Activation of DNA damage checkpoints in CHO cells requires a certain levelof DNA damage

DNA damage activates checkpoint controls in eukaryotic cells. It is not cle ar, however, whether a certain level of DNA damage is required for the acti vation of DNA damage checkpoints. We show here that low levels of DNA damag e in Chinese hamster ovary (CHO) cells induced by short exposure to hydroxy urea (HU) did not trigger checkpoints, whereas higher levels of DNA damage caused by longer exposure to HU resulted in a cell cycle arrest. Our result s argue that a threshold of DNA damage is necessary for activation of DNA d amage checkpoints. (C) 2001 Academic Press.