The chromosomes of Crepis capillaris were labelled with thymidine-2-C14 in their DNA fraction. Quantitative analysis of the distribution of newly synthesized DNA in postmetaphase stages of the division following the period of label incorporation led to the conclusion that the new DNA is not necessarily equally distributed by the mitotic process and that, therefore, chromosome duplication does not involve the equal partition of parental DNA. The implications of these findings with respect to DNA duplication are discussed. An attempt is made to translate the pattern of new DNA distribution into a probable number of units of synthesis per chromosome.

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