Video 2
Hydroxyurea-treated embryo coexpressing GFP:γ-tubulin and GFP:histone. Sequence of an embryo coexpressing GFP:γ-tubulin and GFP:histone exposed to 75 mM HU in the culture medium and imaged as in Video 1 at 40-s intervals. Playback is 3 frames/s (i.e., 120× real time). The embryo proceeds through meiosis I and II before arresting in S phase. Pronuclei slowly drift toward each other but later events such as centrosome maturation and nuclear envelope breakdown fail to occur. In embryos exposed to HU from meiosis I (i.e., before egg shell formation), arrest was permanent. When embryos were dissected from parental worms (previously treated with identical doses of HU on bacterial plates) at later stages (after the egg shell has formed), cell cycle progression was strongly delayed but not eliminated, supporting the dose-dependence of the embryonic DNA damage checkpoint observed previously (not depicted; Holway, A.H., S.H. Kim, A. La Volpe, and W.M. Michael. 2006. J. Cell Biol. 172:999–1008). Cell cycle progression was restored by depletion of CHK-1, which eliminates the DNA damage checkpoint (not depicted; Holway et al., 2006).