Most short DNA molecules isolated from 3T3 cells are not nascent

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Abstract
The population of short DNA molecules (less than 10 3 nucleotides) in 3T3 cells has been studied using in vivo and in vitro pulse labeling techniques and in vitro end-labeling. There is a large number of molecules of less than 100 nucleotides present in equal numbers in both G O and S phase cells. In S phase cells, most of these molecules are not replicating intermediates because they do not become density-labeled after a moderate period of substitution of BrdUMP, although they are detected by end-labeling in vitro This population includes the nascent Okazaki pieces that can be labeled in a short pulse with [ 3 H]dThd or [ 3 H]dTTP, however, these represent less than 10% of the total population. Alkaline hydrolysis of the molecules that had been end-labeled with 32 P using [γ 32 P]ATP and polynucleotide kinase did not reveal significant release of [ 32 P] 2′(3′), 5′ ribonucleoside diphosphates.