Question

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

Assertion (A): In a zinc finger about 30 amino acids form an elongated loop held together at the base by a single \(\text{Zn}^{2+}\) ion which is coordinated to form residues (with Cys and/or His).

Reason (R): The Zn does not itself interact with DNA, rather the coordination of Zn with amino acid residues stabilizes the small structural motif.

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

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A) \
• B. Both (A) and (R) are correct but (R) is not the correct explanation of (A) \
• C. (A) is correct but (R) is not correct \
• D. (A) is not correct but (R) is correct

Hint:

Zinc acts purely as a structural anchor or "glue" for the loop. The protein framework does the actual work of recognizing and binding the DNA sequence.

Answer 1

The Correct Option is B

Step 1: Concept Zinc fingers are common structural motifs found in DNA-binding proteins and transcription factors. They use a coordinated transition metal ion to stabilize their folded conformation.

Step 2: Meaning Coordination means that a central zinc ion forms coordinate covalent bonds with the lone pairs of specific cysteine (Cys) and histidine (His) residues, pinning the loop base into a rigid shape.

Step 3: Analysis * Assertion (A) is correct: A classical zinc finger consists of roughly 30 amino acids that form a loop. This loop is stabilized at its base by a single tetrahedral zinc ion bound to cysteine and histidine residues ($\text{Cys}_{2}\text{His}_{2}$ pattern). * Reason (R) is correct: The zinc ion stays buried inside the hydrophobic core to provide structural stability. It does not directly contact the DNA bases; instead, the alpha-helix of the stabilized protein loop inserts into the major groove of the DNA. * Explanation assessment: While both statements are true biological facts, the structural properties described in the assertion are a consequence of the amino acid sequence configuration, not because the zinc ion avoids interacting directly with DNA. Thus, (R) is not the logical explanation of (A).

Step 4: Conclusion Both statements are accurate, but the reason does not explain the assertion. This corresponds to option B. Final Answer: (B)