PATHOLOGY

Detailed explanation-1: -The cyclin A/CDK2 complex terminates the S phase by phosphorylating CDC6 and E2F1, and drives the cell-cycle transition from S phase to G2 phase, and subsequently activates CDK1 by cyclin A leading the cells to enter the transition to M phase. Upon mitosis, CDK1 activity is maintained by the complex cyclin B/CDK1.

Detailed explanation-2: -Cell cycle progression is regulated in part by the sequential activity of various cyclins. The cyclins are regulatory subunits that bind, activate and provide substrate specificity for their catalytic partner serine-threonine kinases, collectively called cyclin-dependent kinases (Cdks) (reviewed in refs.

Detailed explanation-3: -Cyclin regulates the kinase activity by binding to it. In the absence of cyclin, the kinases are inactive and are, therefore, called cyclin dependent kinases (Cdks). Cyclin confers basal kinase activity to the Cdk due to conformational changes. A number of Cdks are known-Cdk 1 to Cdk 7.

Detailed explanation-4: -In positive regulation, active molecules such as CDK/cyclin complexes cause the cell cycle to progress. In negative regulation, active molecules halt the cell cycle. The best understood negative regulatory molecules are retinoblastoma protein (Rb), p53, and p21.

Detailed explanation-5: -The central machines that drive cell cycle progression are the cyclin-dependent kinases (CDKs). These are serine/threonine protein kinases that phosphorylate key substrates to promote DNA synthesis and mitotic progression.