Recombination between the DNA segments
coding for an immunoglobulin molecule is mediated
by a system of enzymes called recombinases.
Their activities are varied and comprise
lymphocyte-specific and more general activities
including exonuclease and ligase functions.
The enzymes are controlled by specific DNA
recognition sequences. These are located in adjacent
noncoding DNA segments at the 3! end of
each V exon (exons for the variable region) and
at the 5! end of each J segment. The D segments
are flanked on both sides by recognition
sequences. Recognition sequences are noncoding
but highly conserved DNA segments of
seven base pairs (heptamer) or nine base pairs
(nonamer). They are separated by precisely defined
intervals, produced by spacers of 23 or 12
base pairs (bp). Upstream (5! direction) and
downstream (3! direction) from a D segment,
the spacers are 12 base pairs long.
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