6 BACTERIAL AND PHAGE GENETICS
2 Bacteria have many advantages as a genetic system
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- Bacteriophage (Phage) are viruses
that infect bacteria.
- A physical map of DNA shows distances between
and within genes or specified markers measured in base pairs of DNA. It is based
on the direct measurement of DNA.
- A genetic map shows the positions of genes or
markers on a chromosome, as determined by genetic mapping of mutations.
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- E. coli and its bacteriophages
were the model system used to develop our basic understanding of molecular biology.
- The rapid growth of bacteria and
the ability to examine many independent organisms makes them an excellent system
for genetic studies.
- Gene transfer in bacteria can occur
by transformation, conjugation, and transduction.
- Bacteria and bacteriophages can be
easily genetically manipulated.
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Bacteria were not always viewed as good model organisms for genetic analysis. Bacteria
are prokaryotes — organisms that do not sequester their DNA into a nucleus.
They are haploid organisms, usually with a single circular chromosome. Furthermore,
bacteria reproduce asexually. Thus, for a long time it was thought that they behaved
in an entirely different manner from eukaryotes. However, the tractability of bacteria
made it an attractive place to begin to understand how DNA exerts its effect on
an organism.
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Bacteria offer an enormous advantage in genetic studies because it is easy to analyze
very large numbers of them. Genetic analysis requires the isolation of mutants.
Mutations do not happen frequently, even when we treat cells with chemicals or ionizing
radiation. To find a mutant in a given process among many normal (wild-type) organisms,
it is necessary to examine many individuals. Bacteria are small and grow rapidly
in simple media; one bacterium today will be 109 by tomorrow, all derived
from that single cell. We can infect a culture of bacteria with bacteriophages
(viruses that infect bacteria), and have 100-fold more phages in an hour. Of course,
it is not practical (in most cases) to examine a single bacterium for its mutant
properties; instead, we look at the colony that results when a single bacterium
divides and grows on an agar plate.
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Figure 6.1
Some bacteria take up DNA naturally (transformation); others can be induced to take
up DNA by special treatments. Conjugation is the process where DNA is transferred
directly from cell-to-cell by a special bridge between the cells. Transfer is generally
unidirectional. In transduction, a bacterial virus carries the DNA from one host
to another.
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Genetic analysis requires the ability to carry out a number of tests on the mutant
organism. Because bacteria multiply only in an asexual manner, one of the fundamental
techniques of genetic analysis cannot be performed: the mating of two organisms.
Mating is often used to transfer mutations from one individual to another, to perform
the complementation test, and to provide an environment in which recombination may
take place. Fortunately, bacterial geneticists have discovered three mechanisms
for gene transfer in bacteria. Figure 6.1 shows the three mechanisms: transformation,
conjugation, and transduction.
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Once the hurdle of gene transfer has been overcome, it becomes evident that there
are many advantages to working with bacteria and bacteriophage. This realization
led to the origins of molecular biology in the study of bacteria and the bacteriophage
that infect them. The demonstration that the primary genetic material is DNA first
came from experiments in which DNA from one strain of the bacteria Pneumococcus
was introduced into a second strain; the second strain acquired a
colony morphology characteristic of the first strain (see DNA is the genetic material
of bacteria). Another classic experiment was the demonstration by S. Luria
and M. Delbruck that mutations preexisted in bacterial populations and were not
caused by the treatment that revealed them (4195).
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In the 1950s and 1960s, scientists used the bacteriophages lambda and T4 and strains
of E. coli to demonstrate that the genetic code is based on nonoverlapping
triplets (4196), to examine more fully the nature of mutagenesis, and to
define the basic characteristics of regulatory circuits (4197), all without
the techniques of DNA sequencing and molecular biology that we take for granted
today. The principles they used are still what define a robust genetic system today
(for review see 4228; 4229).
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This chapter emphasizes the types of approaches used in E. coli and the
general principles of genetic analysis they represent. Although techniques for manipulating
the DNA of all organisms, both in the cell and in a test-tube, continue to advance
and allow ever more sophisticated genetic analyses, the basic principles remain
as they were in the initial days of molecular biology. Now, the small genomes and
the extensive genetic information available for E. coli and B. subtilis
mean that the jump from genetics to a specific, sequenced gene is particularly easy.
The correlation of the physical map (based on DNA sequence) with the genetic map
(based on phenotypes of mutations) is most complete in E. coli.
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Last Revised on November 14, 2003
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- 4228 T.D. Brock (1990). The Emergence of Bacterial
Genetics (Cold Spring Harbor, New York: Cold Spring Harbor Laboratory
Press).
- 4229 J. H. Miller (1996). Discovering Molecular
Genetics (Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press).
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- 4195 Luria, S. E., and Delbruck, M. (1943). Mutations
of bacteria from virus sensitivity to virus resistance. Genetics 28,
491-511.
- 4196 Brenner, S., Stretton, A.O.W., and Kaplan,
S. (1965). Genetic code: The "nonsense" triplets for chain termination and
their suppression. Nature 206, 994-998.
- 4197 Jacob, F. , and Monod, J. (1961). Genetic
regulatory mechanisms in the synthesis of proteins. J. Mol. Biol. 3,
318-356.
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©Jones and Bartlett Publishers (2007)
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