II.

The Structure of DNA

The most important component of a chromosome is the single continuous molecule of DNA. This double-stranded molecule, shaped like a twisted ladder, is composed of linked chemical compounds known as nucleotides. Each nucleotide consists of three parts: a sugar known as deoxyribose, a phosphate compound, and any one of four bases—adenine, thymine, guanine, or cytosine. These parts are linked together so that the sugar and the phosphate form the two parallel sides of the DNA ladder. The bases from each side join in pairs to form the rungs of the ladder—specifically, adenine always pairs with thymine, and guanine always pairs with cytosine.

The genetic code is specified by the order of adenines, thymines, guanines, and cytosines in the DNA ladder. A particular section of the DNA ladder usually has a unique sequence of base pairs. Because a gene is merely one of these sections of the DNA ladder, it too possesses a unique sequence of base pairs, and this sequence can be used to distinguish the gene from other genes and to map its location on the chromosome.