Original sentence: Exposure of eukaryotic cells to ionizing radiation, of high doses of UV radiation or chemical mutagens, induces additional responses such as G1/S, intra-S, and G2/M checkpoints (see cell cycle checkpoint).One possibility is that ionizing radiation and high doses of UV create DNA structures in G1 and G2 phase, such as double-strand breaks and postreplication gaps, that are created by the replication of damaged DNA under physiological conditions.There is mounting evidence that these checkpoints do not ensure the completion of repair or occur under physiological conditions, therefore their functional significance remains unclear