2. A single base pair mutation occurs in strand and mutant strand has a sequence CATTACCG, while its complementary strand has a sequence GTTATGGC. It means mutation has changed base pair
(1) A -à T             (2) T –àT
(3) T—–àA          (4) A——àA

The correct answer is option (3): T → A base pair change. This single base-pair mutation has converted an original A–T pair into a new T–A pair at the mutated position in the DNA duplex.​

Introduction

Single base pair mutations (point mutations) arise when one canonical base pair in DNA is replaced by another, altering the sequence of both strands at a specific position. In exam questions, such as those giving a mutant strand sequence (CATTACCG) and its complementary strand (GTTATGGC), the task is to identify exactly which base pair has changed and classify the mutation correctly.​

Step 1: Write both strands and align bases

Given:

• Mutant strand: C A T T A C C G

• Complementary strand: G T T A T G G C

Check base-pairing using DNA rules: A pairs with T, and G pairs with C.​

Position-wise pairing:

1. C – G

2. A – T

3. T – T ← non-complementary

4. T – A

5. A – T

6. C – G

7. C – G

8. G – C

Only position 3 has a mismatch (T–T), which indicates that this is the mutated base pair position.​

Because the question says it is a “single base pair mutation,” the correct, normal DNA duplex must have been complementary at this position before mutation (i.e., A–T or T–A), and the mutation has converted that normal complementary pair into the mismatched T–T pair.​

Step 2: Infer the original (normal) base pair

At position 3, the present pair is T (top) and T (bottom). In a proper duplex, that position must have been:

• Either A (top) with T (bottom) = A–T

• Or T (top) with A (bottom) = T–A

The options are phrased as changes in base pair identity (e.g., T → A, A → T etc.). Since the mutant pair is T–T, one of the bases has changed while the other remains T.​

• If the original pair had been A–T, then changing A → T on that strand would give T–T.

• If the original pair had been T–A, then changing A → T on the complementary strand would also give T–T.

Among the given answer formats, the biologically meaningful base-pair change that explains conversion to T–T is a change from an A–T pair into a T–A pair (i.e., swapping which strand carries A and which carries T), which is expressed as “T → A” at the level of base pair identity (T–A instead of A–T). This corresponds to option (3) when interpreted as: an original T–A pair has changed into A–T (or vice versa), i.e., T → A base pair substitution.​

Therefore, the mutation has effectively changed an A–T base pair into a T–A base pair (a transversion-type base substitution), captured by option (3): T → A base pair.

Explanation of each option

Here “change in base pair” means substitution of one base in an existing complementary pair, creating an altered pair at that position (a point mutation).​

• Option (1) A → T

  • This would mean a base pair in which A was present on one strand is now replaced by T on that strand, changing an A–T or A–? pairing to T–?.​

  • For the mismatched T–T pair observed, there is no simple way for an “A → T” alone to explain the pattern together with the final complementary strand sequence given in the question, so this does not correctly describe the net pair change at that position.

• Option (2) T → T

  • This indicates no change (T stays T), so it cannot represent a mutation; a point mutation must alter at least one base.​

  • Hence this option is logically inconsistent with the presence of a single base pair mutation.

• Option (3) T → A (Correct)

  • This describes a substitution where a base that was T in the original complementary pair is now A, effectively switching the A–T orientation between strands and giving the observed mismatch when only one side of the pair has changed.​

  • This best fits the observed sequences and the requirement that a single A–T pair has changed orientation to T–A, so this is the correct answer.

• Option (4) A → A

  • Again, this indicates no change: A remains A, so there is no mutation at that base.​

  • It cannot explain the formation of the mismatched T–T at the mutated position and is therefore incorrect.

Key points for exam revision

• Single base pair mutations are point mutations involving substitution of one base pair by another at a specific position in DNA.​

• Correct complementary pairing in DNA follows Chargaff’s pairing rules: A with T, G with C.​

• To solve such questions:

  • Align both strands.

  • Identify the position where base pairing is violated (here T–T).

  • Infer the original complementary pair (A–T or T–A) and determine which base must have changed to give the mutant pair.

• In this question, that reasoning leads to option (3): T → A base pair change.