Agctaagctatc

^Replication of this single-stranded DNA creates the flanking direct repeats.

Flanking direct repeats

^Replication of this single-stranded DNA creates the flanking direct repeats.

111.16 Flanking direct repeats are generated when a transposable element inserts into DNA.

The presence of flanking direct repeats indicates that staggered cuts are made in the target DNA when a transposable element inserts itself, as shown in FIGURE 11.16. The staggered cuts leave short, single-stranded pieces of DNA on either side of the transposable element. Replication of the single-stranded DNA then creates the flanking direct repeats.

At the ends of many, but not all, transposable elements are terminal inverted repeats, which are sequences from 9 to 40 bp in length that are inverted complements of one another. For example, the following sequences are inverted repeats:

5' - ACAGTTCAG ... CTGAACTGT - 3' 3' - TGTCAAGTC ... GACTTGACA - 5'

On the same strand, the two sequences are not simple inversions, as their name might imply; rather, they are both inverted and complementary. (Notice that the sequence from left to right in the top strand is the same as the sequence from right to left in the bottom strand.) Terminal inverted repeats are recognized by enzymes that carry out transposition and are required for transposition to take place. I FIGURE 11.17 summarizes the general characteristics of transposable elements.

Concepts J"

Transposable elements are mobile DNA sequences that often cause mutations. There are many different types of transposable elements; most generate short, flanking direct repeats at the target site as they insert. Many transposable elements also possess short terminal inverted repeats.

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Transposition

Transposition is the movement of a transposable element from one location to another. Although our understanding of transposition is still incomplete, it's clear that, rather than a single mechanism, several different mechanisms are required for transposition in both prokaryotic and eukaryotic cells. Nevertheless, all types of transposition have several features in common: (1) staggered breaks are made in the target DNA (see Figure 11.16); (2) the

Transposable element

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