The short-sequence designs of isochores from the human genome.

The human genome, a typical mammalian genome, is made up of long (approximately 1-Mb, on average) regions, the isochores, that are fairly homogeneous in base composition and belong in five families characterized by different GC levels. An analysis of di- and tri-nucleotide densities in the isochores from the five families has shown large differences. These different "short-sequence designs:" (i) account for the fractionation of human DNA (and vertebrate DNA in general) when using sequence-specific ligands in density gradients, (ii) are very similar in whole isochores and in the corresponding intergenic sequences and introns, (iii) are reflected in different codon usages, (iv) lead to amino acid differences that increase the thermal stability of the proteins encoded by genes located in increasingly GC-rich isochore families, and (v) correspond to different chromatin structures.