FIG. 6.

p21^−/− MEFs do not accumulate nuclear cyclin B1 before mitosis. (A) Nuclear colocalization of cyclin B1 and p21 in wild-type (a to i) and p21^−/− (j to l) MEFs. The nuclei were counterstained with Hoechst 33258 (DNA) to assess the state of DNA condensation. Representative cells exhibiting cytoplasmic and nuclear (a) and nuclear (d and g) cyclin B1 localization are shown. Micrograph I shows a p21^−/− cell with nuclear cyclin B1 with ongoing DNA condensation. (B) Quantitative analysis of p21-cyclin B1 colocalization in exponentially growing MEFs. Note that only cyclin B1-positive cells were scored. We distinguished cells with apparent accumulation of cyclin B1 in both in the cytoplasm and the nucleus without (CN) and with (CN*) visible nucleoli, predominantly in the nucleus (Nuc) as well as those in different stages of prophase (Pro). (C) Quantitative analysis showing cyclin B1 localization in wild-type (W.T.) and p21^−/− MEFs. Only cells showing nuclear signal were scored.