Immunology Fall 2026: Lecture 11 Generation of Antibody Diversity Part 2

Overview

Brianne Barker explains how RAG1/2-mediated V(D)J recombination joins antibody gene segments under the 12/23 rule, and how Artemis, DNA polymerases, and TdT create junctional diversity by opening hairpins and adding or removing nucleotides. She then connects rearranged variable regions to constant-region expression, showing how RNA splicing produces IgM or IgD and membrane-bound or secreted antibodies, while class-switch recombination enables other antibody classes later in B-cell development.

Key takeaways

The 12/23 rule pairs RSSs with different spacer lengths, enforcing V-to-J joining in light chains and V-to-D-to-J assembly in heavy chains.
RAG1/2 cleavage creates hairpin coding ends and signal ends; signal ends are simply ligated, while coding ends undergo the processing that generates junctional diversity.
Artemis opens coding-end hairpins imprecisely, and template-based fill-in creates palindromic P nucleotides; loss of Artemis can block B- and T-cell development and cause SCID.
TdT adds random, template-independent N nucleotides after hairpin processing, while nucleotide trimming can further alter the junction sequence.
Junctional insertions can shift the reading frame because their lengths are not restricted to multiples of three, so many developing B cells produce nonfunctional receptors.
Alternative RNA splicing can generate IgM or IgD and membrane-bound or secreted forms from one rearranged antibody gene; switching to IgG, IgE, or IgA instead requires later DNA-level class-switch recombination.

Chapters

0:00 V(D)J Recombination and the 12/23 Rule

Light-chain variable regions join one V segment to one J segment; heavy chains join V, D, and J segments.
Recombination signal sequences (RSSs) carry either a 12-base-pair or 23-base-pair spacer.
The 12/23 rule permits joins between segments with unlike spacers, preventing V-to-V and J-to-J joins and guiding heavy-chain assembly through V-D-J.

4:58 RSS Cleavage and Deletional Joining

DNA is cut at the boundary between each gene segment and its RSS, and the intervening DNA is removed.
Barker focuses on deletional joining, in which discarded DNA includes the intervening segments and RSSs.
Inversional joining also exists, but is outside the lecture's scope.

7:00 RAG1/2 Recognize RSSs and Initiate DNA Cleavage

RAG1 and RAG2 act together as the RAG recombinase complex, bringing a 12-RSS and a 23-RSS into proximity.
The complex initiates cleavage at the gene-segment/RSS boundaries; a DNA nick is a break in only one of the two strands.
The nick exposes a free 3′ hydroxyl group, which drives the next DNA-strand exchange reaction.

12:00 Strand Transfer Produces Coding Hairpins and Signal Ends

The free 3′ hydroxyl attacks a phosphate on the opposite DNA strand, converting the initial nicks into double-strand breaks.
The antibody-coding ends form hairpin-shaped DNA, while the RSS-containing signal ends are ordinary broken double-stranded ends.
A complete recombination event creates two coding ends and two signal ends, even when diagrams show only one side for clarity.

17:30 Signal-End Ligation Removes Intervening DNA

The two signal ends are ligated together without nucleotide addition or deletion.
Their joining forms a circular piece of discarded DNA containing the excised intervening region and RSSs.
The coding ends require additional processing because their hairpin structures cannot be directly ligated.

22:00 Artemis Opens Hairpins Required for Adaptive Immunity

Artemis cleaves the coding-end hairpins so the V, D, or J segments can ultimately be joined.
Loss of hairpin opening prevents completion of V(D)J recombination and can cause severe combined immunodeficiency (SCID), affecting both B-cell and T-cell development.
Artemis was identified through mutations found in SCID patients and named for the Greek goddess associated with protecting children.

26:00 Imprecise Artemis Cleavage Generates Palindromic P Nucleotides

Artemis can cut several bases away from the ideal hairpin-opening position rather than at a single exact bond.
Asymmetric opening creates overhangs; DNA polymerase fills them using the exposed strand as a template.
The added bases form palindromic sequences called P nucleotides, creating new sequence at the gene-segment junction.

30:00 P-Nucleotide Addition Creates Junctional Diversity

Different Artemis cut positions can produce different P-nucleotide sequences even when two B cells use the same V and J segments.
P nucleotides arise from asymmetric hairpin opening and template-based fill-in; they are distinct from random N-nucleotide additions.
Junctional diversity therefore adds variation beyond the combinatorial choices of V, D, and J segments.

38:40 End Trimming and TdT Add Random N Nucleotides

After P-nucleotide formation, enzymes may remove bases from coding ends; Barker notes that this trimming will not appear in her examples.
Terminal deoxynucleotidyl transferase (TdT) adds nucleotides without reading a template, unlike ordinary template-directed DNA polymerases.
These random additions are called N nucleotides; the processed coding ends are then ligated together.

44:50 Imprecise Joins Increase Diversity but Can Disrupt Reading Frames

P- and N-nucleotide additions, along with base removal, can change the coding sequence at each junction.
Because additions are not constrained to multiples of three, recombination can shift the reading frame and render an antibody gene nonfunctional.
A heavy-chain variable region requires V-D and D-J joins, while a light chain requires a V-J join; cells tolerate many failed outcomes in exchange for broad diversity.

49:20 Heavy-Chain Constant Regions Follow the Rearranged VDJ

The rearranged variable region sits upstream of antibody constant-region genes, which determine the antibody class.
The heavy-chain locus includes constant regions for IgM (μ), IgD (δ), IgG (γ), IgE (ε), and IgA (α).
Light chains use either κ or λ constant regions; the lecture distinguishes these constant regions from the V(D)J-generated variable region.

54:10 RNA Splicing Produces IgM, IgD, and Membrane or Secreted Forms

RNA polymerase transcribes the rearranged VDJ and nearby μ and δ constant-region sequences into a primary RNA transcript.
Alternative RNA splicing can produce either IgM or IgD from the same rearranged DNA, allowing a B cell to express both without another DNA rearrangement.
Additional splice choices produce membrane-bound B-cell receptors or secreted antibodies, yielding IgM-membrane, IgM-secreted, IgD-membrane, and IgD-secreted forms.

56:10 Class Switching and Practice Mapping Recombination Products

IgG, IgE, and IgA expression requires later class-switch recombination, which removes intervening DNA and places a different constant-region gene downstream of VDJ.
Class switching uses enzymes distinct from RAG1/2 and occurs later in B-cell development.
The worked exercise tracks a heavy-chain example using V2, D2, and J3, separates retained DNA from deleted segments, and identifies possible P- versus N-nucleotide origins.
Barker closes by reminding students that the homework is due Monday.

Summary, takeaways, and chapters were generated by AI from the video's transcript and may contain errors. The video belongs to its creator, Brianne Barker.