³H-thymidine labeling patterns in polytene chromosomes of mitomycin-treated Drosophila melanogaster: evidence of continuous-type labelling as beginning of DNA replication

Abstract

Ninety-hour-old larvae and about 1 h old prepupae, fed for 2 h on mitomycin C, were subjected to ³H-thymidine autoradiography for analysis of the DNA replication patterns. Analysis of the autoradiograms which were broadly categorized into 2C (light continuous), 3C (heavy continuous), 3D (heavy discontinuous), 2D (medium discontinuous), 1D (light discontinuous), revealed that in comparison with the control series there was a significant reduction in the continuous type of labeling in the treated larvae, both male and female, and in both larvae and prepupae. Detailed analysis of the X and autosome 2 R fragments in control and mitomycin-treated larvae and prepupae revealed a higher number of unexpectedly labeled sites in the latter, in the higher-sites-labeled nuclei. An analysis of these results in the light of the action of mitomycin C on DNA structure and synthesis lends considerable support toward our earlier postulation that the continuous type of labeling is the beginning, and the discontinuous type the end, of the DNA replication cycle.