Info

Biological Roles of SiteSpeafic

Transposition (p. 310) *

Examples of Tta nsposable Elements end Their Regulation (p 327)

V(D)J Recombination (p 338)

transposabfe element transposabfe element

transposition

FIGURE 11-1 Two classes of genetic recombination. The top panel shows an example of site-specific recombination Here recombination between the red and blue recombination sites inverts the DMA segment carrying the a and 8 genes. The bottom panel shows an example of transposition in which the red transposabfe element excises from tfie gray dna and inserts into an unrelated Stie in the blue DNA

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recombination sites site-specific recombination

FIGURE 11-1 Two classes of genetic recombination. The top panel shows an example of site-specific recombination Here recombination between the red and blue recombination sites inverts the DMA segment carrying the a and 8 genes. The bottom panel shows an example of transposition in which the red transposabfe element excises from tfie gray dna and inserts into an unrelated Stie in the blue DNA

site-specific recombination

The impact of these DNA rearrangements on chromosome structure and function is profound. In many organisms, transposition is the major source of spontaneous mutation and nearly half the human genome consists of sequences derived from transposable elements. Furthermore, as we will see, both viral infection and development of the vertebrate immune system depend critically on these specialized DNA rearrangements.

Conservative site-specific recombination and transposition share key mechanistic features. Proteins known as recombinasus recognize specific sequences where recombination will occur within a DNA molecule. The recombinases bring these specific sites together to form a protein-DNA complex bridging the DNA sites, known as the synaptic complex. Within the synaptic complex, the recombinase catalyzes the cleavage and rejoining of the DNA molecules either to invert a DNA segment or to move a segment to a new site. One recombinase protein is usually responsible for all these steps. Both types of recombination are also care-fidly controlled such that the danger to the ceil of introducing breaks in die DNA, and rearranging DNA segments, is minimized. As we shall see, however, the two types of recombination also have key mechanistic differences.

in the following sections the simpler site-specific recombination reactions are introduced first, followed by the discussion of transposition. Each of these sections is organized to describe general features of the mechanism first and then to provide some specific examples.

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