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A
gene en coding one of the protein chains (light or heavy) of an immunoglobulin.
The germline genes are activated by a special recombination process
that joins a number of different gene segments together. In the case
of immunoglobulin light chains there are three segments (V variable,
J joining and C constant). In the case of immunoglobulin heavy chains
there are four segments (V variable, D diversity, J joining and C
constant). There are many different V segments (light chains ~300,
heavy chains ~500), 4 J segments in both light and heavy chain genes,
12 D segments in the heavy chain gene and 1 C segment in light chain
genes, 5 C segments in heavy chain gene. The recombination process
appears to be able to join any V with any J (light chain) or any V
to D to J (heavy chain). The total number of possible immunoglobulins
(ie the number of possible light chains times the number of possible
heavy chains) that can be generated by this mechanism is thus several
10s of millions (300x4x500x12x4=28800000). It is certain that the
number is greater than this because the recombination events themselves
are not precise and random loss of nucleotides from each recombining
end occurs. |
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