Human chromosomal banding by in situ hybridization of isochores

Further to the classical methods that involve different chromosome treatments followed by staining, in situ hybridization of isochores represents a novel approach to chromosomal banding. Isochores are long compositionally homogeneous DNA segments that, in the human genome, belong to five families, two GC-poor families (L1 and L2) representing 30% and 33% of the genome, respectively, and three GC-rich families (H1, H2 and H3) representing 24%, 7.5% and 4-5- of the genome, respectively. Gene concentration increases with increasing GC levels, reaching an up to 20-fold higher level in H3 compared to L1 isochores. In situ hybridization of DNA from different isochore families on metaphase chromosomes allow to distinguish different sets of Giemsa and Reverse bands. In addition, it also provides information on the chromosomal distribution of genes.