Genes, isochores and bands in human chromosomes 21 and 22

The recently available DNA sequences from chromosomes 21 and 22 enabled us to define the relationships of different band types with isochores and with gene concentration and to compare these relationships with previous result s. We showed that chromosomal bands appear as Giemsa or Reverse bands depen ding not on their absolute GC level, but on the composition GC level relati ve to those of adjacent contiguous bands. We also demonstrated that the GC- richest, and gene-richest H3(+) bands are characterized by a lower DNA comp action compared with the GC-poorest, gene-poorest L1(+) bands. Moreover, ou r results indicate that the human genome contains about 30,000 genes.