Like early Xenopus embryos, extracts made from Xenopus eggs lack the c
ell cycle checkpoint that keeps anaphase from occurring before spindle
assembly is complete. At very high densities of sperm nuclei, however
, microtubule depolymerization arrests the extracts in mitosis. The ar
rested extracts have high levels of maturation-promoting factor activi
ty, fail to degrade cyclin B, and contain activated ERK2/mitogen-activ
ated protein (MAP) kinase. The addition of the purified MAP kinase-spe
cific phosphatase MKP-1 demonstrates that MAP kinase activity is requi
red for both the establishment and maintenance of the mitotic arrest i
nduced by spindle depolymerization. Increased calcium concentrations,
which release unfertilized frog eggs from their natural arrest in meta
phase of meiosis II, have no effect on the mitotic arrest.