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Modern Genetic Analysis Chapter 9. The Genetics of Bacteria and Phages
Introduction

A large part of genetics is concerned with bacteria. Genetic analysis of bacteria has been a source of key insights into the nature and structure of the genetic material, the genetic code, mutation, and DNA technology. However, bacteria are also interesting in their own right as a unique category of organisms. Their genetic systems have attracted many of the great geneticists of the twentieth century, whose studies have revealed a wealth of fascinating and unique hereditary mechanisms. Furthermore, as agents of human, animal, and plant disease, as well as by virtue of their ability to synthesize antibiotics, bacteria are directly relevant to humans. Bacterial viruses, called bacteriophages or phages, also have been analyzed extensively at the genetic level, and these analyses, too, have revealed a unique set of hereditary processes. Some of these specialized genetic properties have led to the key role that phages play in modern DNA technology.

Bacteria and phages have relatively small and simple genomes that make them excellent model organisms for genetics. Because bacteria are prokaryotes, they do not undergo meiosis. However, there are several other ways in which bacterial genomes can unite and recombine. Therefore the approach to the genetic analysis of recombination in these organisms is remarkably similar to that for eukaryotesnamely, combine two different genomes in the same cell to afford an opportunity for recombination, and then look for recombinants in the descendant cells. Of the processes that afford opportunities for recombination, we shall first examine conjugation, a natural union of bacterial cells during which one cell transfers DNA segments to another. Second, a bacterial cell can pick up DNA from the environment and incorporate this DNA into its own chromosome; this process, called transformation, permanently changes the genotype. Third, certain phages can occasionally pick up a piece of DNA from one bacterial cell and inject it into another, where it can be incorporated into the chromosome, in a process known as transduction. In all these processes, the gene transfer is partial and unidirectional. In other words, only a part of the genome of one organism is transferred and becomes incorporated into the complete genome of another. The strain that contributes the partial genomic fragment is called the donor, and the one that contributes the complete genome is the recipient. The donor fragment is called the exogenote, and the recipient genome is the endogenote. A cell containing an exogenote is a partial diploid, or merozygote. This unidirectional and partial system of genetic union in bacteria contrasts with the zygotes of eukaryotes, in which (with the exception of organelle genes) both parents contribute equally and both contribute complete genomes. The merozygote provides an opportunity for genetic recombination to occur between the genes that are in the partially diploid segment. Thus a merozygote provides the same type of recombination opportunities found in the zygotes of eukaryotes, although on a more limited scale. The merozygote also provides opportunities for testing various types of gene interaction.

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