# DBT JRF 2020 — Question 33

Exam: DBT JRF 2020 · Section: A · Topic: Immunology · Difficulty: Medium

## Question

Using only random VDJ recombination, from 40 V, 30 D and 6 J gene segments, the number of possible variable regions of the resulting antibody would be

## Options

- **A.** 76
- **B.** 7,200
- **C.** 1.4 x 106
- **D.** 40

## Answer

Option D

## Explanation

VDJ recombination generates antibody heavy chain diversity by combining one each of V (variable), D (diversity), and J (joining) gene segments. With 40 V, 30 D, and 6 J segments, the number of possible combinations by random VDJ joining = $40 \times 30 \times 6 = 7{,}200$ variable regions. In reality, additional diversity arises from junctional diversity (P-nucleotide addition, N-nucleotide addition by TdT) and somatic hypermutation — expanding the repertoire to >10^{11} possible antibodies. This question tests only combinatorial diversity from gene segment selection.

Source: https://mypyqbuddy.com/questions/using-only-random-vdj-recombination-from-40-v-6a05a8a02adafec470293207