12. In the following diagram, segments A and C are copies of 10 base pair repeat DNA sequences, flanking a unique stretch shown as B, A and C are in inverted orientation as indicated by arrows. Intramolecular recombination between A and C leads to which event:
(1) The complete region encompassing A to C will be inverted
(2) Only A and B will be inverted
(3) Only B will be inverted
(4) Only regions A and C will be inverted
Intramolecular recombination between inverted repeats A and C will invert only the unique intervening segment B, leaving A and C in their original positions and orientations. This corresponds to option (3): only B will be inverted.
Question recap
The DNA fragment contains two identical 10 bp repeat segments A and C in inverted orientation, flanking a unique central region B on the same DNA molecule. Intramolecular recombination between such inverted repeats typically causes inversion of the DNA segment located between them, while the repeats themselves remain as boundary elements.
Correct option: Only B will be inverted (3)
Recombination between inverted repeats acts like a site‑specific recombination event that cuts and rejoins DNA so that the sequence between the two inverted sites flips 180° within the chromosome. Because A and C are the recombination sites, they pair, exchange strands, and rejoin, producing an inversion of B while A and C end up again flanking B in the same outer positions.
Why option (1) is incorrect
Option (1) claims that the complete region from A to C (A–B–C) will be inverted, which would require recombination between sites outside A and C or a whole‑chromosome rearrangement. In site‑specific intramolecular recombination, only the internal segment between the two homologous recombination sites is inverted when those sites are inverted, not the entire flanking block including them.
Why option (2) is incorrect
Option (2) suggests A and B will be inverted, leaving C unaffected. This cannot occur because both recombination sites (A and C) must participate simultaneously in the exchange; one cannot recombine alone. As a result, the recombination outcome is symmetric around the two sites, inverting only the central segment between them and not just one repeat plus part of the internal region.
Why option (4) is incorrect
Option (4) proposes that only A and C will be inverted, with B unchanged. However, when two inverted repeats recombine, the product keeps the repeats as boundary elements but flips the orientation of the intervening DNA; the repeats themselves end up in the same relative orientation as before. Inverting only the repeats without affecting B would require a different arrangement (for example, repeats placed outside A and C), which is not the case here.
Introduction
Intramolecular recombination between inverted repeats is a classic genome rearrangement that generates precise DNA inversions and often appears in CSIR NET Life Sciences questions. In the June 2017 paper, segments A and C are inverted 10 bp repeats flanking a unique region B, and understanding how their recombination affects B is essential for mastering DNA recombination concepts and solving similar exam problems quickly.
