Question

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

A : The ends of the chromosomes contain structures called telomeres
R: DNA dependent DNA polymerase is the enzyme responsible for telomere synthesis

In 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:

Standard DNA polymerase requires a pre-existing 3'-OH primer, which causes the "end-replication problem".
Only Telomerase (an RNA-dependent DNA polymerase) can synthesize telomeres *de novo* using its own internal RNA guide.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Linear eukaryotic chromosomes require specialized mechanisms to replicate their extreme ends to avoid progressive sequence loss during replication (the end-replication problem).

Step 2: Detailed Explanation:
1. Assertion (A): Eukaryotic chromosomes contain repetitive hexanucleotide non-coding sequences at their terminal ends called telomeres, which protect chromosomes from degradation and end-to-end fusion. Thus, Assertion (A) is correct.
2. Reason (R): Replicating these telomeric ends requires a specialized ribonucleoprotein enzyme called telomerase.
Telomerase carries its own internal RNA template and functions as an RNA-dependent DNA polymerase (reverse transcriptase) to extend the 3' end of the G-rich strand.
Standard DNA-dependent DNA polymerases cannot synthesize DNA without a primer, and are responsible for general DNA replication, not telomere synthesis. Thus, Reason (R) is incorrect.

Step 3: Final Answer:
The correct option is 3, which corresponds to (A) is correct but (R) is not correct.