DISPROPORTIONATE SYNTHESIS OF DNA IN A POLYTENE CHROMOSOME REGION

Abstract

Recent studies of the morphology of the giant polytene chromosomes in dipteran larvae have emphasized the variability from tissue to tissue and from one developmental stage to another of localized swellings: Balbiani rings, bulbs, and puffs. The cytological measurements reported here show that the DNA in the puff region at least doubles with respect to non-puffing regions. The rule that the DNA content of a nucleus is directly proportional to a whole number of chromosome sets is derived from measurements made on whole nuclei. Since the region involved in the present case (puff a) is shorter than one one-hundredth of the total length of all the chromosomes, the twofold increase observed there would add less than 1% to the total nuclear DNA. A difference of this magnitude could not be detected by existing methods. Clearly, the disproportionate DNA synthesis on these chromosomes could not be deduced from measurements of total DNA per nucleus. This fact raises serious difficulties for inferences as to the relation of DNA metabolism to gene action derived from the so-called DNA constancy rule. On the other hand, the present case is not inconsistent with the theory that there is a DNA component replicated in a regular fashion in these chromosomes. This "constant"component is then especially active in puffed regions, where it is augmented by the "extra"DNA in the case studied here. The relation between this extra amount and the material originally present is a question now open to experimental attack.