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Concept
Cohesin
Summary
Cohesin is a protein complex that mediates sister chromatid cohesion, homologous recombination, and DNA looping. Cohesin is formed of SMC3, SMC1, SCC1 and SCC3 (SA1 or SA2 in humans). Cohesin holds sister chromatids together after DNA replication until anaphase when removal of cohesin leads to separation of sister chromatids. The complex forms a ring-like structure and it is believed that sister chromatids are held together by entrapment inside the cohesin ring. Cohesin is a member of the SMC family of protein complexes which includes Condensin, MukBEF and SMC-ScpAB.
Cohesin was separately discovered in budding yeast by Douglas Koshland and Kim Nasmyth.
Cohesin is a multi-subunit protein complex, made up of SMC1, SMC3, RAD21 and SCC3 (SA1 or SA2). SMC1 and SMC3 are members of the Structural Maintenance of Chromosomes (SMC) family. SMC proteins have two main structural characteristics: an ATP-binding cassette-like 'head' domain with ATPase activity (formed by the interaction of the N- and C- terminals) and a hinge domain that allows dimerization of SMCs. The head and the hinge domains are connected to each other via long anti-parallel coiled coils. The dimer is present in a V-shaped form, connected by the hinges.
The N-terminal domain of RAD21 contains two α-helices which forms a three helix bundle with the coiled coil of SMC3. The central region of RAD21 is thought to be largely unstructured but contains several binding sites for regulators of cohesin. This includes a binding site for SA1 or SA2, recognition motifs for separase cleavage and a region that is competitively bound by PDS5A, PDS5B or NIPBL. The C-terminal domain of RAD21 forms a winged helix that binds two β-sheets in the Smc1 head domain.
Once RAD21 binds the SMC proteins, SCC3 can also associate with RAD21. When RAD21 binds on both SMC1 and SMC3, the cohesin complex forms a closed ring structure. The interfaces between the SMC subunits and RAD21 can open to allow DNA to pass in and out of the cohesin ring.
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Ontological neighbourhood
Related publications (42)
Related concepts (16)
Chromosome segregation
Chromosome segregation is the process in eukaryotes by which two sister chromatids formed as a consequence of DNA replication, or paired homologous chromosomes, separate from each other and migrate to opposite poles of the nucleus. This segregation process occurs during both mitosis and meiosis. Chromosome segregation also occurs in prokaryotes. However, in contrast to eukaryotic chromosome segregation, replication and segregation are not temporally separated. Instead segregation occurs progressively following replication.
Cell cycle checkpoint
Cell cycle checkpoints are control mechanisms in the eukaryotic cell cycle which ensure its proper progression. Each checkpoint serves as a potential termination point along the cell cycle, during which the conditions of the cell are assessed, with progression through the various phases of the cell cycle occurring only when favorable conditions are met. There are many checkpoints in the cell cycle, but the three major ones are: the G1 checkpoint, also known as the Start or restriction checkpoint or Major Checkpoint; the G2/M checkpoint; and the metaphase-to-anaphase transition, also known as the spindle checkpoint.
Sister chromatids
A sister chromatid refers to the identical copies (chromatids) formed by the DNA replication of a chromosome, with both copies joined together by a common centromere. In other words, a sister chromatid may also be said to be 'one-half' of the duplicated chromosome. A pair of sister chromatids is called a dyad. A full set of sister chromatids is created during the synthesis (S) phase of interphase, when all the chromosomes in a cell are replicated.