THE EVOLUTIONARY TRANSITION FROM URACIL TO THYMINE BALANCES THE GENETIC-CODE

A multivariate quantitative physicochemical characterization of the fi ve bases adenine (A), cytosine (C), guanine (G), thymine (T) and uraci l (U), followed by principal component analysis, shows that the relati ve dissimilarities between the bases of DNA (A, C, G and T) are almost the same (i.e. balanced). In contrast, mRNA (containing U instead of T) has a considerably larger relative physicochemical similarity betwe en C and U than between all other pairs of bases and is therefore inhe rently more unbalanced. These results provide a physicochemical explan ation of the presence of thymine instead of uracil as an element of DN A. The principal component scores enable a quantitative description of nucleic acid sequence data to be made for structure-activity modellin g purposes.