Sample Page from Chapter 4
Required Reading:
The Molecular Basis of Heredity

What is the Chemical Composition of a Nucleotide?

Genes are segments of DNA molecules which consist of a linear sequence of subunits known as nucleotides. Each nucleotide has three components: a phosphate group, a sugar and a nitrogen containing base (Fig. 4.4).

In DNA the sugar is always deoxyribose. The different types of nucleotide differ only in the nature of the nitrogen-containing base. In DNA there are four alternative bases: adenine, thymine, guanine, and cytosine. These are the only four "letters" of the DNA genetic "alphabet." Aren't we lucky? When writing out genetic information these bases are abbreviated by convention to: A, T, G and C. The phosphate groups and the deoxyribose sugars form the backbone of each strand of DNA. The bases are joined to the deoxyribose and stick out sideways. A single strand of DNA is shown in Figure 4.5 and Figure 4.6.

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