Evidence for a four micron replication unit in CHO cells

CHO cells in culture were synchronized by mitotic selection, allowed to reattach to plastic flasks, and reach S phase in the presence of fluorode-oxyuridine at concentrations known to completely block the synthesis of thymidylate. The cells were released from the block with 3H-thymidine for pulses of 4, 8, 12, 24 and 40 min and DNA fiber autoradiographs prepared. An analysis of the spacing between origins of replication indicates that sites are available at intervals of about 4 mum along most of the DNA. Chain growth proceeds at about 1,000 nucleotides per minute and some of the closely situated sites become continuous, labeled segments after 8-12 min. However, unlabeled segments are still present between the replicated segments after 40 min. The data may be interpreted as evidence for regularly spaced initiation sites which are available in CHO cells, even though only one in 10-15 of these may be utilized for initiation each cycle under normal growth conditions in these cultures.