In Single Stranded Regions

FIGURE 15.08 Replicative Transposition Forms a Cointegrate

Single-stranded cuts are made flanking the transposon in the donor molecule and a staggered cut is made in the target site on the recipient molecule. The ends are joined as shown, which causes the transposon to "split", resulting in two single-stranded copies of the transposon. The single-stranded DNA alerts the host to repair the defect thus making both transposons double-stranded. The recipient DNA is now joined to the donor DNA via duplicated transposons and forms what is called a cointegrate. Resolvase, produced by the transposon, then resolves the cointegrate at the two IRS sequences and releases the donor and recipient molecules. Notice that a copy of the transposon is now located on each molecule of DNA.



RESOLUTION by crossover AT IRS

transposon. Note that resolution further scrambles the two copies of the transposon as shown in Figure 15.08.

Most of the known complex transposons carry other genes in addition to those involved in transposition and resolution. For example, Tn1 and Tn3 are complex transposons that carry resistance to antibiotics of the penicillin family and are found in both the plasmids and chromosomes of many bacteria. Movement of complex transposons is traced by the expressed phenotype of these genes.

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