Complex Transposons Move by Replicative Transposition

Transposition does not always leave behind damaged DNA with double stranded breaks. Some transposons are capable of replicative transposition, during which the transposon creates a second copy of itself (Fig. 15.06). Consequently, both the original home site and the newly selected target location end up with a copy of the transpo-son. The original home DNA molecule is not abandoned or damaged. Transposons using this mechanism are known as complex transposons because the process is more complex than the simple cut-and-paste mechanism described above.

Complex transposons have a transposase that recognizes their inverted repeats and the host target sequence just like other kinds of transposons. In addition, replicative transposons needs an extra enzyme, resolvase, and an extra DNA sequence, the internal resolution site (IRS) which is recognized by the resolvase (Fig. 15.07). Although complex transposons are replicated while moving, they are not replicons, as they have no origin of replication. The transposon does not make a new copy complex transposon A transposon that moves by replicative transposition internal resolution site (IRS) Site within a complex transposon where resolvase cuts the DNA to release two separate molecules of DNA from the cointegrate during replicative transposition replicative transposition Type of transposition in which two copies of the transposon are generated, one in the original site and another at a new location resolvase An enzyme that cuts apart a cointegrate releasing two separate molecule of DNA

FIGURE 15.05 Movement by Conservative Transposition

A) Transposase, produced by the transposon, recognizes and cuts the inverted repeats, freeing the transposon from the chromosome. The chromosome is left with a double-stranded break that needs to be repaired. B) The transposon bound by transposase identifies a target sequence, and transposase directs a staggered cut into the target DNA. The ssDNA ends of the target sequence are joined to the inverted repeats of the transposon. The resulting single-stranded regions are filled in by the host cell, thus duplicating the target sequence.

A) TRANSPOSON IS CUT OUT BY TRANSPOSASE

Chromosome

Inverted repeat iTransposon

Inverted repeat

Chromosome

Inverted repeat

Transposon

Transposon

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