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Molecular Events in the Reproduction of Animal Cells

II. The Focalized Synthesis of DNA in the Chromosomes of HeLa Cells^*

Elton Stubblefield and Gerald C. Mueller

( McArdle Memorial Laboratory, University of Wisconsin Medical School, Madison, Wisconsin)

HeLa cells from synchronized cultures were pulsed with tritiated thymidine during the synthesis of specific portions of their DNA. The distribution of this labeled DNA was determined subsequently in specific chromosomes. Evidence was obtained for the nonrandom and focalized nature of DNA synthesis among both the various chromosomes of a single nucleus and the individual segments along a particular chromosome. It is proposed that the reproducible patterns of early and late labeling DNA in chromosomes reflects the existence of two different states of association of DNA with some other chromosomal constituent. The possible biological significance of this finding is discussed.

^* This work was supported by grant CY-1897 from the United States Public Health Service and by funds from the Alexander and Margaret Stewart Trust.

Predoctoral Fellow of the National Science Foundation.

Received 5/11/62.

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