Molecular Genetics

A special protein complex is responsible for condensation of replicated cohering sister chromatids into metaphase chromosomes.

Facts

• Metaphase chromosomes have consistent lengths and banding patterns, regardless of tissue, cell type or other variables.
• Phosphorylation of histone H3 is correlated spatially and temporally with condensation of interphase chromatin into metaphase chromosomes. Evidence in Tetrahymena indicates that H3 phosphorylation is required for condensation (ref). The requirement holds during establishment of condensation but phosphorylation is not required for maintenance of the condensed state (ref).
• ScII, a protein of the metaphase chromosome scaffold, belongs to the SMC2 family of structural maintenance of chromosomes proteins.
• A 13S condensin complex has been purified from Xenopus extracts. It contains 5 polypeptides, principal ones of which are members of SCM2 and SCM4 families.
• Extracts of Xenopus cells can convert chromatin into microscopically recognizable metaphase chromosomes. However, if the 13S condensin complex is removed by antibody treatment, the immunodepleted extracts no longer convert chromatin into condensed chromosomes. The condensation activity can be restored by addition of puriifed 13S condensin (ref).
• Two kinds of condensi, I and II exist in humans (ref). They alternate along the length of the chromosome arms. Only condensin II is found near centromeres.
• The mixture of 13S condensin and topoisomerase I will induce positive supercoils in closed circular DNA (ref).

Interpretations

• Chromosome condensation is a highly ordered and active process.
• 13S condensin mediates condensation by actively coiling the chromatin.
• Mutations in the genes for the condensin polypeptides should be unable to condense chromosomes for mitosis.
• Condensin II may play a special role in the kinetochore.

Further information

• Observation of knot topology of condensin-topoisomerase II interaction revealed positive trefoils, consistent with the induction of a positive writhe in the DNA by condensin (ref).
• Another complex, cohesin, is responsible for keeping sister chromatids together (cohesion) prior to mitosis.
• Two classes of titan embryo lethal mutants of Arabidopsis represent condensin genes.
• Saccharomyces cerevisiae chromosomes do not condense appreciably in mitosis, though those of the fission yeast, Schizosaccharomyces pombe do.
• Cohesin and condensin are regulated opposingly.
• Other molecules involved in chromatin segregation include cohesins, separase, securins, monopolins, microtubule motors, and regulatory proteins.

This is page 13612 of Molecular Genetics by Ulrich Melcher, © 2001, 2004