Q.32 Which one or more of the following statements correctly describe(s) the addition
of N–nucleotides during the rearrangement of the immunoglobulin heavy chain–
encoding gene?
(A) Addition of N–nucleotides is template encoded.
(B) N–nucleotides are added by terminal deoxynucleotidyl transferase.
(C) The added N–nucleotides are common in V–D and D–J junction.
(D) N–nucleotides are added by the DNA polymerase II.

Correct answer: (B) and (C)

N-nucleotides are non-templated nucleotides added randomly during V(D)J recombination of immunoglobulin heavy chain genes to enhance junctional diversity at CDR3. This process occurs specifically at V-D and D-J junctions by the enzyme terminal deoxynucleotidyl transferase (TdT).​

Option Analysis

(A) Addition of N-nucleotides is template encoded

N-nucleotides are explicitly non-templated, meaning they are not copied from any DNA template strand during rearrangement. They arise from random addition to 3′ overhangs created after RAG-mediated cleavage.​

(B) N-nucleotides are added by terminal deoxynucleotidyl transferase

Terminal deoxynucleotidyl transferase (TdT), a template-independent DNA polymerase, catalyzes the addition of these random deoxyribonucleotides to coding ends in heavy chain rearrangements. TdT expression peaks in pro- and pre-B cells, enabling N-region insertion absent in TdT-deficient models.​

(C) The added N-nucleotides are common in V-D and D-J junction

N-nucleotides routinely appear at both V-D (N1 region) and D-J (N2 region) junctions during sequential heavy chain rearrangement (first D-J, then V to DJ). Studies confirm their presence across these sites, contributing equally to CDR3 diversity.​

(D) N-nucleotides are added by the DNA polymerase II

DNA polymerase II plays no role in N-nucleotide addition; it functions in general DNA repair and replication, not V(D)J junctional processing. TdT uniquely handles this template-independent synthesis.​

N-nucleotides addition during immunoglobulin heavy chain rearrangement is a critical mechanism generating antibody diversity through V(D)J recombination. This process, driven by terminal deoxynucleotidyl transferase (TdT), inserts random non-templated nucleotides at V-D and D-J junctions, shaping the hypervariable CDR3 region essential for antigen recognition.​

Mechanism Overview

V(D)J recombination assembles variable (V), diversity (D), and joining (J) gene segments in B-cell progenitors. After RAG1/2 cleavage creates hairpin coding ends, Artemis opens them, exposing 3′ overhangs. TdT then adds N-nucleotides randomly (typically 0-15 nt, averaging 5-10), followed by non-homologous end joining (NHEJ) via Ku, DNA-PK, XRCC4, and ligase IV.​

• D-J joining first: Forms DJ intermediate with N-nucleotides at D-J junction.

• V to DJ joining: Adds N-nucleotides at both V-D and prior D-J sites.

• Diversity impact: N-regions contribute >10^6 combinations, far exceeding combinatorial VDJ joining.​

Biological Significance

N-nucleotide addition peaks postnatally when TdT expresses in bone marrow pro-B cells, absent in fetal/neonatal stages for longer CDR3s. Biases exist (e.g., G/C gradients from dCTP/dGTP preferences), but randomness dominates. Deficiencies (TdT-/- or XLF mutants) reduce junctional diversity, impairing repertoire breadth.​

This suits CSIR NET preparation, linking molecular immunology to evolutionary adaptation in adaptive immunity.​