<prompt> The transition from G2 to M phase of the cell cycle is controlled by :Retinoblastoma gene productp53 proteinCyclin ECyclin B  <response>: Ans. is D i.e. Cyclin BRef Robbins Illustrated 6the 284Cyclins control the passage of cells through specific phases of the cell cycleThe entry and progression of cells through the cell cycle are controlled by changes in levels and activities of cyclinsSo the levels of several of the cyclins peak during specific phases of the cell cycle, they are rapidly degraded as the cell enters the next phase of the cell cycleCyclins perform their function by forming a complex with a group of constitute expressed proteins called cyclin-dependent kinases CDKs.Different combination of cyclins and CDKs are associated with each of the important transitions in the cell cycle.As the cell moves into any phase the cyclin specific for that phase is synthesized and then it binds to its constitutive CDK, creating the cyclinCDK complex, whose activity is necessary for cells to enter the next phase.This complex when activated participates in the reactions and after the reactions are over these cyclins are degraded.Examples of cyclinCDK complexes controlling the cell cycle cyclins BCDK1Regulates the transition from G2 to M phaseCyclin DCDK4 Cyclin DCDK6 Cyclin ECDK2regulates the transition from G1-SCyclin ACDK2 and cyclin BCDK1Active in S phase 