In addition to mutation, the genetic information in a cell can be altered if the cell gains genes from other cells. The movement of DNA from one cell to another accounts for the rapid spread of resistance to antimicrobial medications and heavy metals in bacterial populations, as described for Shigella earlier in this chapter. The transfer of genes from one bacterium to another is called horizontal gene transfer or lateral gene transfer. DNA passed from the parental cell to daughter cells following DNA replication is termed vertical gene transfer.
Gene transfer between bacteria can best be studied using mutants. They make it possible to determine whether genetic recombination, the combining of DNA or genes from two different cells, has occurred. Recombination in the DNA from two different bacteria mixed together can be readily recognized because the resulting cells, termed recombinants, have certain properties of each of the original bacteria. For example, when cells that are streptomycin resistant (StrR) and require histidine (His:) and tryptophan (Trp:) to grow are mixed with cells that are killed by streptomycin (StrS) and require leucine (Leu:) and threonine (Thr:), rare recombinants appear. These are resistant to streptomycin and grow on a glucose-salts medium—that is, the recombinants are His+, Trp+, Leu+, Thr+ and StrR (figure 8.13). These recombinant cells contain a mixture of genes from the two original types of cells. Since the bacteria are usually hap-loid, such recombinants can arise only if DNA has been transferred from one cell, the donor, to another cell, the recipient. Reversion of the auxotrophs cannot account for the result because at least two genes would have had to revert in the same cell, an extremely rare event.
Genes are naturally transferred between bacteria by three different mechanisms:
1. DNA-mediated transformation, in which DNA is transferred as "naked" DNA
2. Transduction, in which bacterial DNA is transferred by a bacterial virus
3. Conjugation, in which DNA is transferred from one bacterium to another when the cells are in contact with one another
Although these processes differ from one another in how the DNA is delivered, they share certain features:
■ Only a part of the chromosome is usually transferred. This contrasts with the situation in eukaryotic cells,
Glucose-salts medium + streptomycin
Figure 8.13 General Experimental Approach for Detecting Gene Transfer in Bacteria
Glucose-salts medium + streptomycin
Figure 8.13 General Experimental Approach for Detecting Gene Transfer in Bacteria in which cells, such as the egg and the sperm, fuse so that two sets of all chromosomes are present.
■ Although DNA may be transferred as either a single- or double-stranded molecule, single-stranded DNA replaces the homologous, or identical genes in the recipient cell in the integration process. This is the process of homologous recombination.
To detect gene transfer, one can select directly for recombinant cells, by inoculating the mixture of cells on a medium on which only the recombinants will grow and form colonies. Non-recombinant bacteria are unable to grow, so that only recombinants form colonies. Since several billion bacteria can be inoculated onto the agar contained in a single Petri dish, a few recombinants in the billion cells can be detected readily.
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