Abstract
The DNA of a murine lymphoma cell was pulse-labelled with radioactive thymidine for different periods of time. The cells were X-irradiated in order to introduce breaks at random into the DNA molecules, and the apparent molecular weight distribution of the labelled DNA in the denatured form was measured by rate sedimentation in alkaline sucrose. It has been demonstrated that the shape and position of the radioactivity distribution on a sucrose gradient after a given X-ray dose is dependent on the length of the label in the DNA. Mathematical analysis has shown that this is to be expected for a distribution of molecules of different lengths labelled only at the ends; it does not indicate that the nascent DNA has a different size from the bulk of the chromosomal DNA. The results have been used to calculate the average length of label and hence the average replication rate.
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