THE DISTRIBUTION OF GENES ON CHROMOSOMES - A CYTOLOGICAL APPROACH

Studies during the last 20 years have shown that the chromosomes of ma ny organisms, especially those of higher vertebrates, consist of a ser ies of segments having different properties. These can be recognized a s, for example, G- and R-bands. Recent studies have indicated that gen es tend to lie in the R-bands rather than in the G-bands, although the number of genes that has been mapped with high precision is, as yet, only a very small proportion of the total, probably much less than 1%. We have therefore sought to study the distribution of genes on chromo somes using a cytological approach in conjunction with ''universal'' m arkers for genes. Such markers include mRNA and the gene-rich, G + C-r ich H3 fraction of DNA, both of which can be localized using in situ h ybridization, and DNase I hypersensitivity, and digestion by restricti on enzymes known to show selectivity for the CpG islands associated wi th active genes, both of which can be detected using in situ nick tran slation. We have chosen to use the approaches involving in situ nick t ranslation and have shown that the patterns of DNase I hypersensitivit y and of CpG islands on human chromosomes show a strict correspondence to R-banding patterns: Deviations from R-banding patterns reported by previous investigators who have made similar studies appear to be att ributable to excessive digestion. On the other hand, we have not found the expected differentiation between the active and inactive X chromo somes; this may perhaps be attributable to such factors as the demethy lation of some non-island CpGs in the inactive X and the possible alte rations of chromatin structure caused by methanol-acetic-acid fixation affecting DNase I hypersensitivity.