Assembly of compact mitotic chromosomes and resolution of sister chromatids are 
two essential processes for the correct segregation of the genome during 
mitosis. Condensin, a five-subunit protein complex, is thought to be required 
for chromosome condensation. However, recent genetic analysis suggests that 
condensin is only essential to resolve sister chromatids. To study further the 
function of condensin we have depleted DmSMC4, a subunit of the complex, from 
Drosophila S2 cells by dsRNA-mediated interference. Cells lacking DmSMC4 
assemble short mitotic chromosomes with unresolved sister chromatids where 
Barren, a non-SMC subunit of the complex is unable to localise. Topoisomerase 
II, however, binds mitotic chromatin after depletion of DmSMC4 but it is no 
longer confined to a central axial structure and becomes diffusely distributed 
all over the chromatin. Furthermore, cell extracts from DmSMC4 dsRNA-treated 
cells show significantly reduced topoisomerase II-dependent DNA decatenation 
activity in vitro. Nevertheless, DmSMC4-depleted chromosomes have centromeres 
and kinetochores that are able to segregate, although sister chromatid arms form 
extensive chromatin bridges during anaphase. These chromatin bridges do not 
result from inappropriate maintenance of sister chromatid cohesion by DRAD21, a 
subunit of the cohesin complex. Moreover, depletion of DmSMC4 prevents premature 
sister chromatid separation, caused by removal of DRAD21, allowing cells to exit 
mitosis with chromatin bridges. Our results suggest that condensin is required 
so that an axial chromatid structure can be organised where topoisomerase II can 
effectively promote sister chromatid resolution.