Structural Analysis of Polytene Chromosome Bands and Interbands

Abstract

Beermann (1972) estimated that at least 95% of the DNA of polytene chromosomes of Drosophila is located in bands and only less than 5% in the interband regions. Recent studies of Laird et al. (1980) on high-voltage electron micrographs of whole-mounted polytene chromosomes have provided arguments which call for a reevaluation of the previous results. Densitometric determination of cross sectional relative dry mass in polytene chromosome bands and interbands from the EM negatives has suggested that the earlier estimates of the proportion of interband DNA are too low (Laird, 1980). According to the results obtained from the densitometric studies on the whole-mounted chromosomes at least 26$ of the total DNA is located in the interbands of polytene chromosomes. This new result, if it holds true, has several interesting and important implications. It favors the hypotheses that the interband regions are the sites of permanently active “housekeeping” genes in polytene chromosomes (e.g., Crick, 1971; Speiser, 1974). The high number of interbands, more than 5,060 in the genome of Drosophila melanogaster (Bridges, 1942), and obviously the high variability in their number in polytene chromosomes of related species, like Drosophila hydei (Berendes, 1963), contradict the view, that all interbands are active genes.