The Fifth Letter

Abstract

In the biological sciences lew 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 arc sometimes qualified with accents. We should not be surprised to find that there are similar qualifications in the DNA language. The most evident of these is methy Icytosine, where the base C acquires a chemical grouping (mcthyl) [2]. Thus, in many organisms DNA has five letters — A, C, Me-C, G and T. Apart from the pattern of the four regular bases, there is a pattern of methylation at intervals along a DNA sequence. A brief consideration of the fifth letter is needed to conclude our discussion of evolutionary bioinformatics.

“We have had some absurd attempts ... to apply mathematics to biology, but ... my hope is still that I may live to see mathematics applied to biology properly. The most promising place for beginning, I believe, is the mechanism ofpattern.” W. Bateson to G. H. Hardy (1924) [1].