Chapter 6 Genetic Analysis and Mapping in Bacteria and ...

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Chapter 6 Genetic Analysis and Mapping in

Bacteria and Bacteriophages



Section 6.1

• Bacteria Mutate Spontaneously and Grow at an Exponential Rate.

• Spontaneous mutation that occurs in the

presence or absence of phage is considered the primary source of genetic variation in bacteria.


Section 6.1

• Selection is the growth of the organism under conditions in which only the mutant of interest grows well, whereas the wild type does not.


Section 6.1

• Prototroph • can synthesize all essential organic

compounds, and therefore can be grown on minimal medium.

• Auxotroph • through mutation, has lost the ability to

synthesize one or more essential compounds, and must be provided with them in the medium if it is to grow.


Section 6.1

• Bacteria have 4 phases when grown in culture • lag phase • log phase (exponential growth) • stationary phase • death phase


Section 6.2

• Conjugation Is One Mean of Genetic Recombination in Bacteria.

• Bacteria undergo conjugation, in which • genetic information from one bacterium is

transferred to another • it recombines with the second bacterium’s

DNA


• In bacterial conjugation in E. coli, F+ cells serve as DNA donors and F– cells are the recipients.

• F+ cells contain a fertility factor (F factor) that confers the ability to donate DNA during conjugation.

• Recipient cells are converted to F+.


• An Hfr (high-frequency recombination) strain has the F factor integrated.

• An Hfr strain can donate genetic information to an F– cell, but the recipient does not become F+.

Recombination between two circles can make a bigger circle

F factor can be integrated into E. coli genome to make Hfr strains Plasmids that can integrate are called episomes. Location of insertion of F factor determines the frequency of recombination


• Interrupted matings demonstrated that specific genes in an Hfr strain are transferred and recombined sooner than others.

Mechanism of gene transfer

Genetic distances in E. coli measured can be minutes or cM

*donor DNA is Linear, recipient DNA is Circular


• In some cases, an F factor is excised from the chromosome of an Hfr strain.

• In the process, the F factor (referred to as F’) often brings several adjoining genes with it.

• Transfer of an F’ to an F– cell results in a partially diploid cell called a merozygote.


Section 6.4

• The F Factor Is an Example of a Plasmid.

• Plasmids contain one or more genes and replicate independently of the bacterial chromosome.


• F factors confer fertility. • R plasmids confer antibiotic resistance. • Col plasmids encode colicins that can kill

neighboring bacteria.


Section 6.5

• Transformation Is Another Process Leading to Genetic Recombination in Bacteria.

• In transformation, small pieces of extracellular DNA are taken up by a living bacterial cell and integrated stably into the chromosome.


Section 6.6

• Bacteriophages Are Bacterial Viruses.

• Bacteriophages can infect a host bacterium by injecting their DNA.

• Transduction Type of bacterial genetic recombination caused by the infection of a bacteriophage


Lytic life cycle: T4 phage infecting E. coli


• Lysogeny occurs when • the phage DNA integrates into the bacterial

chromosome to become a prophage • it is replicated along with the chromosome • it is passed to daughter cells

• Bacteria containing a prophage are

lysogenic and can grow and divide stably until viral reproduction is induced.


Section 6.7

• Transduction Is Virus-Mediated Bacterial DNA Transfer.

• Bacteriophages, which can themselves undergo genetic recombination, can be involved in a mode of bacterial genetic recombination called transduction.

• The Lederberg-Zinder experiment led to the discovery of phage transduction in bacteria.


• In generalized transduction, bacterial DNA instead of phage DNA is packaged in a phage particle and transferred to a recipient host .

• In specialized transduction, a small piece of bacterial DNA is packaged along with the phage DNA.


Section 6.8

• Bacteriophages Undergo Intergenic Recombination.

• Phage mutations often affect plaque morphology.

• Such mutations have been important in understanding genetic phenomena in phages.


Section 6.8

• Mapping in Bacteriophages

• Mixed infection experiments demonstrated that intergenic recombination occurs in bacteriophages.


• Intragenic Recombination Occurs in Phage T4.

• Seymour Benzer’s (1950s) detailed examination of the rII locus of phage T4 allowed him to produce a genetic map of this locus.

Section 6.9

Complementation testing

Intragenic recombination in T4 phage

Two mutant viral strains, neither can lyse K12

Calculation of intragenic recombination frequency

Deletion testing was used to provide a rough and quick localization of each mutation Benzer studied

Chapter 6 Genetic Analysis and Mapping in Bacteria and ...

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