Cytochemical staining reactions showed that instead of normal requirement of 10–15 seconds acetic-acid treatment, 5-minute treatment was required for the extraction of chromosomal histone proteins of salivary-gland cell chromosomes of Drosophila melanogaster. After this the chromosomes which have been treated with acetic acid become stretched during squashing In three chromosome regions, the stretched region could be determined on the basis of the banding pattern. The lengths of these regions were about 12–14 % of the haploid DNA content of the same regions. Thus the increase of the length of a particular region was threefold and the change in the package ratio of DNA from 20:1 to 7:1. On the basis of the effect of acetic acid on salivary chromosomes a model of the uncoiling of chromatids is presented.
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