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Figure 8.
Figure 8. Model of the Yeast Cohesin Complex(A) Smc1 and
Smc3 form a heterodimer with intramolecular coiled coils. Scc1
bridges the head domains of Smc1 and Smc3 and links them to
Scc3. For comparison, a schematic 10 nm chromatin fiber of DNA
wrapped around nucleosomes and a DNA double helix are shown in
scale to the Smc1/3 ring.(B) Hypothetical “embrace” model of
how the cohesin complex might confer sister chromatid cohesion.
Before the commencement of replication, the cohesin complex is
loaded onto DNA. The arms of the Smc1/3 molecules embrace the
DNA, thereby forming a ring of approx. 40 nm diameter. The head
domains of Smc1 and Smc3 are locked together by Scc1. Now,
cohesion might be generated as the replication fork passes
through the ring, entrapping both sister chromatids inside. At
the metaphase to anaphase transition, Scc1 is cleaved by
separase, thereby opening the lock of the Smc1/3 head domains.
The ring opens and sister chromatids can be pulled to opposite
spindle poles.
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