Chromosome duplication (DNA synthesis) was studied in cultured cells of Chinese hamsters by means of autoradiography following thymidine-H3 incorporation. The technique used was to expose an asynchronously dividing population of rapidly growing cells for a 10 minute interval to a medium with thymidine-H3. Cells were then transferred to a medium with excess unlabeled thymidine. The population was sampled at intervals thereafter and studies made of the frequency of labeled interphases and division figures, and the patterns of labeling of specific chromosomes.

The average generation time during these experiments was about 14 hours. DNA synthesis occurred during an interval of about 6 hours and stopped 2 to 3 hours before metaphase. After metaphase the chromosomes usually begin duplication again within 5 to 6 hours.

Grain counting, to estimate the amount of tritium incorporated after a short contact with thymidine-H3 and at intervals after transfer to a medium with excess unlabeled thymidine, indicated that the intracellular pool of labeled precursors was diluted within less than a minute so that further labeling would not be detected. The chromosomes labeled during the contact period retained their precise pattern of labeling through another duplication cycle and no turnover of DNA or loss of tritium was detectable.

Five or 6 chromosomes of the complement have segments typically late in duplication. Two of these are the X and Y chromosomes. The long arm of the X chromosome and the whole Y chromosome are duplicated in the last half of the interval of DNA synthesis. The short arm of the X chromosome in a male strain is duplicated in the first half of the interval. In another strain (female), one X chromosome had the same timing, but the other one was all duplicated in the last half of the period of DNA synthesis. The DNA in the short arms of 2 medium sized chromosomes, as well as most of the DNA in 1 or 2 of the smallest chromosomes of the complement was replicated late.

The study has led to the hypothesis that various chromosomes or parts of chromosomes have a genetically controlled sequence in duplication which may have some functional significance.

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