THYMIDINE AS A SYNCHRONIZING AGENT .I. NUCLEIC ACID AND PROTEIN FORMATION IN SYNCHRONOUS HELA CULTURES TREATED WITH EXCESS THYMIDINE

Synchronous cultures of HeLa cells obtained by selective detachment of mitoses were treated with high concentrations of thymidine. The inhibitor was added soon after completion of cell division and rates of cell enlargement and accumulation of DNA, RNA and protein were compared for untreated and thymidine‐treated cultures at various points of the cell cycle. It was found that concentrations of thymidine which in randomly growing cultures inhibit the rate of cell division by more than 90% allowed a considerable degree of DNA synthesis and did not affect the rate of accumulation of RNA and protein, when applied to cells in the G1 phase of synchronous culture. Treated and untreated cells enlarged at the same rate throughout their life cycle. The results show that concentrations of thymidine commonly employed to produce cell synchrony do not arrest the cells at the G1‐S boundary, but allow slow progress through S in respect to DNA synthesis, and near‐normal progress towards G2 as regards RNA and protein accumulation and cell enlargement. Copyright © 1969 Wiley‐Liss, Inc.