Distribution of condensins and chromosome architecture

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Eukaryotic cells possess two different condensin complexes, known as condensin I and II. Both play essential yet distinct functions in the processes of mitotic chromosome assembly and segregation. Interestingly, condensin II tends to be enriched in G-band regions. It has long been known that G-band regions are replicated later in S phase and condense earlier in prophase. This set of information allows us to propose the working hypothesis that condensin II may play a role in linking DNA replication in S phase to chromosome condensation in M phase. Quantitative cell imaging analyses combined with immunostaining show that condensin II start to associate with chromosomes during S phase. Interestingly, in the presence of replication stress, the depletion of condensin II, but not of condensin I, induced a critical abnormality in chromosome in M phase. These results suggest that condensin II may play a crucial role in structural conversion of replicated chromosomes during S phase to facilitate chromosome condensation in M phase.