COMPOSITIONAL MAPPING OF MOUSE CHROMOSOMES AND IDENTIFICATION OF THE GENE-RICH REGIONS

The mouse genome is a mosaic of isochores, consisting of long (> 300 k b), compositionally homogeneous DNA segments that can be divided into two GC-poor families, L1 and L2, representing 56% of the genome, and t wo GC-rich families, H1 and H2, representing 26% and 7% of the genome, respectively, the remaining 11% being formed by satellite and ribosom al DNAs. (GC is the molar fraction of guanine + cytosine in DNA.) The mouse genome differs from the human genome (which is representative of most mammalian genomes) because it shows a narrower compositional spe ctrum of isochores and it has a karyotype formed exclusively by acroce ntric chromosomes. The chromosomal distribution of the four isochore f amilies, as investigated here by in situ hybridization of single-copy sequences from compositional DNA fractions, has shown that G(iemsa) ba nds are essentially composed of sc-poor isochores, whereas R(everse) b ands comprise three subsets of bands: R' bands, containing GC-poor iso chores and GC-rich isochores of the H1 family, and T and T' bands, con taining all H2 isochores (in addition to other isochores), the former containing a higher proportion of H2 isochores than the latter. Mouse T and T' bands are generally syntenic with, and are compositionally re lated to, human T and T' bands and have the highest gene concentration s. These findings indicate that the distribution of isochore families and genes in chromosomal bands is basically similar in mouse and in hu man genomes, in spite of their remarkable differences and their extrem ely large phylogenetic distance.