Figure 4.

Cell division timing asynchrony in the two-cell embryo. (Top) Combined fluorescence and DIC microscopy image of a two-cell-stage embryo in which the DNA is visualized by GFP::H2B expression. The anterior blastomere (AB) initiated mitosis almost 2-min earlier than the precursor of the germline P1. (Bottom) Several cell cycle-regulatory mechanisms have been found to underlie the timing asynchrony. One is based on the anterior enrichment of the MEX-5/6 cytoplasmic determinants, which interact with the PLK-1,2 Polo kinases. The other involves preferential activation of the ATL-1^ATR-CHK-1 DNA replication checkpoint pathway in P1. See text for further information.