DNA synthesis in phytohemagglutinin-stimulated human leucocyte cultures treated with β-mercaptoethanol

Abstract Human peripheral-blood leucocyte cultures stimulated with phytohemagglutinin extracts were treated with β-mercaptoethanol and compared to untreated control cultures by microspectrophotometric measurement of DNA content of individual cells. Phytohemagglutinin produced mitosis primarily by stimulating DNA synthesis rather than by initiating mitosis in pre-existing G2 cells. The peak mitotic index coincided with the time of appearance of the maximum number of G2 cells. In control cultures, no cells contained more DNA than did diploid mitoses. The eventual cessation of cell division was due to a failure of cells to re-enter DNA synthesis, rather than lack of onset of mitosis in G2 cells. In β-mercaptoethanol-treated cultures, cells containing DNA exceeding that in diploid mitoses appeared on the third day and reached amounts equivalent to the completion of synthesis for an octoploid-level mitosis by the sixth day. DNA measurements were in agreement with cytologie determination of ploidy in that tetraploid and octoploid mitoses, including endoreduplicated mitoses, contained amounts of DNA proportional to the chromosome number.