A plot of G + C content against sequence length of 640 bacterial chromosomes shows the points are widely scattered in the upper triangular area.

In this work, we study the distribution of G + C content against chromosome size among 640 fully sequenced bacterial chromosomes. When %G + C is plotted versus increasing chromosome length, it is shown that the points denoting 640 bacterial chromosomes are widely scattered in the upper triangular area and the triangle is right-angled. The mean G + C content increases while the range or variance of G + C contents decreases with length. The net effect of these two trends is to exclude data from a triangular area that should represent long A + T-rich chromosomes. Thus, the shorter chromosomes have a wide, 20-70%, range of G + C contents while the longest chromosomes are severely restricted to a G + C content near 70%. To generate the distribution type of right-angled triangle, two conditions are requisite. Consequently, two possible related explanations are presented to interpret the relationship. One aims to give the reason why most large bacterial chromosomes have high G + C contents and the other tries to interpret why most A + T-rich bacterial chromosomes tend to be small in size. Both of the two interpretations are proposed from the selectionist viewpoint and the latter is an addition to the former one. According to the theories, the bacteria with large chromosomes and bacteria with A + T-rich chromosomes have different tactics to cope with ultraviolet radiation and other physical or chemical damage.