Post on 30-May-2018
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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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Genetic transfer and
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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Genetic transfer and
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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Genetic transfer and
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