because it is the chokepoint of regulation. Most of the control over whether a gene contributes to a trait is exercised right here — at the decision of whether transcription happens at all, and how often. Almost everything the environment does to the genome, it does by influencing this step.
Why it matters
How it works
An enzyme called RNA polymerase docks at a gene's promoter and moves along the DNA, reading one strand and assembling a complementary RNA strand base by base. Whether the polymerase can dock and proceed depends on transcription factors bound nearby — proteins that recruit or block it — and on how tightly the DNA is packed. Environmental signals shift those conditions, dialing transcription up or down.