Kornberg was the first to produce an actual picture of this process at the molecular level, in the important group of organisms called eukaryotes, which, as opposed to bacteria, have well-defined cell nuclei.
Mammals, as well as ordinary yeast, belong to this group of organisms.
In its citation announcing the award, the Royal Swedish Academy said: "Understanding of how transcription works also has a fundamental medical importance. Disturbances in the transcription process are involved in many human illnesses such as cancer, heart disease and various kinds of inflammation."
The 59-year-old is part of the Stanford University school of medicine in Palo Alto, California.
His father, Arthur Kornberg, won the Nobel Prize in medicine in 1959 for his studies of how genetic information was transferred from one DNA molecule to another.
He is the lone winner of the prize, and the fifth American to win a Nobel prize this year.
So far, all the awards, for medicine, physics and chemistry, have gone to Americans.
Each prize includes a cheque for $1.4m, a diploma and a medal, which will be awarded by King Carl XVI Gustaf of Sweden at a ceremony in Stockholm on December 10.