Q.For Questions number 13 to 16, two statements are given — one labelled as Assertion (A) and the other labelled as Reason (R). Select the correct answer to these questions from the codes (A), (B), (C) and (D) as given below. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Assertion (A) is false, but Reason (R) is true. Assertion (A) : RNA is unstable and can mutate at a faster rate. Reason (R) : The presence of 2' – OH group in every nucleotide of RNA makes it labile and easily degradable.

Concept: Chemical stability of RNA vs. DNA.

Reasoning:

1. The 2'–OH group in RNA’s ribose sugar makes it chemically more reactive than DNA (which has 2'–H). This hydroxyl group can participate in intramolecular attack on the phosphodiester bond, making RNA labile and easily degradable.
2. This inherent instability means RNA mutates at a faster rate than DNA, as errors during replication or damage are less efficiently repaired. …

Concept and Intuition

The question tests your understanding of the fundamental chemical difference between RNA and DNA — and how that difference drives biological consequences. The 2'–OH group in RNA is not just a structural detail; it’s the reason RNA is more reactive, less stable, and mutates faster than DNA. Think of it this way: DNA is like a sturdy, well-protected library book (stable, rarely damaged), while RNA is like a working draft on loose paper (easily torn, frequently revised). The 2'–OH group is the chemical “weak spot” that makes RNA labile.

Step-by-Step Reasoning

1. Examine the Assertion (A): “RNA is unstable and can mutate at a faster rate.”

   This is a well-established fact. RNA molecules have a short half-life (minutes to hours in cells) compared to DNA (years in some cells). Their instability means they are constantly being degraded and replaced. This rapid turnover, combined with the lack of a proofreading mechanism in RNA synthesis, leads to a much higher mutation rate — roughly 100 to 1000 times faster than DNA replication.

2. Examine the Reason (R): “The presence of 2'–OH group in every nucleotide of RNA makes it labile and easily degradable.”

   This is chemically accurate. In RNA, each ribose sugar has a hydroxyl group (–OH) attached to the 2' carbon. This –OH group is a nucleophile — it can attack the adjacent phosphodiester bond (specifically the phosphorus atom) in a reaction called intramolecular transesterification. This reaction breaks the RNA backbone, especially under alkaline conditions or in the presence of certain enzymes (RNases). DNA lacks this 2'–OH (it has just a hydrogen atom), so it is far more resistant to hydrolysis.

3. Check if Reason (R) correctly explains Assertion (A): …