Key Points
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Although distinct in structure and function, cohesin and condensin both contain conserved chromosomal ATPases of the structural maintenance of chromosomes (SMC) protein family. Present models indicate that cohesin might encircle two sister chromatids to promote their cohesion, whereas condensin might stabilize supercoiled DNA loops on a single chromatid to promote its condensation. Whether the diversity of functions exhibited by each complex during interphase and mitosis reflects a common molecular mechanism remains unknown.
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Cohesin and condensin affect gene expression through influences on control mechanisms that operate globally at a chromosome-wide level, regionally over a subchromosomal domain, or locally on an individual gene.
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As well as a mitotic condensin complex, C. elegans contains a second condensin-like complex that mediates the global twofold repression of genes along hermaphrodite X chromosomes to achieve dosage compensation, a vital process that equalizes X-linked gene products between males (one X chromosome) and hermaphrodites (two X chromosomes). The complex also mediates the local 20-fold repression of a single autosomal gene.
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A condensin subunit might help maintain repression over regulatory domains of the Drosophila body-patterning gene Abd-B.
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Cohesin might need to be removed during mitosis to establish transcriptional silencing of the S. cerevisiae mating-type locus, whereas condensin might facilitate silencing over this domain.
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In S. cerevisiae, cohesin localizes to several DNA segments that border regions of active and silent chromatin and might be required for their chromatin insulation function.
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A cohesin regulatory protein might facilitate contact between a distant enhancer and the promoter of the Drosophila wing gene cut.
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In several organisms, cohesin subunits associate with DNA-repair proteins to form new complexes with roles in DNA repair. Accordingly, mutations in several cohesin subunits show hypersensitivity to DNA damage.
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Condensin and some cohesin subunits have been implicated in the DNA-damage checkpoint-signalling pathway.
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Cohesin, and possibly condensin, might help organize the structure and orientation of the centromere, so that sister chromatids are properly captured by microtubules from opposite poles of the spindle. Mutations in cohesin subunits trigger the spindle checkpoint.
Abstract
Two related protein complexes, cohesin and condensin, are essential for separating identical copies of the genome into daughter cells during cell division. Cohesin glues replicated sister chromatids together until they split at anaphase, whereas condensin reorganizes chromosomes into their highly compact mitotic structure. Unexpectedly, mutations in the subunits of these complexes have been uncovered in genetic screens that target completely different processes. Exciting new evidence is emerging that cohesin and condensin influence crucial processes during interphase, and unforeseen aspects of mitosis. Each complex can perform several roles, and individual subunits can associate with different sets of proteins to achieve diverse functions, including the regulation of gene expression, DNA repair, cell-cycle checkpoints and centromere organization.
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Acknowledgements
We thank R. Heald, S. Wignall, N. Bhalla and A. Severson for critical reading of the manuscript.
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FURTHER INFORMATION
Glossary
- UBIQUITYLATION
-
The covalent addition of the small protein ubiquitin to another protein. Ubiquitin conjugation generally targets proteins for degradation by proteases.
- PARALOGUES
-
Homologous genes in the same organism that have evolved from a gene duplication and a subsequent divergence of function.
- POSITIVE SUPERCOILING
-
Twisting of the DNA about its own axis. Twisting opposite to the direction of the double-helix turns produces negative supercoils, whereas twisting in the same direction produces positive supercoils.
- ATOMIC-FORCE MICROSCOPY
-
A method that maps submicroscopic surfaces to give information about their nature at the atomic level. A mechanical probe with a sharp tip is tracked over the surface of interest, and the deflection of the cantilevered tip is measured.
- GENE CONVERSION
-
A specific type of recombination, which results in non-reciprocal genetic exchange, in which the sequence of one DNA strand is used to alter the sequence of the other.
- PARASEGMENT
-
In Drosophila development, body-patterning genes are expressed in parasegmental units that are out of register with morphologically visible segments. A parasegment contains the posterior portion of one segment and the anterior section of the next segment.
- TOPOISOMERASE II
-
An ATP-dependent enzyme that creates transient breaks in both strands of the DNA sugar-phosphate backbone, then passes one strand through the other and reseals the break. Such enzymes can remove or create supercoils in duplex DNA.
- SYNTHETIC LETHALITY
-
A phenomenon that refers to lethality that is caused by the combination of two mutations, neither of which causes lethality by itself. Synthetic lethality of double mutants can indicate that two genes might function in related processes.
- DNA LIGASE III
-
Ligases are enzymes that seal nicks in one strand of double-stranded DNA by creating an ester bond between adjacent 3′OH and 5′PO4 ends on the same strand. DNA ligase III acts in DNA repair to heal single-stranded DNA breaks.
- DNA POLYMERASE-ε
-
Polymerases are enzymes that synthesize new DNA strands using a DNA template. DNA polymerase-ε acts in DNA repair when nucleotides have been excised, and also associates with the replication fork as a lagging-strand polymerase.
- THYMINE DIMERS
-
A pair of abnormal covalently bonded adjacent thymine residues in DNA that are caused by DNA damage.
- SPINDLE MIDZONE
-
A region of overlapping microtubules at the centre of the spindle-microtubule apparatus. The midzone forms when chromosomes segregate, and is required for proper spindle bi-polarity and elongation, chromosome movement and cytokinesis.
- SPINDLE ASTER
-
A star-shaped cluster of microtubules that emanate towards the cell cortex from the microtubule-organizing centres at the poles of the spindle. Astral microtubules help position the mitotic apparatus.
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Hagstrom, K., Meyer, B. Condensin and cohesin: more than chromosome compactor and glue. Nat Rev Genet 4, 520–534 (2003). https://doi.org/10.1038/nrg1110
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DOI: https://doi.org/10.1038/nrg1110
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