Chromosomes 21 and 22: Gene Density

Abstract Isochores are long DNA (deoxyribonucleic acid) segments that are fairly homogeneous in base composition. The GC (guanine+cytosine)‐richest and GC‐poorest isochores of the genomes from warm‐blooded vertebrates are characterised by the highest and the lowest gene concentrations, respectively. They correspond to specific types of chromosomal bands, namely the GC‐richest isochores are in the subset of R‐positive bands generally located at the telomeric chromosomal end and the GC‐poorest are in the darker G‐positive bands. The distribution of the isochores along the chromosomes determines the formation of the light/dark G/R bands, with the compositional difference at the local level having a great relevance. Chromosomes 21 and 22 can be used to show some properties of both the GC‐richest and GC‐poorest isochores of the human genome, being composed by chromosomal bands covering the entire range of GC composition, from the GC‐poorest L1 + bands to the GC‐richest H3 + bands. Key Concepts The human genome is composed of two compartments endowed with very different genomic properties. The GC‐richest genomic compartment is prevalently composed of isochores from the H2 and H3 families. The GC‐poorest genomic compartment is prevalently composed of isochores from the L1 family. The GC‐richest and the GC‐poorest genomic compartments are located, in the mitotic chromosomes, distantly from one another. The GC‐richest and the GC‐poorest genomic compartments correspond to the gene‐richest and the gene‐poorest region, respectively. The GC‐richest and the GC‐poorest genomic compartments are located in the inner part and at the periphery of the cell nuclei. Human chromosomes 21 and 22 summarise all the properties endowed with the GC‐richest and the GC‐poorest regions.