FURTHER STUDIES ON THE PATTERN OF CHROMOSOME DUPLICATION IN CULTURED MAMMALIAN LEUCOCYTES

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Abstract
Cultured leucocytes from a domestic cow (Bos taurus) were exposed to thymidine-H3 continuously for 5-6 hours and were harvested directly from the isotope containing medium. One of the two X-chromosomes, which are the only submetacentric chromosomes in the whole complement, was the last to complete duplication, followed by one of the larger auto somes. The other X terminated replication much earlier than the late-X, although the morphologic sequence of DNA replication in both Xs appeared to be the same. Autosomes of this animal also showed a characteristic pattern of DNA duplication. This study confirms unequivocally that the chromosome which replicates its DNA later than any other chromosomes in a mammlian female complement is indeed an X-chromosome.