Condensins

A fundamental part of life is a cell’s ability to replicate its genome and divide; these processes occur over two main phases of the cell cycle.

First, during the S phase, chromosomal DNA is duplicated. During the M phase, the duplicated chromosomes are separated and distributed to two genetically identical daughter cells.

Following the S phase, the DNA of sister chromatids is very long and tangled. Separating the sister chromatids in this state could lead to chromosome breaks, improper segregation, and even cell death.

To avert this potential crisis, the cell devotes a substantial amount of energy during early mitosis to gradually reorganizing the sister chromatids into shorter structures that separate more easily.

This reorganization relies on condensin, a protein complex involved in the condensation of chromosomes.

Condensin consists of five subunits. In eukaryotes, the two major subunits, SMC2 and SMC4, are connected at their ATPase head domains by the three other subunits: one kleisin and two HEAT-repeat subunits.

Condensins use energy generated by ATP hydrolysis to promote two major processes that facilitate sister chromatid separation: chromosome condensation and sister chromatid resolution.

During chromosome condensation, chromatids become tightly packed. During sister chromatid resolution, the condensed sister chromatids become distinct structures when sister DNAs are unlinked, or decatenated, by the partial removal of cohesins.

By the time the cell reaches metaphase, the sister chromatids are only loosely connected along the arms but are still tightly linked at the centromeres.  

Condensin can alter DNA coiling and catalyze chromosome condensation and sister chromatid resolution by forming ringed structures that encircle loops of DNA.