Q.When RNA is hydrolysed, there is no relationship among the quantities of different bases obtained. What does this fact suggest about the structure of RNA?

The key idea is that RNA is single-stranded, unlike the double-stranded DNA where base pairing (A=T, G=C) forces a fixed 1:1 ratio between complementary bases.

1. In a double-stranded nucleic acid, the bases pair specifically — adenine with thymine (or uracil), guanine with cytosine. This means the amount of A always equals T (or U), and G always equals C.
2. If RNA were double-stranded, hydrolysis would yield equal quantities of A and U, and equal quantities of G and C. …

This is a classic question that tests your understanding of the structural difference between DNA and RNA — and specifically, why Chargaff’s rules apply to one but not the other.

The core idea

When you hydrolyse a nucleic acid, you break it down into its constituent bases. If the nucleic acid is double-stranded, the two strands are held together by specific base pairs: A pairs with T (or U in RNA), and G pairs with C. This pairing is complementary — wherever there is an A on one strand, there must be a T on the opposite strand. As a result, in any double-stranded molecule, the total number of A’s equals the total number of T’s, and G equals C. This is Chargaff’s rule.

If you hydrolyse such a molecule, the bases you recover will reflect this equality: you’ll get roughly equal amounts of A and T, and equal amounts of G and C.

Now, if hydrolysis gives you no fixed relationship between the amounts of different bases — meaning A can be much more than U, or G much less than C, with no pattern — then the molecule cannot be double-stranded. There is no complementary strand forcing those equalities.

Step-by-step reasoning

1. Recall Chargaff’s rules for DNA.

   In double-stranded DNA, $A = T$ and $G = C$. This is a direct consequence of base pairing: each A on one strand hydrogen-bonds to a T on the other, and each G to a C. So the total counts must match.

2. What about RNA?

   RNA uses uracil (U) instead of thymine (T). If RNA were double-stranded, you would expect $A = U$ and $G = C$ for the same reason. But the problem states that when RNA is hydrolysed, the quantities of different bases show no relationship — they are not equal, not in any fixed ratio.

3. What does “no relationship” mean?

   It means that in one RNA sample you might find 30% A, 20% U, 25% G, 25% C; in another, 40% A, 10% U, 30% G, 20% C. There is no constraint like $A = U$ or $G = C$. This is exactly what you would expect if the RNA molecule consists of a single strand — because there is no complementary strand to enforce pairing.

4. Doesn’t RNA sometimes form double-stranded regions? …

Method: Base Composition Analysis (Chargaff's Rule Comparison)

This method uses the principle of Chargaff's rules — which apply to double-stranded DNA — to infer structural differences in RNA.

Steps:

1. Recall Chargaff's rules for DNA

   In double-stranded DNA, base pairing is complementary:

   • A = T (adenine equals thymine)
   • G = C (guanine equals cytosine) This equality arises because each base on one strand pairs with a specific base on the opposite strand.

2. Observe the given fact for RNA

   When RNA is hydrolysed, the quantities of A, U, G, and C show no fixed relationship — for example, A ≠ U and G ≠ C in general.

3. Apply the logic of base pairing …

Here are the common mistakes students make on this question, along with how to avoid each one.

Mistake 1: Saying RNA is "Unstable" or "Degrades Easily"

• The Mistake: Students read "hydrolysed" and think the question is about RNA being chemically fragile. They answer: "This shows RNA is unstable and breaks down easily."
• Why it's wrong: The question is not about how easily RNA breaks, but about the result of breaking it. The key fact is that the quantities of bases are unrelated (e.g., A ≠ U, G ≠ C). Instability doesn't explain this pattern.
• How to avoid: Focus on the relationship between the products. The fact that the amounts of A, U, G, and C are not equal is the clue. Ask yourself: "What structure would give me unequal amounts of bases?"

Mistake 2: Confusing RNA with DNA (Applying Chargaff's Rule Incorrectly)

• The Mistake: Students remember Chargaff's rule for DNA (A=T, G=C) and think RNA must follow the same rule. They answer: "This suggests RNA is double-stranded, just like DNA."
• Why it's wrong: This is the most common error. Chargaff's rule (A=T, G=C) is a property of double-stranded DNA due to base pairing. RNA is single-stranded, so there is no complementary strand to enforce equal base ratios.
• How to avoid: Memorise this key contrast:
  • DNA (double-stranded): A = T, G = C (Chargaff's rule holds).
  • RNA (single-stranded): A ≠ U, G ≠ C (no fixed relationship).
  • The logic: If the amounts were equal, it would suggest base pairing (double strand). Since they are not equal, it suggests no complementary strand → single-stranded.

Mistake 3: Giving a Vague or Incomplete Answer

• The Mistake: Students write: "RNA is single-stranded." without explaining why the data leads to that conclusion.
• Why it's wrong: In exams, you must connect the evidence to the conclusion. A one-line answer often loses marks.
• How to avoid: Use a clear cause-and-effect structure:
  1. Observation: Hydrolysis gives unequal amounts of A, U, G, C.
  2. Reasoning: If RNA were double-stranded with complementary base pairing (A-U, G-C), the amounts would be equal (A = U, G = C).
  3. Conclusion: Since the amounts are not equal, RNA cannot have a complementary strand. Therefore, it must be single-stranded.

Mistake 4: Forgetting to Mention "No Complementary Base Pairing" …