Chromosome Condensation and Cohesion

Abstract

The diploid human genome consists of 46 chromosomes, which collectively contain about 2 m of deoxyribonucleic acid (DNA). During mitosis, the genome is packaged into 46 pairs of sister chromatids, each less than 10 μm long. Two fundamental mechanisms govern the formation and maintenance of chromosome structure during mitosis: (i) the dramatic and progressive compaction of chromosomes in the process of chromosome condensation and (ii) the establishment of the physical linkages or cohesion between the sister chromatids. Central to the processes of chromosome condensation and cohesion are the type II topoisomerases and the Structure Maintenance of Chromosomes (SMC) family of proteins, which form the condensin and cohesin complexes. These proteins cooperate to ensure chromosome condensation and cohesion, thus promoting the accurate partition of the genome during mitosis. Key Concepts: Chromosome condensation and cohesion are necessary for accurate chromosome segregation. Condensin and topoisomerase IIα are the major regulators of chromosome condensation. Sister‐chromatid cohesion is required for the bipolar attachment of chromosomes to the spindle microtubules and counteracts the spindle pulling forces generated at the kinetochores. The cohesin complex is critical for sister‐chromatid cohesion. Both DNA catenation and sister‐chromatid cohesion have to be resolved for proper sister‐chromatid separation. Centromeric cohesion is regulated by the spindle checkpoint. Its removal occurs at the onset of anaphase.