Any sequence in the genome might become an IS element, therefore these elements are randomly selected throughout the chromosome. A copy of the transposon is made and inserted at any position in the head of a gene, except at the start position.
Typically, an IS transposition rate (pis) of 0.1 and a set of three IS elements of different length are used. The transposition operator randomly chooses the chromosome, the start of the IS element, the target site, and the length of the transposon. Consider the 2-genic chromosome below:
012345678901234567890012345678901234567890
*-+*a-+a*bbabbaabababQ**+abQbb*aabbaaaabba
Suppose that the sequence “bba” in gene 2 (positions 12 through 14) was chosen to be an IS element, and the target site was bond 6 in gene 1 (between positions 5 and 6). Then, a cut is made in bond 6 and the block “bba” is copied into the site of insertion, obtaining:
012345678901234567890012345678901234567890
*-+*a-bba+babbaabababQ**+abQbb*aabbaaaabba
During transposition, the sequence upstream from the insertion site stays unchanged, whereas the sequence downstream from the copied IS element loses, at the end of the head, as many symbols as the length of the IS element (in this case the sequence “a*b” was deleted). Note that, despite this insertion, the structural organization of chromosomes is maintained, and therefore all newly created individuals are syntactically correct programs. Note also that transposition can drastically reshape the ET, and the more upstream the insertion site the more profound the change. Thus, this kind of operator (IS transposition and RIS transposition
in the next section) may be seen as having a high hit rate at the lowest levels of ETs
[7].
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