Question

What are the differences between junctional and combinational diversity Give 2 examples of mechanisms that contribute to junctional diversity?

Answer 1

During the maturation of B cells in bone marrow, the genes in the DNA for variable region of the antibody undergoes recombination whereby some intervening nucleotides are removed so that V (variable), D (diversity) and J (joining) genes join. This is called somatic recombination. During this process, the existing nucleotides may be lost or new nucleotides may be added at the junction of the genes. This is called junctional diversity.This is responsible for a variety of antibodies that differ in variable region. This recombination occurs only within the same chain thus differs from combinatorial diversity.

Mechanisms contributes to junctional diversity include

1) imprecise joining of V,D and J genes which results in different V gene expression.

2) addition of non-template encoded (N) and palindromic (P) nucleotides at imprecise joints V-D-J genes.

Combinatorial diversity is due to different combinations of heavy (H) and light (L) chain V (variable) regions. Multiple germline gene segments combine randomly and different combinations produce different antigen receptors. It is generated by the random formation of many different VJ_L and VDJ_H combinations. Combinatorial diversity is increased by the ability of any V_H region to pair with any V_L region to bind antigen.  Actual combinatorial diversity is probably less, because some gene segments rearrange more than others and some V_H-V_L pairings bind antigen better than others.