Molecular Genetics

Replicated sister chromatids remain associated with one another from S phase to anaphase.

Facts

• Sister chromatid exchanges are routinely observed after exposure of cells to DNA damaging agents.
• The double-helical nature of DNA results in intertwining of daughter double helices unless the intertwining is destroyed by topoisomerases.
• Genetic and biochemical approaches led to the identification of four polypeptides required for sister chromatid cohesion.
  • The genetic approach used Saccharomyces cerevisiae (ref).
    • Mutants were sought with two properties:
      • They lost chromosomes at high frequency when grown at 25 C. Chromosome loss was detected by red-sectored colonies.
      • They could still separate sister chromatids when the temperature sensitive anaphase-promoting complex was inactivated by raising the temperature to 37 C. Non-separating chromosomes remain in the bud neck and can be detected by microscopy of cells.
    • Four genes were identified initially. Two further genes were identified later (ref).
  • The biochemical approach used extracts of Xenopus oocytes (ref).
    • cDNAs for the Xenopus homologues of two structural maintenance of chromosome (scm) proteins were used to make and purify XSCM1 and XSCM3.
    • Antibodies were made against each of the proteins. Antibodies against XSCM1 also immunoprecipitated XSCM3, and vice versa. Two complexes were found, the larger (named 14S cohesin) contained both proteins and three other proteins.
    • Sperm chromatin was incubated with extracts of nuclei in interphase. When extracts had been pretreated with antibodies to remove cohesins, the chromatin did not assemble and sister chromatids did not cohere properly while extracts that were not pretreated allowed chromatin assembly and cohesion of sister chromatids.
    • Cohesin subunits oligomerize into a donut shape (rev1, rev2, rev3)
• Cohesin protein complexes must be present during S phase for sister chromatid cohesion to be apparent in later phases (ref)

Interpretations

• Sister chromatids remain closely connected to one another through replication and chromatin condensation. The cohesion must be destroyed at anaphase of mitosis and of meiosis II.
• Daughter DNA strand intertwining can be one explanation of sister chromatid cohesion. Thus, topoisomerases must play important roles in chromosome dynamics.
• A protein complex, the 14S cohesin, is required for cohesion of sister chromatids during G2 and into mitosis. DNA intertwining left over from DNA replication may help maintain cohesion.
• The cohesin complex may work by encircling both strands of DNA.
• Sister chromatid cohesion is important for proper segregation of chromosomes

Further information

• SCM1 and SCM3 form a heterodimer by anti-parallel association so that a DNA binding domain is located at each tip of a V-shaped molecule.
• The Drosophila melanogaster mei-5332 gene is required for sister chromatid cohesion in meiosis I. It localizes to centromeres through meiosis until anaphase II (ref).
• Cohesin binds preferentially to AT-rich DNA (ref), possibly accounting for some chromsome banding patterns.
• Chiasmata are responsible for holding homologous chromosomes together in the beginning of meiosis I.
• Other molecules involved in chromatin segregation include condensins, separase, securins, monopolins, microtubule motors, and regulatory proteins.

This is page 13611 of Molecular Genetics by Ulrich Melcher, © 2001, 2004