Are late replicating regions in polytene chromosomes of Drosophila enriched by repeated nucleotide sequences?

THE ordered highly coordinated pattern of replication of different bands is characteristic of salivary gland chromosomes of Drosophila^1,2. During the initial stage of polytene chromosome replication labelled precursor is found over Puff/interband sections while prominent bands incorporate label later and need in general a longer time for DNA synthesis^3,4. It is significant that regions of ectopic pairing in the chromosomes of D. melanogaster are of the late replicating type. We report here that late replicating DNA from Drosophila salivary gland cells reassociates after denaturation more rapidly than the bulk of the DNA from the same cells. A high level of repeated nucleotide sequences may be involved.