Introduction to Genetic Analysis 163

Introduction to Genetic Analysis 163 - 162 Chapter 5 The...

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Unformatted text preview: 162 Chapter 5 The Genetics of Bacteria and Their Viruses First we need to understand some special features of the recombination event in bacteria. Note that recombi- nation does not take place between two whole genomes, as it does in eukaryotes. In contrast, it takes place between one complete genome, from the F , called the endogenote, and an incomplete one, derived from the Hfr donor and called the exogenote. The cell at this stage has two copies of one segment of DNAone copy is the exogenote and one copy is part of the endogenote. Thus at this stage the cell is a partial diploid, called a merozygote. Bacterial genetics is merozygote genetics. A single crossover in a merozygote would break the ring and thus not produce viable recombinants, as shown in Figure 5-15. To keep the circle intact, there must be an even number of crossovers. An even number of cross- overs produces a circular, intact chromosome and a frag- ment. Although such recombination events are repre- sented in a shorthand way as double crossovers, the actual molecular mechanism is somewhat different, more like an invasion of the endogenote by an internal section of the exogenote. The other product of the dou-section of the exogenote....
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