Correct option is C
Explanation-
Statement A: "De novo synthesis and destruction of Cyclin B are essential for cell cycle progression in yeast."
This is mostly true, especially in the context of mitotic control. In yeast, Cyclin B (Clb proteins) is synthesized and degraded to control progression. While Cyclin B requires a CDK (Cdc28 in yeast) to function, this statement may be acceptable if the context assumes CDKs are inherently involved.
Statement B: "De novo synthesis and destruction of Cyclin B and the related Cyclin dependent Kinase (CDK) are essential for cell cycle progression."
It incorrectly implies that CDKs themselves are also synthesized and degraded de novo during each cycle — this is not true. The error in Statement B lies in suggesting that CDKs undergo the same regulated synthesis/destruction as Cyclins, which is not accurate.
Statement C: "CDK activity is regulated by both activating and inhibitory phosphorylation."
This is a core regulatory mechanism involving activating phosphorylation by CAK, inhibitory phosphorylation by Wee1 and removal of inhibition by Cdc25 phosphatases.
Statement D:"Retinoblastoma (Rb) functions as an inhibitor of G2 to M transition".
Rb inhibits G1 to S transition by binding E2F transcription factors. It is phosphorylated by Cyclin D/CDK4/6, releasing E2F and allowing progression into S phase. Not involved in G2 to M transition.
Statement E: "Inactivation of Sic1 is essential for transition into S phase."
In budding yeast, Sic1 inhibits S-phase CDKs. Its phosphorylation and degradation allow DNA replication to start.
Final answer - Option c - A, C and E
