National Cancer Institute
Science Behind the News Understanding Cancer
A drawing of the DNA molecules in the cell nucleus, and a diagram of one gene.

DNA Structure

Genes reside within large DNA molecules, which are composed of two chemical strands twisted around each other to form a "double helix." Each strand is constructed from millions of chemical building blocks called "bases." DNA contains only four different bases (abbreviated A, T, G, and C), but they can be arranged in any sequence. The sequential order of the bases in any given gene determines the message the gene contains, just as the letters of the alphabet can be combined in different ways to form distinct words and sentences.

page 1page 2page 3page 4page 5page 6page 7page 8page 9page 10page 11page 12page 13page 14page 15page 16page 17page 18page 19page 20page 21page 22page 23page 24page 25page 26page 27page 28page 29page 30page 31page 32page 33page 34page 35page 36page 37page 38page 39page 40page 41page 42page 43page 44page 45page 46page 47page 48page 49page 50page 51page 52page 53page 54page 55page 56page 57page 58page 59page 60page 61page 62