Genetic activity of the G- and R-band DNA of human mitotic chromosomes

The concept of genetic inactivity of G-band DNA had been reinvestigated using the modified approach of Korenberg et al (1978). Coefficients of correlation and partial correlation between the relative gene density (g'), the relative G-band material richness (kH/C) and the relative chromosome size (s') were calculated. The kH/C was calculated as the ratio of brightness of fluorescence of chromosomes stained by Hoechst 33258 (Hi) and by chromomycin A3(Ci). The kH/C is the characteristics of G-band chromosome richness, because G-bands become bright after Hoechst 33258 staining and R-bands are bright after chromomycin A3 staining, while no significant C-bands in chromosomes which may be stained by these fluorochromes are discovered. For the kH/C determination the flow cytometry data of Langlois et al (1982) were used. The relative size of chromosomes was determined, based on the flow cytometry data of Young et al (1979). According to Korenberg, the "gene density" (g') in a chromosome was calculated as a ratio of the number of genes located in the chromosome before 1984 (Human Gene Mapping 7) to the relative size of this chromosome. Correlation between the "gene density" and the G-band richness was rs = -0.65. Out of 107 genes located in either G- or R-bands (Human Gene Mapping 7), 90 were mapped in the R-band and only 17 were ascribed to the G-band in metaphase chromosomes. The data on gene replication time show that all genes of the general cell activity and a portion of tissue-specific genes replicate during the early S-phase, together with R-band materials. These three independent lines of evidence are consistent with the notion that the R-band DNA is more genetically active than G-band DNA. The nature of "junk" DNA of G-bands is discussed.