Figure 4.6. V-region gene segments are joined by recombination.

Figure 4.6V-region gene segments are joined by recombination

In every V-region recombination event, the signals flanking the gene segments are brought together to allow recombination to take place. For simplicity, the recombination of a light-chain gene is illustrated; for the heavy-chain gene, two separate recombination events are required to generate a functional V region. In some cases, as shown in the left panels, the V and J gene segments have the same transcriptional orientation. Juxtaposition of the recombination signal sequences results in the looping out of the intervening DNA. Heptamers are shown in orange, nonamers in purple, and the arrows represent the directions of the heptamer and nonamer recombination signals (see Fig. 4.5). Recombination occurs at the ends of the heptamer sequences, creating a signal joint and releasing the intervening DNA in the form of a closed circle. Subsequently, the joining of the V and J gene segments creates the coding joint. In other cases, illustrated in the right panels, the V and J gene segments are initially oriented in opposite transcriptional directions. Bringing together the signal sequences in this case requires a more complex looping of the DNA. Joining the ends of the two heptamer sequences now results in the inversion and integration of the intervening DNA. Again, the joining of the V and J segments creates a functional V-region exon.

From: The generation of diversity in immunoglobulins

Cover of Immunobiology
Immunobiology: The Immune System in Health and Disease. 5th edition.
Janeway CA Jr, Travers P, Walport M, et al.
New York: Garland Science; 2001.
Copyright © 2001, Garland Science.

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