Nascent DNA from phytohemagglutinin-stimulated human lymphocytes

DNA synthesis was studied in human peripheral blood lymphocytes that had been stimulated with phytohemagglutinin. DNA pulse-labeled with [3H]thymidine was fractionated by sucrose gradient centrifugation or by chromatography on hydroxylapatite columns. Nascent DNA was identified as a single-stranded species that sedimented at 4-5S in neutral sucrose gradients and appeared to be precursor to chromosomal DNA in pulse-chase experiments. At least two-thirds of the nascent DNA was released as single strands from high molecular weight DNA without employing a denaturation step. It is concluded that synthesis of DNA by phytohemagglutinin-stimulated lymphocytes involves a low molecular weight, single-stranded, short-lived intermediate similar to that described for other eukaryotic cells.