Replicative Behaviour of Bovine X-chromosomes during Early DNA Synthesis

Abstract

THE patterns of DNA replication in several mammalian chromosome complements have been investigated by the use of labelled DNA precursors and autoradiography^1–5. Such work has established that replication of the sex chromosomes is asynchronous during the terminal stages of DNA synthesis. Several investigations have attempted to elucidate whether or not the chromosomes which terminate their replication later than all other chromosomes in a mammalian complement also start their synthesis late. In cultured cells from male and female Chinese hamsters Taylor¹ observed that the late-replicating sex chromosomes duplicate only during the last half of the 6 h synthetic period. Using 5-fluorodeoxyuridine (FUdR) to synchronize cultures of diploid Chinese hamster cells, Hsu⁶ obtained evidence which suggested that the late-replicating Y and long arm of the X-chromosome begin DNA synthesis approximately 4.5 h after DNA synthesis has commenced in the rest of the complement. Using a comparable technique, Petersen⁷ independently identified a “late-starting” chromosome in cultured leucocytes from a human female, which he presumed to be the late-replicating X- chromosome. In cultured cells from bovine males, Gartler and Burt⁸ observed that the “late synthesizing Y- chromosome” begins replication later than the rest of the chromosomes in the complement. These observations suggest that “late-replicating chromosomes” or chromosome segments are correspondingly late in beginning DNA synthesis. Stubblefield⁹, however, using a modified autoradiographic technique, came to the conclusion that the “late-replicating Y” and the long arm of the X in the male Chinese hamster complement do “incorporate a significant amount of isotope during the first 15 min of synthesis”. Making use of a continuous labelling technique and a human complement with X/iso-X sex chromosome constitution, where the iso-X is known to be “late-replicating”, it has been shown¹⁰ that the iso-X chromosome begins its replication along with autosomes of comparable length and is therefore not the last chromosome in the complement to initiate replication.