MICROBIOLOGY

Microbial   Recombination  and Plasmids: Mapping the Genome: Finding the location of genes in any organism’s genome is a very complex task. This section surveys approaches to mapping the bacterial genome, using E.coli as an example. All three modes of   gene  transfer  and  recombination  have  been  used  in  mapping.  Hfr conjugation is frequently used to map the relative location of bacterial genes. This technique rests on the observation that  during  conjugation  the  linear  chromosome  moves  from donor to recipient at a constant rate. In an interrupted mating experiment the conjugation bridge is broken and Hfr F mating is stopped at various intervals after the start of conjugation by mixing the culture vigorously in a blender. The order and timing of gene transfer can be determined because they are a direct reflection of the order of genes on the bacterial chromosome. For example, extrapolation of the curves in figure back to the x-axis will give the t

Microbial   Recombination  and Plasmids:  DNA Transformation: The second way in which DNA can move between bacteria is through  transformation, discovered  by  Fred  Griffith  in  1928. Transformation is the uptake by a cell of a naked DNA molecule or fragment from the medium and the incorporation of this molecule into the recipient chromosome in a heritable form. In natural transformation the DNA comes from a donor bacterium. The process is random, and any portion of a genome may be transferred between bacteria.   When bacteria lyse, they release considerable amounts of DNA into the surrounding environment. These fragments may be relatively large and contain several genes. If a fragment contacts a competent cell, one able to take up DNA and be transformed, it can be bound to the cell and taken inside. The transformation frequency of very competent cells is around 10^-3 for most genera when an excess of DNA is used. That is, about one cell in every thousand will take up an