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The RAG [(Recombination-Activating Gene)] genes [which assist in the facilitation of V(D)J recombination] lack the introns that characterize eukaryotic genes. In this unusual feature they resemble the transposase gene of a transposon, a type of genetic element that can make and move copies of itself to different chromosomal locations. The essential components of a transposon are a transposase—an enzyme that cuts double-stranded DNA—and regions of repetitive DNA, called the terminal repeat sequences, that are recognized by the transposase. These two features allow the transposon to be excised from one location and inserted into another. The similarity of the RAG recombinase to a transposase has led to the hypothesis that the mechanism now used to rearrange immunoglobulin and T-cell receptor gene segments originated in a vertebrate ancestor with the insertion of a transposon into a gene encoding a receptor of innate immunity. The inserted transposase genes evolved to encode RAG proteins, and the terminal repeat sequences evolved to become the recombination signal sequences for the first rearranging gene segments. During this evolution, the transposase gene and the long terminal repeats of the transposon were separated and became components of different genes, both expressed specifically in lymphocytes. Today, the human RAG genes are on chromosome 11[,] and on four other chromosomes are the much-expanded sets of rearranging antigen-receptor genes.
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