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Modern Genetic Analysis Chapter 13. Transposable Genetic Elements
Review of Transposable Elements in Prokaryotes

Let's examine what we have learned up to this point about prokaryotic transposable elements:

1. There are several different types of transposable elements, including insertion sequences (IS1, IS2,...) and transposons (Tn1, Tn2,,...).
2. Two copies of a transposable element can act in concert to transpose the DNA segments in between them. Some of the antibiotic-resistance-conferring transposons are formed in this manner, with two insertion sequences flanking the genes for antibiotic resistance.
3. Most of the transposable elements have recognizable inverted repeat (IR) structures, some of which can be observed under the electron microscope after denaturation and renaturation.
4. Transposable elements are found in bacterial chromosomes, as well as in plasmids.
5. After insertion into a new site on the DNA, transposable elements generate a short repeated sequence, commonly consisting of 9 or 5 bp.
6. The detailed mechanism of transposition is not known, but two different pathways for transposition have been identified. In some cases, transposition takes place by replication of a new copy of the element into the target site, with one copy being left behind at the original site. In other cases, transposition consists of the excision of the element from the original site and its reintegration into a new site. These two modes of transposition are called replicative and conservative, respectively.

Transposable elements have been found in eukaryotes, and some have close similarities to those observed in bacteria, transposing through DNA intermediates. Interestingly, other mobile elements transpose through RNA intermediates and, in certain cases, resemble mammalian retroviruses.

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