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GATE 2025 BT Q.15: solution and answer

GATE 2025 · BTQ.15 · 1 mark · MCQ

The diversity in T-cell receptors is generated by

  1. (A)

    gene rearrangements

  2. (B)

    somatic hypermutation of rearranged V region

  3. (C)

    gene conversion

  4. (D)

    class switching

See the question in the paper
GATE 2025 BT question 15Tap to open full size

Answer and solution

ANSWER

The diversity in T-cell receptors is generated by gene rearrangements, option (A).

Official answer: ASource: Official answer key, GATE 2025 BT (IIT Roorkee)

  1. T-cell receptor (TCR) chains are encoded by gene segments: V, D and J for the and chains, and V and J for the and chains. These segments are joined by V(D)J recombination in the developing thymocyte, catalysed by the RAG1/RAG2 recombinase.

  2. Option A, gene rearrangements: correct. Random combinatorial choice of V, D and J segments, together with junctional diversity (random loss of nucleotides and addition of P and N nucleotides at the joints), produces a very large TCR repertoire.

  3. Option B, somatic hypermutation of rearranged V region: wrong. This is AID-dependent and occurs in activated B cells in germinal centres, where it refines antibody affinity. Rearranged TCR genes are not hypermutated in this way.

  4. Option C, gene conversion: wrong. It diversifies immunoglobulin genes in species such as chickens and rabbits. It is not the main source of TCR diversity.

  5. Option D, class switching: wrong. It exchanges the heavy-chain constant region of an antibody (for example IgM to IgG, IgA or IgE) in B cells. TCRs have no isotypes to switch.

  6. So TCR diversity is generated by gene rearrangements. The answer is (A).

KEY IDEA

TCR diversity comes from V(D)J gene rearrangement, with combinatorial and junctional diversity, by RAG1/RAG2 in the thymus. Somatic hypermutation, gene conversion and class switching are features of B-cell and immunoglobulin biology.

WATCH OUT FOR

Choosing somatic hypermutation because it is the best-known diversity mechanism in immunology. It acts only on rearranged immunoglobulin V regions in B cells, not on TCR genes.

From the GATE 2025 BT paper. Every question of this paper · Download the full paper · All 7 Adaptive immunity questions, with notes

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