Genetic Recombination Help
There are three basic mechanisms by which DNA can be transferred from one bacterial cell to another: transformation, conjugation or transduction. These processes are known generally as horizontal gene transfer. If the donor DNA is incorporated, or recombined, into the genome of the recipient cell, a recombinant organism with one or more new phenotypes may result.
Bacterial transformation is the transfer of naked DNA from one bacterial cell to another. When a bacterial cell ruptures (in a process called lysis), its circular DNA is released into the environment and can be taken in by competent cells. Competence is the ability of a cell to take up naked DNA from its environment. Not all bacteria are capable of competence and those that are only become competent during certain parts of their life cycle. To become competent, the cell produces competence proteins that modify the cell wall to allow binding of naked DNA and aid in the absorption and/or incorporation of the foreign DNA into the cell's chromosome. DNA that has been transferred from a donor cell to a recipient cell is called donor DNA, or an exogenote; the recipient DNA is the native DNA of the recipient cell, also called the endogenote. A bacterial cell that has received donor DNA is initially diploid for part of its genome, and is said to be a merozygote. Once taken in by a competent cell, the donor DNA becomes coated with a protein (such as the RecA-protein of E. coli) that aids in recombination of the donor DNA with the native chromosome. Once the donor DNA is integrated into the recipient DNA, the cell is no longer a merozygote.
If the donor DNA contains a different allele than the recipient chromosome, the resulting recombinant DNA will contain one or more mismatched base pairs, and is referred to as a heteroduplex. If progeny cells are to inherit the new allele, a process called mismatch repair must occur by excising a segment of the recipient DNA strand and using the donor DNA strand as a template for its replacement. Also, more than one gene may be contributed by the donor DNA, and if this occurs, the recipient cell is cotransformed. The frequency of cotransformation is a function of the linkage distance between the two genes.
EXAMPLE 10.5 In 1928 Fred Griffith discovered the first example of bacterial transformation. Streptococcus pneumoniae is a bacterium that causes human pneumonia and can also kill mice. The virulent strain of this bacterium contains a polysaccharide capsule that tends to resist destruction by immune cells of the host species. A nonvirulent strain of the pneumococcus does not have a capsule. The virulent strain forms colonies with smooth borders on nutrient agar plates and is thus designated the S (smooth) strain; the nonvirulent strain forms colonies with rough borders and is designated the R (rough) strain. When mice were injected with both heat-killed S strain and live R strain, they died, and live S-strain bacteria were recovered from their bodies. Griffith did not know how to explain these results, but he called the process ''transformation'' and named the responsible substance ''transforming principle.'' Later studies by Avery and others demonstrated that the transforming principle was naked DNA. In the Griffith experiment, the exogenote from the S strain contained the gene responsible for capsule formation. When this exogenote was incorporated into the endogenote of the R strain the transformant cells had the ability to make the capsule and thus became virulent S-type cells.
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