Abstract
We have combined the techniques of pulse-labeling and DNA autoradiography to investigate the mechanism of DNA replication in the chromosomes of Chinese hamster and HeLa cells. Our results prove that the long fibers of which chromosomal DNA is composed are made up of many tandemly joined sections in each of which DNA is replicated at a fork-like growing point. In Chinese hamster cells most of these sections are probably less than 30 μ. long, and the rate of DNA replication per growing point is 2.5 μ per minute or less. In addition, we have taken advantage of the apparent slowness of equilibration with external thymidine of the internal thymidine triphosphate pool in Chinese hamster cells to determine the direction of DNA synthesis at the conclusion of the pulse in pulse-chase experiments. We have found, unexpectedly, that replication seems to proceed in opposite directions at adjacent growing points. Furthermore, adjacent diverging growing points appear to initiate replication at the same time.
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