Lecture 6 - Microbial Genetics

8/14/2019 Lecture 6 - Microbial Genetics

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Microbial genetics

Edet E. Udo PhD

Department of Microbiology

Faculty of Medicine. Kuwait University.Kuwait.


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Microbial genetics

Objectives: Define genetics and heredity

Discuss the mechanisms of genetic exchangein bacteria

Discuss mutation and mutagenic agents

Discuss the concepts and application of

genetic engineering in Medicine


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Definitions 1. Genetics:

is the science of the study of heredity. It includes thethe study of gene replication and transmission.

2. Gene : a segment of DNA that codes for a functional product

or a linear sequence of DNA that forms a functionalunit of a chromosome.

The location of a characteristic is the Locus. Genes with different information at the same

locus are alleles 3. Genotype:

the genetic composition of an organism- its entire DNA

4. Phenotype: the expression of the genes the proteins and the

properties they confer on the organism


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Bacterial DNA

Bacterial DNA:

DNA stores information used

to guide the replications ofDNA in preparation for celldivision

1. Chromosomal DNA,

2. Extra chromosomalDNA----Plasmid DNA


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Bacterial DNA

Bacterial DNA Replication:

by the semi conservative mode,

Replication of DNA usually begins at a specific pointorigin of replication)

During binary fission each daughter cell receives achromosomal DNA like the one in the parent cell.


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Mutations Definition: Mutations are alterations

(changes) in the nucleic acidsequence. accounts for variations in

the genotypes and

phenotypes ofmicroorganisms

Types of mutations : Point mutations:

consists of changes in a

single nucleotide Frameshiftmutations: consists of ofthe insertion or deletion of asingle nucleotide


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Mutations

Effects of mutation:Phenotypic variationse.g lethal mutation leading to arrest of protein synthesis

No effect: Spontaneous mutation: Occurs in the absence of known mutagen May be due to errors in base pairing

Induced mutation:

Produced by agents known as mutagens Antibiotic resistance, Generation of new phenotypes.


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Mutagens Chemical agents: 5-bromouracil, nitrous acid, ethidium

bromide, nitroguanidine, acridines, etc.

Physical agents: X-rays, UV-rays: causes formation of dimers

Biological mutants: Transposons, insertion sequences.


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Mutants

Repair of DNA damage:

Light repair:

involves enzymes that is activated by visiblelight and breaks bonds between pyrimidines of adimer

( role in skin cancer)

Dark repair:

involves several enzymes that do nor requirelight for DNA repair.


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The AMES test: It is used to identify possible chemicalcarcinogens in chemicals applied to human bodysuch as body creams, shampoos etc.

It is based on the ability of bacteria to mutate by

reverting to their original synthetic ability(reverse mutation). E.g. Histidine auxotroph of Salmonella are

exposed to potential carcinogen and revertantsto the non mutant state shows that the chemical

is a potential carcinogen.


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Genetic transfer and

recombination Definition:

Gene transfer refers to the movement of geneticinformation between organisms.

Occurs by transformation, transduction and

conjugation. Transformation:

involves the uptake of naked DNA by bacteria.Uptake of DNA .

It was first demonstrated in 1928 by Griffith while

studying pneumococcal infections in mice. Occurs naturally in Streptococcus pneumoniae,

Heamophilus species and some Bacillus species


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Transformation

Transformationis significantbecause:

It contributes togenetic diversity

It can be used tointroduce DNA intoorganisms, observe

its effect and studygene location

It can be used tocreate recombinantDNA


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recombination Transduction:

mode of transfer involving bacteriophages.

Significance of transduction.

Transfers genetic materials and demonstrates aclose evolutionary relationship between host cellDNA and prophage.

Its persistence in a cell suggests a mechanism

for viral origins of cancer. Provides a mechanism for studying gene linkage


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Phages can be virulent (lytic) or temperate.

1. Virulent phages

destroy a host cells DNA, and cause lysis of thehost cell in a lytic cycle

2. Temperate phages (prophages)

produces a repressor substance that prevents destruction ofhost DNA.

Cells containing prophages are called Lysogeniccells because they have the potential to enter thelytic cycle


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Phage cycle


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Transduction Transduction can

be specialized or

generalized.

Generalizedtransduction,

the phage canincorporate any part ofthe chromosomal orplasmid DNA and

transfer them.


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Transduction

In specializedtransduction thephage is incorporatedinto the chromosomeand can transfer onlygenes adjacent to thephage.e.g. phagelambda in E. coli


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Lysogenic or phage conversion

The alteration of a bacterialphenotype resulting from theacquisition of a phage.

It may confer virulence

property . Examples include:The production of diphtheriatoxin after the acquisition ofphage BThe acquisition of Shiga-liketoxin by E.coli after acquisitionof a phage

Production of botulinum toxin-Cby phageProduction of scarlet fever toxinby lysogenic Streptococcuspyogenes


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recombination Conjugation: A process requiring cell to cell contact. 1. Contact between donor and recipient cells is required

2. Larger amount of DNA is transferred.

Mediated by conjugative plasmids or conjugative transposons.

Sex pilli is involved In E. coli and other Gram-negative bacilli,

Sex pheromones may be involved in Streptococci

Significance of conjugation:

It increases genetic diversity

May represent an evolutionary stage between asexual and

sexual reproduction It provides a means of mapping genes in bacterial chromosome


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Conjugation

Conjugation in E. coli


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recombination Characteristics of plasmids: Double stranded extra chromosomal DNA.

Plasmids are self-replicating

They are identified by virtue of some recognizablefunction that they serve in a bacterium e.g. F-plasmids(fertility factors) direct the synthesis of proteins thatself-assemble into sex pili

R-plasmids (resistance factors) carry genes that

provide resistance to various antibiotics Other plasmids direct the synthesis of bactericidal

proteins called bacteriocin, toxin plasmids ( metabolicplasmids (Tol plasmids)


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Plasmids

Classification: based on size, copy number,

phenotype, incompatibility

Restriction endonucleases

analysis ( physical mapping) Plasmids are used in the study

of epidemiology of pathogenicbacteria.

Agarose gel electrophoresis of plasmids.


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Genetic engineering

Definition:the purposeful manipulation of geneticmaterial to alter the characteristics of anorganism.

Techniques include genetic fusion, protoplastfusion and recombinant DNA.

Recombinant DNA makes it possible to fusegenes with vectors and clone them in host

cells.


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Making Recombinant DNA involves:The manipulation of DNA in vitro,

The cloning of DNA from other organism s inbacteria DNA with phage or plasmid.

The production of many genetically identicalprogeny of phages or plasmids.


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Applications of Recombinant DNA

technology in Medicine 1. Treatment or management of disease

Production of growth hormones, insulin, anti sera, bloodcoagulation proteins etc

Gene therapy

2. Diagnosis of disease DNA probes for detection of genetic defects, identification of

pathogens DNA amplification (PCR)

3. Prevention and control of infections Recombinant vaccines Molecular epidemiology- for the determination of source and extent

of spread of an infectious agent

Safety concerns: Environmental impact, health effect of recombinant plants and

vegetables, Production of Biological warfare agents, ethical concern with

use on human subjects

Lecture 6 - Microbial Genetics

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