The most important aspect of cohesin

 The most important aspect of cohesin

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It is a protein complex that forms a ring-like structure, and sister chromatids are thought to be held together by trapping within the cohesin ring.

Cohesin is formed by  multi-subunit protein complex

1.      SMC1, SMC3,

2.       RAD21 and

3.      SCC3 (SA1 or SA2

·         SMC proteins have two main structural characteristics

·         a head domain : an ATP-binding cassette with ATPase activity (formed by the interaction of the N- and C- terminals of RAD21)

·         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. 

·         RAD21 is a protein that in humans is encoded by the RAD21 gene

·         Rad21 binds to the ATPase heads of Smc3 and Smc1 via its N- and C- terminus, 

Once RAD21 binds the SMC proteins, SCC3 can also associate with RAD21. 

Function of cohesion

1. It is used to keep the sister chromatids connected with each other during metaphase ensuring that during mitosis (and meiosis), each sister chromatid segregates to opposite poles.

•      2. It facilitates  attachment of microtubules of spindle  onto chromosomes.

3. It facilitates DNA repair by recombination.

4. Cohesin has been shown to be responsible for transcription regulation, DNA double strand break repair

Mechanism of Sister Chromatid Cohesion

•      it is not clear how the cohesin ring links sister chromatids together. There are two possible scenarios:

•      Cohesin subunits bind to each sister chromatid and form a bridge between the two.

•      cohesin has a ring structure, it is able to encircle both sister chromatids.

•      Localization of cohesin rings

•       

•      A few cohesin rings are found in chromosome arms that have AT-rich DNA sequences

•      Cohesin rings, are also located in the region surrounding the centromere,  especially in budding yeast,