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 concentrations. In-situ hybridization of DNA from different isochore families provides, therefore, information not only on the correlation between isochores and chromosomal bands, but also on the distribution of genes in chromosomes. Three subsets of R(everse) bands were identified: H3+, H3* and H3-, that contain large, moderate, and no detectable amounts, respectively, of 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 levels, replication timings and DNA compaction of G(iemsa) bands. We showed that G bands comprise two different subsets of bands, one of which is predominantly composed of L1 isochores, replicates at the end of the S phase, has a higher DNA compaction relative to H3+ bands and corresponds to the darkest G bands of Francke (1994). In contrast, the other subset is composed of L2 and H1 isochores, has less-extreme properties in replication and composition and corresponds to the less-dark G bands of Francke.