Question

Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A: Attenuation of the trp operon is possible in bacteria.
Reason R: Transcription and translation are coupled.

• A. Both A and R are true and R is the correct explanation of A
• B. Both A and R are true but R is not the correct explanation of A
• C. A is true but R is false
• D. A is false but R is true

Hint:

No nucleus in bacteria = Coupled = Attenuation possible.

Answer 1

The Correct Option is A

Step 1: Concept
Attenuation is a mechanism for regulating gene expression in prokaryotes that uses translation to control the completion of transcription.

Step 2: Meaning
"Coupled" means that in bacteria, ribosomes begin translating the mRNA while the RNA polymerase is still synthesizing the tail end of that same mRNA.

Step 3: Analysis
In the trp operon, the speed of the ribosome moving through a "leader sequence" (determined by tryptophan levels) dictates whether the nascent RNA forms a terminator or an anti-terminator loop. This can only happen if the ribosome is physically right behind the RNA polymerase. In eukaryotes, transcription occurs in the nucleus and translation in the cytoplasm, so they are not coupled and attenuation cannot occur.

Step 4: Conclusion
Coupled transcription-translation is the physical prerequisite that allows attenuation to work. Final Answer: (A)