The gene-richest bands of human chromosomes replicate at the onset of the S-phase.

Previous investigations on the correlations between isochore organization and human chromosomal bands have identified three sets of R(everse) bands: H3+, H3* and H3-, endowed with large, moderate, and no detectable amounts of the gene-richest H3 isochores, respectively. In the present work we compared the replication timing of these three sets of bands and showed that the chromosomal bands containing H3 isochores replicate almost entirely (in the case of H3+ bands) or largely (in the case of H3* bands) at the onset of S phase, whereas chromosomal bands not containing H3 isochores (H3- bands) replicate later. The existence, at a resolution of 400 bands per haploid genome, of at least three distinct subsets of R bands is, therefore, not only supported by their GC and gene concentration but also by their replication times.