An organic substance that encodes and carries genetic information and is the fundamental element of heredity.

The thousands of genes that make up each chromosome are composed of deoxyribonucleic acid (DNA), which consists of a five-carbon sugar (deoxyribose), phosphate, and four types of nitrogen-containing molecules (adenine, guanine, cytosine, and thymine). The sugar and phosphate combine to form the outer edges of a double helix, while the nitrogen-containing molecules appear in bonded pairs like rungs of a ladder connecting the outer edges. They are matched in an arrangement that always pairs adenine in one chain with thymine in the other, and guanine in one chain with cytosine in the other. A single DNA molecule may contain several thousand pairs. As the transmitter of inherited characteristics, DNA replicates itself exactly and determines the structure of new organisms, which it does by governing the structure of their proteins.

The specific order...