Somatic recombination during the formation of lymphocytes

Somatic recombination occurs within the genes
for the L and theHchains during the maturation
of B cells and within the genes for the four T-cell
receptor chains in the maturing T cells. By
means of this process, different coding DNA
sequences (exons) of the various domains of the
particular Ig molecule are in unique combinations
in each cell. This provides each molecule
with an antigen-binding specificity that differs
from that of all other cells. Here, an example of
the genetic processes during formation of an
immunoglobulin H chain is shown. The exons
(V1 to Vn) of the variable region (V) lie at the 5!
end of the IgH locus. They are separated from
each other by different lengths of noncoding
DNA. A small exon (60–90 bp) that codes for a
signal to initiate translation (leader or signal
peptide L) lies more than 90 base pairs (bp) in
the 5! direction of the V-region exons. Signal
peptides guide the growing polypeptide into
the lumen of the endoplasmic reticulum before
they are cleaved off. The D genes of the constant
region (C) lie at different distances in the 3!
direction fromthe V genes. Each C segment consists
of different exons, corresponding to the
domains of the complete C region and different
isotypes (Cμ, C!), etc.