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Transposable elements, which are found in virtually all cells, are segments of DNA that can move from one location in the genome to a target sequence in another.

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A simple type of transposable element called an insertion sequence or IS element is only 700 to 1500 base pairs long and consists of two basic parts,

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a coding region for an enzyme called transposase, and two short inverted repeat flanking sequences.

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The cellular machinery transcribes the transposase coding region

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and translates it into the transposase enzyme.

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Transposase catalyzes the transposition process.

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The transposase randomly selects one of many possible target sequences

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where it will move the insertion element.

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The IS element, depending on the particular type,

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may enter the target sequence by one of two general mechanisms.

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replicative transposition or non-replicative transposition.

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In replicative transposition, transposase nicks the transposable element

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and the nick dens attack the target sequence, making a staggered cut in the target DNA.

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DNA polymerase fills in the large gaps and DNA ligase seals the remaining nicks.

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A recombination event resolves the two molecules.

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As a result of replicative transposition, the cell has a copy of the transposable element, flanked by duplicated target sequences.

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Non-replicative transposition is sometimes called cut-and-paste replication.

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In this process, transposase brings inverted repeats and flanking DNA together.

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One phosphodiester bond is cleaved on each strand at opposite ends of the IS element, yielding free 3' OH groups.

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Each 3' OH end attacks the other strand of the double helix, creating a new bond that results in hairpin structures.

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Host carrier DNA is ejected and the hairpins on the IS element are re-nicked and attack target DNA.

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From the 3' hydroxyl groups, the cell's DNA polymerase molecules fill in the gaps and DNA ligase seals the remaining nicks.

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In this way, the element jumps from one DNA molecule to another.

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To summarize, in non-replicative transposition, the transposable element is cut and pasted from one DNA molecule to another.

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In replicative transposition, the transposable element is copied to yield two transposable elements,

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one in the original position and one in the target DNA.

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Thank you.