The Fifth Letter

Abstract

In the biological sciences few generalizations are absolute and we have already noted strange bases in RNA in addition to the usual ‾ A, C, G and U (Fig. 5.1). For many purposes DNA can be considered solely in terms of its four major bases ‾A, C, G and T. However, in written languages single letters are sometimes qualified with accents. The words gène and gêne have very different meanings in French. We should not be surprised to find that there are similar qualifications in the DNA language. The most evident of these is methylcytosine, where the base C acquires a chemical grouping (methyl) [2]. Thus, in many organisms DNA has five letters ‾ A, C, Me‾C, G and T. Apart from the four regular bases, there is a pattern of methylation at intervals along a DNA sequence.