The crystal structure of PCNA closely resembles the E. coli b subunit: A trimer forms a ring that surrounds the DNA.  The sequence and subunit organization are different from the dimeric b2 clamp; however, the function is likely to be similar.

TLS Online TPP Program

#Id: 8878

#Unit 2. Cellular Organization

At the same time that Cdc25C is being activated, mitotic CDKs and Plk1 phosphorylate Wee1, which targets it for destruction through a phosphobinding SCF protein-ubiquitin ligase. This forms positive feedback loop 2

TLS Online TPP Program

#Id: 8879

#Unit 2. Cellular Organization

This combination of two positive feedback loops:

one that activates a CDK1 activator (Cdc25C)

one that inhibits a CDK1 inhibitor (Wee1)

results in the abrupt and irreversible transition from G2 into mitosis.

TLS Online TPP Program

#Id: 8880

#Unit 2. Cellular Organization

Active cytoplasmic cyclin B-CDK1 translocates into the nucleus, where it phosphorylates nuclear lamins to cause nuclear envelope breakdown and, together with Plk1 and the Aurora kinases, induces chromosome condensation and formation of the mitotic spindle.

TLS Online TPP Program

#Id: 8881

#Unit 2. Cellular Organization

Plk1 is critical for formation of the mitotic spindle as well as for chromosome segregation.

TLS Online TPP Program

#Id: 8882

#Unit 2. Cellular Organization

The Aurora kinase family members Aurora A and Aurora B play key roles in mitotic spindle formation and in ensuring that chromosomes attach correctly to the mitotic spindle so that they are segregated accurately during mitosis.

TLS Online TPP Program

#Id: 8883

#Unit 2. Cellular Organization

Once mitotic CDKs are activated at the end of G2 , they phosphorylate specific serine residues in all 3 nuclear lamins.