Given below are a few statements about the λ infection cycle:
A. Competition between cl and cll gene products determines the establishment of lysogeny versus lysis.
B. cl binds O_R1 first while cro binds to O_R3 first
C. Cro binding to O_R represses cl transcription
D. Rich medium favours lytic cycle because cll is protected from cellular proteases

Which one of the following options represents all correct statements?

Correct option is A

Explanation-

Statement A - Competition between cI and cII gene products determines the establishment of lysogeny versus lysis  - INCORRECT
While cII plays a critical role in establishing lysogeny by activating transcription of cI (the repressor), cI and cII do not directly compete. Instead, Cro and cI compete for control of the lytic vs lysogenic decision. cII is more of a regulator than a competitor. So, this is misleading.

Statement B - cl binds O_R 1 first while cro binds to O_R 3 first -  CORRECT
cI binds O_R1 with highest affinity to repress cro and promote lysogeny. Cro binds O_R3 with highest affinity to block cI expression, favoring the lytic cycle. This differential binding is essential in determining the fate of the phage.

Statement C - Cro binding to O_R represses cI transcription  - CORRECT
Cro protein binds O_R3, which blocks the promoter PRM (from which cI is transcribed). This prevents cI synthesis, promoting the lytic cycle. Hence, this is a correct and key mechanism.

Statement D - Rich medium favours lytic cycle because cII is protected from cellular proteases  - INCORRECT
Actually, rich medium increases host cell metabolism, which leads to increased protease activity. This degrades cII, preventing it from activating cI transcription. Therefore, lysogeny is disfavored and lysis is favored, but not because cII is protected—it is degraded more rapidly in rich medium.

 Final Answer: Option a – B and C only

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