Chromosomes and cellular radiosensitivity-II

Abstract Data regarding chromosome volumes, nucleotide content per chromosome and concentration (nucleotides/ μ 3 ) of chromosomal nucleic acid were compiled for 120 diverse organisms ranging from viruses to higher plants and animals. Interrelationships among these variables were studied both with respect to each other and to cellular radiosensitivity ( D 0 ). It was found that the nucleotide content of most prokaryotes is linearly correlated with chromosome volume with a slope =1. Another direct relationship also exists between these two variables among eukaryotes with a line of the same slope but with a different intercept. These correlations appear to hold over seven orders of magnitude. The nucleotide content of single-stranded viruses followed a line of shallow slope ( When D 0 was plotted against chromosome volume on a log-log scale, the organisms formed eight regression groups (radiotaxa) whose slopes were = − 1. However, when D 0 was plotted against total nucleotide content per chromosome, the organisms with the exception of myxoviruses became aligned into four parallel regressions also having slopes = − 1, but the groupings of organisms on these lines were not the same as those based on chromosome volume. Thus, depending on the parameter used, the same organisms formed a series of four or eight regressions. For either parameter, however, D 0 was inversely related to the parameter used. The groupings of organisms in both cases were mostly independent of their usual taxonomical classification, ploidy level, or genetic complexity. The correlations between D 0 and chromosome volume may not be simply the consequence of the direct proportionality between nucleotide content and chromosome volume although in many cases, i.e., plants, such a proportionality did exist. Concentration of chromosomal nucleic acid did not bear any obvious relationship to D 0 Compared to the ranges of the other variables, the range in concentration of nucleic acid in the chromosomes of many organisms was relatively small. The various interrelationships described may be useful in estimating cellular radiosensitivity. In terms of energy absorption at D 0 however, the regressions of D 0 against nucleotide content could be considered of little significance because energy must be absorbed within some critical volume and a volume calculated from numbers of nucleotides is thought to be highly artificial and, in most cases, unrealistic. It was concluded that the groupings of organisms based on interphase chromosome volume are probably more significant than those based on nucleotide content per chromosome. This is because the effective volume of the nucleic acid as far as ionizing radiation and energy absorption are concerned is probably its volume as it actually exists dispersed throughout the volume of the intact interphase chromosome. This effective volume appears to be adequately represented by the volume of the ‘interphase’ chromosome. These nuclear parameters appear to be related to D 0 in such a way that, in general, a difference occurring in either chromosome volume or nucleic acid content should be expected to lead to a difference in radiosensitivity.