Chapter 8The Genetics of Bacteria and

Their Viruses

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Multi-Drug-Resistant Bacteria (MDRB)

Chapter Outline

Viruses and Bacteria in GeneticsThe Genetics of Viruses and BacteriaMechanisms of Genetic Exchange in

BacteriaThe Evolutionary Significance of

Genetic Exchange in Bacteria

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Small size (~2 um)Rapid reproduction (~ hrs)Selective media (e.g., antibiotics)Relative Simple structures and

physiologyComplete genome sequences

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Bacteria and Viruses in Genetics

The Genetics of Viruses

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Viruses (DNA-RNA) can only reproduce by infecting living host cells.

Bacteriophages are viruses that infect bacteria.

Several important genetic concepts have been discovered through studies of bacteriophages.

Bacteriophage T4

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Double-stranded linear DNA genomeProtein headGenome contains 168,800 base pairs and 150

characterized genesLytic phage

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T4 DNAse:(Specific)5-hydroxymethylcytosine(HMC)

Lysosyme:(Specific)Cell wall

Bacteriophage Double-stranded linear DNA genome Genome contains, 48,502 base pairs and about 50

genes May be lytic or lysogenic (inserted in the bacterial

chromosome)

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Differential genes expression

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Recombination process:

---site-specificattP (virus) and attB (bacteria)

--- int gene (integrase)

--- GCTTTTTTATACTAA--- CGAAAAAATATGATT

--homologous recombination

The Genetics of Bacteria

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Bacteria contain genes that mutate to produce altered phenotypes.

Gene transfer in bacteria is unidirectional—

from donor cells to recipient cells.

Energy (Lac / lac); Synthesis Trp / trp; Resistant Amp / TetS r

Monopliod to monoploid?

BacteriaOne main chromosome with a few thousand

genes.

Variable number of plasmids and episomes.

Asexual reproduction by simple fission.

(Para) sexual transformation of DNA.

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Phenotypes in BacteriaColony =one bacteria.Colony color and morphology.Nutritional mutants for energy sources.

(any sugars-galactose-Lac+ and Lac-)Prototrophs-produce any metabolites-

and auxotrophs- do note produce specific metabolites.

Antibiotic resistance (Ampr, Tetr, Purr).

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Bacterial colonies:Serratia marcencens P. aeruginosa.

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Recombination in Bacteria

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--Small fragment to larger fragment (chromosome)

--Monoploid (to partial diploid)

-Donor to recipient cell

Mechanisms of Genetic Exchange in Bacteria

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Bacteria exchange genetic material through three different (para)sexual

processes.

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Bacteria exchange genetic material through three different (para)sexual processes.

The U-tube Experiment

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Transformation

Conjugation

Transduction

TRANSFORMATIONStreptococcus pneumoniae---Phenotypes

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Polysaccharide capsule (S, type I,II, III, IV, V)Type II, agglutination

Non- Polysaccharide capsule (R)

Transformation

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Type IIIS to type IIR

Transfer DNA

Hydrophilic

Non-membrane permeable

Transporter (competence-Com-protein)

Competent bacteria

Transformation in Bacillus subtilis

A heteroduplex is a double-stranded (duplex) molecule of DNA originated by recombination of single complementary strands derived from exogenous sources.

Conjugation in E. coli

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DNA is transferred from a donor to acceptor cell,

F: Fertility factor ; F pili

F+

The F Factor in E. coli

F: Fertility factor

F+ factor: --Autonomous and integrated states

Hfr: high frequency recombination

F’ factor: --Similar to F+ but with bacterial genes

Formation of Hfr Cells

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seq A -seq B

seq A -seq B

seq A -seq B

seq A -seq B

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Conjugation

MODELReplication: Rolling-cycle

strr

gal-

lac-

leu-

thr-

azir

tonr

strs

gal+

lac+

leu+

thr+

azis

tons

F-Hfr

Selected medium:-Thr, -Leu,+Streptomycin (str)

Interrupted Mating

Experiments

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Plasmids

A plasmid is a genetic element that can replicate independently of the main chromosome in an extrachromosomal state.

Most plasmids are not required for the survival of the host cell.

Plasmids in E. coli– F Factor (Fertility Factor)– R Plasmids (Resistance Plasmids)– Col Plasmids (synthesize compounds that kill

sensitive cells)

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Episomes

An episome is a genetic element that is not essential to the host and that can either replicate autonomously or be integrated into the bacterial chromosome.

Integration depends on the presence of IS elements.

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Formation of an F ’ Factor

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Homologous DNA recombination, but some bacterial DNA is translocated to the plasmid.

Transduction In transduction, a bacteriophage transfers DNA

from a donor cell to a recipient cell.

In generalized transduction, a random fragment of bacterial DNA is packaged in the phage head in place of the phage DNA (normal Excision).

In specialized transduction, recombination between the phage chromosome and the host chromosome produces a phage chromosome containing a piece of bacterial DNA (abnormal Excision).

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Auxotrophic Salmonella typhimurium:

tyr, phe, trp and met, his.

Infected with P22 bacteriophage

Normal Excision of Prophage

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Anomalous Excision of Prophage

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Specialized Transduction

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Three parasexual processes—transformation, conjugation, and transduction—occur in bacteria.

These processes can be distinguished by two criteria: whether the gene transfer is inhibited by deoxyribonuclease (DNAase) and whether it requires cell contact.

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The Evolutionary Significance of Genetic Exchange in

Bacteria

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Genetic exchange is as important in bacteria as it is in other

organisms.

Genetic Exchange in Bacteria

Mutation is the source of new genetic variation.

(UV radiation)Recombination produces new combinations of allele.

(normal and abnormal recombination)Transformation, conjugation, and transduction

generate new combinations of genes in bacteria to allow bacteria to adapt to new environments.

(soil, sewers and polluted waters) (animal/plants)

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Bacteria genetically engineer plants with the Ti plasmid to control their differentiation (tumorigenic- Crown gall disease).