nucleotide base pairs

The
sequences of the nucleotide base pairs of the
spacers are not conserved. The characteristic
rearrangement between neighboring signal
sequences for Ig molecules and TCR receptors
requires spacers of different lengths, i.e., 12 and
23 base pairs (so-called 12/23 rule). When an H
chain is formed, nonhomologous pairing of the
heptamer of a D segment and of a J segment occurs.
These D and J segments are then joined
(D–J joining) by means of recombination: the
spacer of 12 or 23 base pairs and all of the intervening
DNA forms a loop. This is excised, and
the D and J segments are joined. By pairing and
recombination of the recognition sequences at
the 5! end of a DJ segment and the recognition
sequence at the 3! end of a V gene, a V segment
is joined to the DJ segment. The recombination
of genes coding for T-cell receptors (see p. 308)
proceeds in a similar manner. Two genes, recombination-
activating genes 1 and 2 (RAG-1
and RAG-2) have been identified as stimulating
Ig gene recombination in pre-B cells and immature
T cells. Mutations in these genes cause
severe combined immune deficiency