Figure 1.
The double helix. a. Deoxyribonucleic acid (DNA) is a double-stranded, helical molecule composed of nucleotide units. Replication of DNA occurs through the pairing of nucleotide bases to form new strands. b. Diagram of the molecular structure of the base pairs of DNA. “Sugar” indicates the bond to deoxyribose. Phosphates connect the sugars to make the “backbone” of DNA. A nitrogen base plus deoxyribose and a phosphate group constitutes a nucleotide. Adenine (A) always pairs with thymine (T), and cytosine (C) always pairs with guanine (G). c. Diagram showing pairing of the nucleotide bases in a short DNA segment. (Courtesy G.L. Schumann)