A person has 500 V and 50 J light-chain gene segments and 600 v, 60 D and 6 J heavy-chain gene segments in its haploid genome Calculate the theoretical number of light-chain genes and heavy-chain genes that can be generated if no nucleotides are gained or lost during recombination within heavy and light-chain DNA.
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Answer:
Explanation:
generation of diversity in immunoglobulins
Virtually any substance can elicit an antibody response. Furthermore, the response even to a simple antigen bearing a single antigenic determinant is diverse, comprising many different antibody molecules each with a unique affinity, or binding strength, for the antigen and a subtly different specificity. The total number of antibody specificities available to an individual is known as the antibody repertoire, or immunoglobulin repertoire, and in humans is at least 1011, perhaps many more. The number of antibody specificities present at any one time is, however, limited by the total number of B cells in an individual, as well as by each individual's encounters with antigens.
Before it was possible to examine the immunoglobulin genes directly, there were two main hypotheses for the origin of this diversity. The germline theory held that there is a separate gene for each different immunoglobulin chain and that the antibody repertoire is largely inherited. By contrast, somatic diversification theories proposed that the observed repertoire is generated from a limited number of inherited V-region sequences that undergo alteration within B cells during the individual's lifetime. Cloning of the immunoglobulin genes revealed that the antibody repertoire is, in fact, generated by DNA rearrangements during B-cell development. As we will see in this part of the chapter, a DNA sequence encoding a V region is assembled at each locus by selection from a relatively small group of inherited gene segments. Diversity is further enhanced by the process of somatic hypermutation in mature activated B cells. Thus the somatic diversification theory was essentially correct, although the concept of multiple germline genes embodied in the germline theory also proved true.
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4-1. Immunoglobulin genes are rearranged in antibody-producing cells
In nonlymphoid cells, the gene segments encoding the greater part of the V region of an immunoglobulin chain are some considerable distance away from the sequence encoding the C region. In mature B lymphocytes, however, the assembled V-region sequence lies much nearer the C region, as a consequence of gene rearrangement. Rearrangement within the immunoglobulin genes was originally discovered 25 years ago, when it first became possible to study the organization of the immunoglobulin genes in both B cells and nonlymphoid cells using restriction enzyme analysis and Southern blotting. In this procedure, chromosomal DNA is first cut with a restriction enzyme, and the DNA fragments containing particular V- and C-region sequences are identified by hybridization with radiolabeled DNA probes specific for the relevant DNA sequences. In germline DNA, from nonlymphoid cells, the V- and C-region sequences identified by the probes are on separate DNA fragments. However, in DNA from an antibody-producing B cell these V- and C-region sequences are on the same DNA fragment, showing that a rearrangement of the DNA has occurred. A typical experiment using human DNA is shown in Fig. 4.1.
Figure 4.1. Immunoglobulin genes are rearranged in B cells.
Figure 4.1
Immunoglobulin genes are rearranged in B cells. The two photographs on the left (germline DNA) show a Southern blot of a restriction enzyme digest of DNA from nonlymphoid cells from a normal person. The locations of immunoglobulin DNA sequences are identified (more...)
This simple experiment showed that segments of genomic DNA within the immunoglobulin genes are rearranged in cells of the B-lymphocyte lineage, but not in other cells. This process of rearrangement is known as somatic recombination, to distinguish it from the meiotic recombination that takes place during the production of gametes.
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4-2. The DNA sequence encoding a complete V region is generated by the somatic recombination of separate gene segments
The V region, or V domain, of an immunoglobulin heavy or light chain is encoded by more than one gene segment. For the light chain, the V domain is encoded by two separate DNA segments. The first segment encodes the first 95–101 amino acids of the light chain and is termed a V gene segment because it encodes most of the V domain. The second segment encodes the remainder of the V domain (up to 13 amino acids) and is termed a joining or J gene segment.
The rearrangements that lead to the production of a complete immunoglobulin light-chain gene are shown in Fig. 4.2 (center panel). The joining of a V and a J gene segment creates a continuous exon that encodes the whole of the light-chain V region. In the unrearranged DNA, the V gene segments are located relatively far away from the C region. The J gene segments are located close to the C region, however, and joining of a V segment to a J gene segment also brings the V gene close to a C-region sequence. The J gene segment of the rearranged V region is separated from a C-region sequence only by an intron.
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