gene segment

First, a D gene segment and a J segment are
joined (D–J joining). Next, this D–J segment is
joined to one of the V segments (V–D–J joining).
The combination of V, D, and J segments is
different in each cell, with at least 25000 different
possible combinations due to the number of
different segments present (100–125 V segments,
12D segments, and 4 J segments). The
joined VDJ segments form the primary RNA
transcript. At this stage, noncoding segments
(introns) are still present. In the example
shown, a D2 is joined to a J1, but not to J2–4.
J segments not directly connected to a D segment
(here, J2–4) are removed. After the RNA is
processed by splicing, the mature messenger
RNA (mRNA) is formed as template for the
translation of an H chain polypeptide. By
further processing, such as removal of the
leader segment (L) and glycosylation of the protein
at certain sites, the definitive H chain is finally
produced (of type μ in the example
shown). Unlike the H chains, the L chains have
no diversity (D) genes, so that a J and a V gene
are directly joined by somatic recombination
during DNA rearrangement in the lymphocytes.
Thus, rearrangement of the genes for the H
chain and two types of L chains in the lymphocyte
DNA leads to a new combination of genes
in each cell.