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

Gene transfer and recombination can take place between conjugating strains of bacteria. However, genetic material is passed in only one direction; that is, from a donor cell (F+ or Hfr) to a recipient cell (F). Donor ability is determined by the presence in the cell of F, a plasmid called the fertility factor.

On occasion, the F factor present in a free state in F+ cells can integrate the E. coli chromosome and form an Hfr cell. When this occurs, bacterial gene transfer and subsequent recombination take place. The F factor can insert at different places on the host chromosome, resulting in different Hfrs. Interruptions of the conjugation at different times have provided geneticists with a method for constructing a linkage map of the single chromosome of E. coli and other similar bacteria. The map of the E. coli chromosome is a single circle, or ring.

By transformation, genes can be transferred from one bacterial cell to another in the form of purified DNA. For transformation to occur, DNA must be taken into a recipient cell, and recombination between a recipient chromosome and the incorporated DNA must then take place.

Bacteria can be infected by bacteriophages. In one method of infection, the phage chromosome enters the bacterial cell and, using the bacterial metabolic machinery, produces progeny phages that burst the host bacterium. The new phages can then infect other cells. If two phages of different genotypes infect the same host, recombination between their chromosomes can take place during this lytic process.

In lysogeny, another infection method, the injected phage lies dormant in the bacterial cell. In many cases, this dormant phage (the prophage) integrates into the host chromosome and replicates with it. Either spontaneously or under appropriate stimulation, the prophage can arise from its latency and can lyse the bacterial host cell.

Phages can carry bacterial genes from a donor to a recipient. In generalized transduction, random host DNA is incorporated alone into the phage head during lysis. In specialized transduction, faulty outlooping of the prophage from a unique chromosomal locus results in the inclusion of specific host genes as well as phage DNA in the phage head.

© 1999 by W. H. Freeman and Company. All rights reserved.