Gene density in the Giemsa bands of human chromosomes

The human genome is formed by isochores belonging to five families, L1, L2, H1, H2 and H3, that are characterized by increasing GC levels and gene con centrations. In-situ hybridization of DNA from different isochore families provides, therefore, information not only on the correlation between isocho res and chromosomal bands, but also on the distribution of genes in chromos omes. Three subsets of R(everse) bands were identified: H3(+), H3* and H3(- ), that contain large, moderate, and no detectable amounts, respectively, o f the gene-richest H2 and H3 isochores, and replicate very early and early, respectively, in S phase of the cell cycle. Here, we investigated the GC l evels, replication timings and DNA compaction of G(iemsa) bands. We showed that G bands comprise two different subsets of bands, one of which is predo minantly composed of L1 isochores, replicates at the end of the S phase, ha s a higher DNA compaction relative to H3(+) bands and corresponds to the da rkest G bands of Francke (1994). In contrast, the other subset is composed of L2 and H1 isochores, has less-extreme properties in replication and comp osition and corresponds to the less-dark G bands of Francke.