AP BIOLOGY

Detailed explanation-1: -The second group comprises four CDKs-Kin28, Srb10, Bur1 and Ctk1-each activated by a single specific cyclin (Figure 1). These cyclins are usually not regulated in a cell-cycle-dependent manner, and the members of this second group of CDKs are involved in the control of gene transcription.

Detailed explanation-2: -The main families of regulatory proteins that play key roles in controlling cell-cycle progression comprise the cyclins (cyc) family, their substrates, the cyclin dependent kinases (cdks), the different families of cdk inhibitors (CKI) and the pocket protein retinoblastoma (pRB) family.

Detailed explanation-3: -These regulatory molecules either promote progress of the cell to the next phase (positive regulation) or halt the cycle (negative regulation). Regulator molecules may act individually or they can influence the activity or production of other regulatory proteins.

Detailed explanation-4: -Cdks, cyclins, and the APC/C are direct regulators of cell cycle transitions, but they aren’t always in the driver’s seat. Instead, they respond to cues from inside and outside the cell. These cues influence activity of the core regulators to determine whether the cell moves forward in the cell cycle.