A model for chromosome condensation based on the interplay between condensin and topoisomerase II.

The compaction of chromatin that occurs when cells enter mitosis is probably the most iconic process of dividing cells. Mitotic chromosomal compaction or 'condensation' is functionally linked to resolution of chromosomal intertwines, transcriptional shut-off and complete segregation of chromosomes. At present, understanding of the molecular events required to convert interphase chromatin into mitotic chromosomes is limited. Here, we review recent advances in the field, focusing on potential chromosomal compaction mechanisms and their importance to chromosome segregation. We propose a model of how metaphase chromosomes could be shaped based on the enzymatic activities of condensin and topoisomerase II in overwinding and relaxation of the DNA fiber during mitosis. We suggest that condensin overwinding is an important requirement for intertwine resolution by topoisomerase II and, together with the inhibition of transcription, contributes to cytological mitotic chromosome appearance or 'condensation'.