Effects of Ionizing Radiation on Cyclin Expression in HeLa Cells

The levels of cyclin B mRNA and protein rise rapidly in <tex-math>${\rm G}_{2}+{\rm M}$</tex-math> phase, and fall at the end of mitosis. The studies described here were initiated to determine the effects of ionizing radiation on the level of cyclin B bearing in mind that the division delay induced by ionizing radiation might be influenced by the expression of cyclin B. After irradiation in S phase, the cyclin B mRNA in HeLa cells was measured as the cells proceeded through the cell cycle. Instead of the usual rise, after irradiation cyclin B mRNA levels remained low during the G₂ delay. After irradiation in G₂ phase, cyclin B mRNA was readily detectable although at slightly lower levels than in the controls. However, cyclin B protein was markedly decreased in amount.