When light strikes the outer segment of Rod cells: 

Na + channels closed 

Hyperpolarizing receptor potential

TLS Online TPP Program

#Id: 8874

#Unit 2. Cellular Organization

Initial pool of active cyclin B-CDK1 phosphorylates Cdc25C, this recruits Pin1 and Plk1, which recognize Cdc25C through their phospho-binding domains. 

TLS Online TPP Program

#Id: 8875

#Unit 2. Cellular Organization

Pin1 and Plk1 modify and phosphorylate Cdc25C to stimulate its phosphatase activity

TLS Online TPP Program

#Id: 8877

#Unit 2. Cellular Organization

The pool of active Cdc25C is created, that activates the large pool of cytosolic cyclin B-CDK1, which activates more cytosolic Cdc25C as part of positive feedback loop 1

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.