Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A): In RNA molecule guanine content is equal to the cytosine content.
Reason (R): Mostly RNAs are single stranded molecule.
In light of the above statements, choose the correct answer from the options given below.

• 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:

Chargaff's rules (\(\text{A}=\text{T}\), \(\text{G}=\text{C}\)) apply strictly to double-stranded nucleic acids.
Since RNA is mostly single-stranded, it is not bound by these stoichiometric ratios.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Chargaff's rules state that in double-stranded DNA, the concentration of purines equals pyrimidines, specifically \([\text{A}] = [\text{T}]\) and \([\text{G}] = [\text{C}]\), due to obligatory base-pairing.

Step 2: Detailed Explanation:
- Assertion (A) evaluation: Because RNA is typically single-stranded, it does not have a complementary strand matching its bases throughout its entire length. Therefore, Chargaff's base-pairing rules do not apply, and the concentration of guanine is not necessarily equal to cytosine (\([\text{G}] \neq [\text{C}]\)). Thus, Assertion (A) is false.
- Reason (R) evaluation: Unlike double-stranded genomic DNA, the majority of functional biological RNA molecules (mRNA, tRNA, rRNA) exist as single polynucleotide chains. Thus, Reason (R) is true.

Step 3: Final Answer:
Assertion (A) is false but Reason (R) is true, corresponding to option (D).