Sequence replication and banding organization in the polytene chromosomes of Drosophila melanogaster

The relative proportions of cloned DNA fragments from all known hierarchies of sequence organization in polytene and diploid chromosomes were compared. It was found that unique sequences of varying sizes and chromosomal locations are equally replicated in salivary gland chromosomes. Sequences of euchromatic polydisperse gene families are also replicated proportionately in polytene and diploid tissues. Perhaps the most significant finding is that the histone gene repeats, despite their normal banding organization, are under-replicated in the polytene chromosome of Drosophila melanogaster. However, the clustered and well-banded 5S genes are most likely equally replicated. It is therefore concluded that differential sequence replication plays no apparent role in either the assembly or morphology of a band; and likewise, the assembly of polytenic DNA into band units is not affected by either the local abundancy or arrangement of middle repetitive sequences. The likelihood that the clustered arrangement is an important factor in the selection of sequences for under-replication is discussed.