The condensin II complex gives cells a head start on mitosis by helping to separate sister chromatids during S phase, Ono et al. show.
Chromosomes are strung out and invisible for most of the cell cycle, not showing themselves until early in mitosis. But the process of chromosome condensation may begin earlier in the cell cycle. The protein complexes condensin I and condensin II help orchestrate chromosome condensation during mitosis. Unlike condensin I, which is trapped in the cytoplasm until prometaphase, condensin II dwells in the nucleus throughout interphase, raising the possibility that it acts before the onset of mitosis.
Ono et al. spurred cells to undergo premature chromosome condensation, in which the chromosomes compact and become visible before mitosis. G1 cells showed a single set of chromatids, but paired sister chromatids began to show up late in S phase and were clearly visible by G2. In cells lacking a component of condensin II, however, distinct chromatids rarely formed in S phase.
The team then used fluorescence in situ hybridization to tag and track matching locations on sister chromatids. The fluorescent signals from two sister chromatids were farther apart when cells carried condensin II than when they lacked a component of the complex, suggesting that condensin II helps each newly synthesized chromatid distance itself from its sister. The findings indicate that, during interphase, condensin II helps to separate sister chromatids shortly after DNA duplication, preparing them to condense and then segregate in the next mitosis.
Text by Mitch Leslie