The replicative organization of DNA in polytene chromosomes of Drosophila hydei.

Abstract

Salivary-gland nuclei ofDrosophila hydei were pulse-labeledin vitro with3H-thymidine and studied autoradiographically in squash preparations. The distribution of radioactive label over the length of the polytene chromosomes was discontinuous in most of the labeled nuclei; in some nuclei the pattern of incorporation was continuous. Comparison of the various labeling patterns of homologous chromosome regions in different nuclei showed that specific replicating units are replicated in a specific order. By combining autoradiography with cytophotometry of Feulgen-stained chromosomes, it was possible to correlate thymidine labeling of specific bands with their DNA content. The resulting data indicate that during the S-period many or perhaps all of the replicating units in a salivary-gland nucleus start DNA synthesis simultaneously but complete it at different times. Furthermore, the data support the hypothesis that the chromomere is a unit of replication or replicon. The DNA content of haploid chromomeres was found to be about 5×10-4 pg for the largest bands inDrosophila hydei. From the results of H3-thymidine autoradiography and Feulgen-cytophotometry on neuroblast and anlage nuclei it was concluded that during growth of the polytenic nucleus heterochromatin is for the most part excluded from duplication. The results of DNA measurements in interbands of polytene chromosomes do not agree with a multistrand structure for the haploid chromatid. A chromosome model is proposed which is in accordance with the reported results and with current views concerning the replicative organization of chromosomes.