The germline organization of Ig and TCR loci described in the preceding section exists in all cell types in the body. The germline genes cannot be transcribed into mRNAs that encode functional antigen receptor proteins. Functional antigen receptor genes are created only in developing B and T lymphocytes after DNA rearrangement events that bring randomly chosen V, (D), and J gene segments into contiguity. Figure 8-8 schematically depicts a Southern blot, including an Ig light chain locus in germline configuration in a nonlymphoid cell. The V segment shown in this configuration is a considerable distance away from the J segments depicted. A hypothetical rearrangement event in a B cell clone is also shown. In this clone, a specific upstream V segment has been joined to one of the J segments during the rearrangement process.
The process of V(D)J recombination at any Ig or TCR locus involves selection of one V gene, one J segment, and one D segment (when present) in each lymphocyte and rearrangement of these gene segments together to form a single V(D)J exon that will code for the variable region of an antigen receptor protein (Fig. 8-9). In the Ig light chain and TCR a and y loci, which lack D segments,
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