CHO cells are extensively used for:
Logarithmic or exponential phase follows lag and is characterized by maximal and constant specific growth rate where population doubles at regular intervals, producing straight line on semi-logarithmic plot of cell number versus time. Birth rate greatly exceeds death rate. Cellular activities peak including DNA replication by DNA polymerases alpha and delta, rRNA transcription, protein synthesis, high glucose consumption via aerobic glycolysis producing lactate, and oxygen uptake by mitochondria. For most continuous lines generation time 18-24 hours though HeLa approximately 20 hours. Contact inhibition not yet engaged, space and growth factors remain sufficient. This phase represents healthiest period with viability above 95% and high fraction of cells in S/G2/M measurable by BrdU or propidium iodide staining. Optimal window for transfection because nuclear envelope breakdown during mitosis enhances plasmid entry, for viral infection requiring active replication machinery, and for cell cycle studies. Maintaining cultures through regular subculture ensures they remain in log phase, avoiding stress responses associated with plateau and death phases that reduce reproducibility.
Ref: Freshney Ch.13 Log phase exponential growth; Lodish MBoC Ch.12 Cell cycle exponential proliferation and growth curve.