Further Evidence for the Simultaneous Initiation of DNA Replication in both X Chromosomes of Bovine Female


THE existence of differential rates of mammalian sex chromosome replication late in the DNA synthetic period has been well established1, but there is still controversy about the replicative behaviour of sex chromosomes at the beginning of the S period. Several reports suggest that, for a given complement, the sex chromosomes which synthesize their DNA at a higher rate at the end of the S period are late in initiating DNA synthesis2–9. But others suggest that both sex chromosomes begin replication almost simultaneously, even earlier than some autosomes10–13. This experiment was designed to investigate the replicative behaviour of the X chromosomes in the bovine (Bos taurus) female complement at various times during the DNA synthetic period with the hope of finding an explanation for the discrepancy. The chromosome complement of the cow is advantageous for this experiment, because the two X chromosomes are the only submetacentric ones and can easily be distinguished from the fifty-eight acrocentric autosomes.

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