Q.7 Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R.

In the light of the above statements, choose the most appropriate answer from the options given below:

1. Both A and R are correct and R is the correct explanation of A
2. Both A and R are correct but R is NOT the correct explanation of A
3. A is correct but R is not correct
4. A is not correct but R is correct

   A is correct but R is not correct. Riboswitches are structured mRNA elements that regulate gene expression by binding specific ligands, but their high specificity targets individual metabolites, not a broad range.

   Question Breakdown

   This assertion-reason question evaluates riboswitch mechanism understanding. Assertion A describes their function accurately, while Reason R misstates ligand specificity – riboswitches bind one cognate ligand with high precision, causing conformational switches (aptamer + expression platform) for transcription/translation control.

   Option Analysis

   Both A and R correct, R explains A

   Incorrect. While A holds, R’s “broad range of ligands” contradicts riboswitch specificity (e.g., TPP riboswitch binds only thiamine pyrophosphate).​

   Both A and R correct, but R not explanation

   Incorrect. R is factually wrong; riboswitches evolved narrow specificity for single metabolites like SAM, FMN, or glycine.​

   A correct but R not correct

   Correct. A matches riboswitch definition (mRNA elements altering gene expression via ligand-induced folding). R fails as specificity is to specific ligands, not broad ones; broad binding would abolish regulation.

   A not correct but R correct

   Incorrect. A is accurate; R is not.​

   Riboswitches assertion reason questions test if mRNA elements regulating genes via specific ligand binding are understood. Assertion A correctly identifies riboswitches as structured mRNAs altering expression, but Reason R wrongly claims broad ligand specificity.

   Riboswitch Mechanism

   Riboswitches comprise aptamer (ligand-binding) and expression platform domains:

   • Ligand binds aptamer → Conformational switch.​

   • High specificity to one metabolite (e.g., TPP, c-di-GMP).​

   • Regulates via termination, RBS sequestration, or splicing.​

   Broad binding (per R) would prevent precise feedback; evolution selected narrow pockets.​

   Why R Fails

   Specificity ensures metabolic homeostasis (e.g., TPP riboswitch shuts off biosynthesis at high TPP). Broad range would mimic promiscuous aptamers, useless for regulation.​

   Option	Status	Explanation
   1. Both correct, R explains	Wrong	R contradicts specificity​
   2. Both correct, R not explains	Wrong	R factually false​
   3. A correct, R not	Correct	Matches mechanism​
   4. A wrong, R correct	Wrong	A accurate​