Molecular Biology Terms Starting With V
Molecular Biology Glossary: V
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Variable Region
/ VAIR-ee-uh-bul REE-jun / · Latin variabilis (changeable)
Variable Region is the N-terminal domain of an antibody heavy or light chain that contains hypervariable loops forming the antigen-binding site and differs in amino acid sequence between antibody molecules.
Variable regions are encoded by gene segments assembled by V(D)J recombination during B cell development, a process that randomly joins V, D, and J segments from libraries of alternatives at each position. Within each variable region, three hypervariable loops called complementarity-determining regions make direct contact with antigen and account for most of the sequence diversity between antibodies. Pairing of a heavy-chain variable region with a light-chain variable region creates the final antigen-binding pocket, and because any heavy chain can pair with any light chain, combinatorial pairing multiplies diversity further.
Somatic hypermutation after antigen exposure introduces additional point mutations into variable regions, refining binding affinity during the germinal center reaction.
X-ray crystallography of antibody-antigen complexes, pioneered by Roberto Poljak and colleagues in 1973, revealed that only the complementarity-determining region loops of the variable domain make physical contact with antigen. The framework regions that support those loops are structurally conserved enough that researchers now use them as scaffolds to graft foreign binding loops and engineer synthetic antibodies.
Immune System Fun Facts →All regions of an antibody contribute equally to binding the target antigen. Only the complementarity-determining region loops within the variable domain make direct contact with antigen; the constant regions mediate effector functions such as complement activation and Fc receptor binding.
After influenza vaccination, human B cells whose variable regions bind the viral surface protein hemagglutinin undergo clonal expansion and somatic hypermutation in germinal centers. Selected clones can achieve binding affinities in the low nanomolar range, with dissociation constants below 10 nanomolar, enabling effective neutralization of circulating influenza strains.
Vdj Recombination
/ VEE-DEE-JAY reh-kom-bih-NAY-shun / · Variable, Diversity, Joining gene segments; Latin recombinare
Vdj Recombination is the somatic DNA rearrangement process in developing B and T lymphocytes that randomly joins V, D, and J gene segments to assemble functional immunoglobulin and T cell receptor genes.
Recombination-activating proteins RAG1 and RAG2 recognize recombination signal sequences flanking each gene segment and introduce double-strand breaks that allow the segments to be joined by the non-homologous end-joining repair pathway. Random addition and deletion of nucleotides at the junctions by terminal deoxynucleotidyl transferase generates additional sequence diversity beyond what segment combination alone provides. Because any V segment can join any D and any J segment, and because junctional diversity multiplies the possibilities further, the theoretical number of distinct heavy-chain sequences exceeds one trillion.
Errors in this process can fuse a V, D, or J segment to an oncogene, and the translocation t(14;18) found in most follicular lymphomas arises from a mistaken RAG-mediated cut near the BCL2 gene on chromosome 18.
RAG1 and RAG2 share structural and mechanistic features with transposases, the enzymes that move transposable elements through genomes. This similarity led to the transposon hypothesis, first proposed by David Schatz and colleagues in the 1990s, which holds that V(D)J recombination evolved from an ancient transposon insertion event more than 500 million years ago.
Every immune cell inherits a complete, pre-assembled antibody or T cell receptor gene from its parent cell. Developing B and T cells actively cut and rejoin their own DNA during maturation, assembling receptor genes from separate segments that are never joined in the germline.
Immune System Fun Facts →Developing human B cells use VDJ recombination to assemble immunoglobulin heavy-chain genes from roughly 40 functional V segments, 25 D segments, and 6 J segments on chromosome 14. Combinatorial joining of these segments alone generates more than 6,000 possible heavy-chain variable sequences, and junctional nucleotide additions expand that number by several orders of magnitude.