Early replication banding reveals a strongly conserved functional pattern in mammalian chromosomes.

One of the best documented autosomal linkage associations in man is on chromosome 1p and in the mouse on chromosome 4. On mitotic chromosomes this genetic homology is shown more clearly by early replication banding (RBG; induced by incorporation of 5'bromodeoxyuridine (BrdU) in the second half of the S phase) than by structural banding (induced on prefixed chromosomes by denaturation, RFA, or trypsin, GTG). To analyse this phenomenon in more detail, 11 chromosomal regions in man and the domestic cat with known genetic homology were compared. In four chromosome pairs RBG and GTG banding show the same degree of homology. In seven chromosome pairs the homology is more pronounced by RBG than by GTG banding. RFA banding does not reveal the same extent of homology as does RBG banding. These results clearly show a difference between the structural banding pattern, RFA and GTG, and the replication banding pattern, RBG. The following conclusions can be drawn: in chromosomal regions with homologous functions the DNA replicates in the same temporal order. Early replication banding (RBG) reveals a functional pattern in these regions which has been more strongly preserved during evolution than the underlying chromosomal DNA. Differences in chromosomal banding are most prominent in the GTG banding pattern, whereas similarities are most apparent in the RBG banding pattern.