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Chapter 4 - Chapter 4 Immunoglobulin genes are rearranged...

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Chapter 4 Immunoglobulin genes are rearranged in B cells - Locations of immunoglobulin DNA sequences are ID’d by hybridization with V- and C- region probes - V and C regions found in distinct DNA fragments in nonlymphoid DNA. o In B cell DNA, C and V fragments the same, but not in germline V region genes constructed from gene segments - Light chain V-region genes constructed from two segments o Variable (V) and a joining (J) gene segment in the genomic DNA are joined to form a complete light-chain V-region exon. Ig chains are extracellular proteins and the V gene segment is preceded by an exon encoding a leader peptide (L). o L peptide which directs protein into cell’s secretory pathways and is then cleaved. - Light chain C region encoded in separate exon and joined to V region by splicing RNA to remove L-to-V and J-to-C introns. - Heavy chain V regions are constructed from 3 segments. Diversity (D) and J segments join, then V gene segment joins to combined DJ forming Vh exon. - Heavy chain C region encoded by several exons, C region gene w/ leader sequence spliced to the V-domani sequence during processing of RNA transcript. o Leader sequence removed after translation and disulfide bonds form. Numbers of functional gene segments for V regions of human heavy/light chains - Derived from cloning and sequencing of DNA, excluding pseudogenes. o Numbers not the same for all people due to genetic polymorphisms - Variable (v) – Light chains: K = 40, lambda = 30, Heavy chains: H = 40 - Diversity (D) – Light chains: K = 0, lambda = 0, Heavy chains: H = 25 - Joining (J) – Light chains: K = 5, lambda = 4, Heavy chains: H = 6 Germline organization of Ig heavy and light chain loci in human genome - Genetic locus for lambda light chain (chromosome 22) has 30 functional Vlambda segments and four pairs of functional Jlambda segments and Clambda genes. - K locus (chromosome 2) is similar, with 40 function Vk segments and a cluster of 5 Jk segments. o In 50% of individuals, cluster of Vk has undergone an increase by duplication - Heavy chain locus (chromosome 14) has about 40 functional Vh segments and a cluster of 25 Dh segments between Vh and 6 Jh segments.
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