Fig. 1. In vivo DNA replication activity of CHO K1 and xrs-5 cells. CHO K1 or xrs-5 cells were transfected with either p186 or pBR322 DNA. 72 hours post-transfection, plasmid DNA was isolated by the method of Hirt, then purified and digested with DpnI. The DpnI-digested DNA was then used to transform E. coli. The number of bacterial colonies produced was counted, corrected for the amount of DNA recovered and related to the positive control reaction with the CHO K1 cells, which was taken as 100%. The bars represent the error from the average of two experiments performed in triplicate.