LARVAL AGE AND THE PATTERN OF DNA SYNTHESIS IN POLYTENE CHROMOSOMES

Abstract

It has been shown, by autoradiography using H³-thymidine, that the frequency of salivary gland cells where DNA synthesis covers the entire length of a specific polytene chromosomal segment (Chromosome II, 56F-60A, Drosophila melanogaster) drops off some time during the last day of larval life. The frequency of highly discontinuous DNA synthesis over the same region remains at about the same level until a stage closer to puparium formation, when all DNA synthesis stops.If a cycle of DNA synthesis, once initiated, goes to completion, then this finding indicates that the patterns of spatially continuous synthesis tend to occur early in the replicative cycle and that the terminal phases of the cycle involve highly discontinuous patterns of synthesis.