Replication and G2 checkpoints: their response to caffeine.
Under long hydroxyurea treatments, evidence was obtained for the sequential activation of four checkpoints located between the onset of S phase and mitosis in Allium cepa L. root meristems. Biparametric flow cytometry (Br-DNA/total DNA) showed that cells initially accumulated at early S phase but, after a delay, they resumed replication and paused again at mid S phase. Cells not only overrode this second replication block but also any G2 checkpoint they encountered. Thus, a late mitotic wave was produced in the presence of hydroxyurea. The wave was formed by cells that had apparently completed their replication (normal mitoses), while others displayed anaphases/telophases with less than the expected DNA content and with chromosomal breaks (aberrant mitoses). The presence of aberrant mitoses is direct evidence for the undue override of the two G2 checkpoints responsible for surveillance of completion of DNA synthesis and repair, respectively. Caffeine selectively abrogated the G2 block produced by the checkpoint that controls post-replication DNA repair, as it advanced the entry of cells into an aberrant mitosis. However, caffeine proved not to be the universal checkpoint-evading agent as postulated. Caffeine did not modify the spontaneous override of the replication checkpoints. Moreover, it seems to enforce the checkpoint that controls the completion of DNA synthesis, as the appearance of the late wave of normal mitoses produced in the presence of hydroxyurea was prevented by the use of caffeine.