A checkpoint escape

Abstract

In most dividing cells, the spindle assembly checkpoint (SAC), which monitors kinetochore–microtubule interactions, signals to prevent anaphase onset until all chromosomes are correctly attached to the metaphase spindle. Here, Shao et al. reveal that Xenopus laevis oocytes do not respond to a SAC. The authors first developed a karyotyping technique that allows the analysis of meiotic chromosomes after nuclear envelop breakdown. When treating oocytes during meiosis I with microtubule-depolymerizing drugs that destroyed the spindle, they observed that homologous chromosomes could still divide into single pairs of chromatids, and on time. This suggested that oocytes could progress to meiosis II and arrest at metaphase II, as normal, without a spindle. Similarly, defective oocytes exhibiting monopolar spindles proceeded through monopolar anaphase without delay, indicating that, in X. laevis, the metaphase-to-anaphase transition is not under SAC regulation.