The highest gene concentrations in the human genome are in telomeric bands of metaphase chromosomes.
Chromosome in situ suppression hybridization has been carried out on human metaphase chromosomes to localize the G+C-richest human DNA fraction (which only represents 3.5% of the genome), as isolated by preparative equilibrium centrifugation in Cs2SO4/3,6-bis(acetatomercurimethyl)-1,4-dioxane density gradient. This fraction essentially corresponds to isochore family H3. The rationale for carrying out this experiment is that this isochore family has, by far, the highest gene concentration, the highest concentration in CpG islands, the highest transcriptional and recombinational activity, and a distinct chromatin structure. The in situ hybridization results obtained show that the H3 isochore family is localized in two coincident sets of bands of human metaphase chromosomes: telomeric bands and chromomycin A3-positive 4',6-diamidino-2-phenylindole-negative bands. This result is the first step toward a complete compositional map of the human karyotype. Because the G+C gradient across isochore families is paralleled by a gene concentration gradient, such a map has structural, functional, and evolutionary relevance.