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  Section: Genetics » Plasmids, IS Elements, Transposons and Retroelements
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Replication, transfer and recombination in plasmids

Plasmids, IS Elements, Transposons and Retroelements
Classification of plasmids
Replication, transfer and recombination in plasmids
Insertion sequences or IS elements
Transposons and controlling elements
Transposons in prokaryotes
Transposons in eukaryotes
Retrolelements (viral and non-viral)
Mechanism of Transposition
Uses of Transposons
All plasmids identified so far are small circular DNA molecules. The DNA circles may be single or multiple structures linked together in chains. The size of these DNA molecules vary from 30 μm (e.g. F')in a plasmid of E. coli, making 20% of the main chromosome (same size in R plasmids also) to less than 1 μm as in P16C of E. coli. Colicin plasmids Col E1, E2 and E3 are usually small having a length of 2.3 μm. The number of plasmids in a cell varies from a few in case of F and R plasmids to as many as 20-30 in case of Staphylococcus plasmids.

The DNA plasmids replicate in semi-conservative manner. It has been shown that while initiation of replication' is controlled by plasmid genes, other stages of replication i.e. polymerization (elongation) and termination are controlled by bacterial genes. When a plasmid is inserted in the bacterial chromosome, bacterial control on plasmid replication is absolute, but when free, the control is less that absolute.

Plasmids also have the ability of transferring themselves from one bacterium to another. This transfer is facilitated by the formation of a hair like growth known as sex pilus, formed of the outer membrane of the donor bacterium (as in bacterial conjugation; see Sexuality and Recombination in Bacteria and Viruses). The F sex pilus is upto 20 μm (1 μm = 10-3 mm) in length and 8 nm (1 nm = 10-6 mm) wide. The sex pili produced by different plasmids are distinguished by the reaction they show for attachment of certain phages. The sex pili produced by Col I and known as I-like sex pili differ from F-like sex pili on the basis of reaction shown with certain phages. When sex pilus is removed by injury or is absent due to genetic reasons, no transfer of plasmid DNA is possible, suggesting that sex pilus is essential for transfer.

In E. coli, transfer is controlled by plasmid genes tra-A, -B, -C, etc. and mutations in these genes lead to failure of transfer. However, when two plasmids having mutations for different tra-genes are introduced in the same cell, pilus may be formed and transfer may occur due to complementation between mutants. When transfer occurs, one of the two strands of plasmid DNA is nicked (broken) at a site called origin of transfer (Ori-T)arid the linear strand thus formed moves to the recipient bacteria. The single strands in the donor as well as in recipient cell will then synthesize complementary strands, so that duplex DNA plasmids are reconstituted in both the cells (Fig. 15.1). Sometimes due to insertion of plasmid, the transfer process may involve transfer of bacterial genes also, which are carried with plasmid DNA. Plasmid DNA may undergo recombination either with another plasmid DNA or with bacterial DNA. In the bacterial chromosome there are insertion sequences (IS), which are homologous in plasmids and bacteria. These sequences help in recombination. There are atleast 17 such sites in E. coli chromosome. Mapping of genes in plasmid DNA has also been achieved. Methods employed in mapping were not mainly recombination, but complementation (as described above) between strains having overlapping deletions. For more detailed description, the reader may refer to various reviews (Clowes, R.C., 1972, Bacterial Rev., 36 : 361-405; Clowes R.C., 1973, Sci. Amer. April 1974, 228 : 18-27). A condensed account of plasmids is also available in the book Extranuclear Genetics, written by G. Beale and J. Knowles.
Five different stages (1 lo 5) of infection transfer of a bacteria] plasmid, such as F factor or one of the R plasmids (for details see text; figure modified from the original in Sci. Amer., 1973).
Fig. 15.1. Five different stages (1 lo 5) of infection transfer of a bacteria] plasmid, such as F factor or one of the R plasmids (for details see text; figure modified from the original in Sci. Amer., 1973).

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