Asynchronous DNA synthesis in a duplicated chromosomal region of Drosophila melanogaster.

Abstract

THERE is a highly ordered temporal sequence in the replication of DNA in the polytene chromosomes of Drosophila1–10. The mechanism underlying this replicative organization remains unknown, but it has been shown that homologous chromosome regions replicate their DNA synchronously whether or not they are paired11 and, in the one case in which it has been studied, this synchrony remains evident even when one of the two homologous regions is translocated to an abnormal position12. These observations suggest that an essential part of the system controlling replication pattern is located in each of the small chromosome regions, replication of which can be resolved autoradiographically. The simplest model consistent with these assumptions involves a chromosome constituted of numerous “replicons” with replication times geared to a common control mechanism but are independent of the anatomical ordering of the “replicons” within the genome.