Question

Hydrolysis of which of the following nucleic acids does not give equal number of A and T or G and C? \[ \text{(a) m-RNA} \] \[ \text{(b) Single strand DNA} \] \[ \text{(c) r-RNA} \] \[ \text{(d) Double helix DNA} \]

• A. (a), (b), (c)
• B. (b), (c), (d)
• C. (b) only
• D. (a) only

Hint:

Chargaff's rule \((A=T,\ G=C)\) is strictly applicable only to double-stranded DNA. Single-stranded nucleic acids such as m-RNA, r-RNA, and single-stranded DNA generally do not follow this rule.

Answer 1

The Correct Option is A

Step 1: Recall Chargaff's rule.
In double-stranded DNA, complementary base pairing occurs: \[ A=T \] and \[ G=C \] Therefore, after hydrolysis of double helix DNA, equal numbers of A and T as well as G and C are obtained.

Step 2: Examine m-RNA.
m-RNA is a single-stranded nucleic acid.
It contains bases \[ A,\ U,\ G,\ C \] and does not contain thymine (\(T\)).
Since complementary pairing is absent throughout the molecule, \[ A \neq U \] and \[ G \neq C \] in general.
Hence, m-RNA does not give equal numbers of complementary bases on hydrolysis.

Step 3: Examine single-stranded DNA.
Single-stranded DNA does not possess complete complementary base pairing.
Therefore, there is no requirement that \[ A=T \] or \[ G=C \] Thus, hydrolysis of single-stranded DNA does not necessarily yield equal numbers of these bases.

Step 4: Examine r-RNA.
r-RNA is also predominantly single-stranded.
Hence, it does not obey Chargaff's base equivalence rule.
Therefore, \[ A \neq U \] and \[ G \neq C \] in general.

Step 5: Final conclusion.
Among the given nucleic acids, the ones that do not give equal numbers of complementary bases on hydrolysis are \[ \boxed{\text{(a), (b), (c)}} \] Therefore, the correct option is (1).