A complete cell cycle of mature, concanavalin A (Con A) stimulated rat thymocytes was documented by analyzing the cell number as well as the content and synthesis of DNA and RNA. Cell cycle progression is accompanied by an elevation of class I, II and III RNA polymerase activities (about 10-fold) in the S phase maximum, 48 h after stimulation. Moreover, maximal cellular contents of DNA, ATP, ADP and AMP were observed at this culture period, whereas the RNA level peaked at 60 h. The synthesis of purine and pyrimidine nucleotides de novo was detected by use of [14C]HCO3-. Maximal incorporation rates of [14C]HCO3- into nucleotides (de novo synthesis) and of [3H]adenine into adenylates ('salvage pathway') occur during the S phase. However, the de novo synthesis rates were markedly lower than those of the 'salvage pathway'. The highest cellular level of the nucleotide precursor 5-phosphoribosyl-1-pyrophosphate (8.4-fold increase) also coincided with the S phase.
Animals Cell Cycle