No significant overreplication occurs in Ehrlich ascites cells during and after reversal of hypoxia

Labeling with bromodeoxyuridine and analysis by isopycnic banding for emergence of DNA-helices density-labeled in both strands (HH-DNA) were used to examine whether transient controlled hypoxia induces significant overreplication in the DNA of cultured Ehrlich ascites cells within a single cell cycle. Diverse situations of the hypoxic period (4-8 h) within the BrdUrd labeling time (8-16 h) were tested. If transversal of more than one complete cell cycle by very fast cycling individual cells was avoided by using short BrdUrd labeling periods or by addition of colcemid, HH-DNA was never detected. This indicates that hypoxia does not induce significant overreplication under the conditions chosen and, in particular, that the burst of replicon initiations occurring after reoxygenation of hypoxic cells principally is not due to overreplication.