WEEK 4 Microbiall genetics 2013.ppt
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Transcript of WEEK 4 Microbiall genetics 2013.ppt
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OUTCOMES
Bacterial geneticDefine genetics, genome, chromosome, gene, genetic code,
genotype, and phenotype
Describe the process of DNA replication
Explain the mechanisms of gene transfer in bacteria.Name & define transposable genetic elements and plasmids
Describe the functions of transposable genetic elements and
plasmids
Name the components of transposable genetic elements and
plasmids
Define mutation
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GeneticTerminology
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Genetics:The science which defines and analyzes heredity.
Gene:
the unit of heredity, it is a segment of DNA that carriesinformation for a specific biochemical or physiologic property.
Bacterial genome:
the total set of genes present inside a bacterial cell. These genesare carried on (mainly the chromosome):
1- Bacterial chromosome:
a double stranded circular DNA molecule that encodes up to 4000genes necessary for bacterial growth & multiplication
There is no true nucleus in bacteria as they are prokaryotic cells,a single chromosome is present in the cytoplasm as a nucleoid.
2- Plasmids
3- Transposable genetic elements
4- Prophage (bacteriophage DNA)
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Structure and Function of
Genetic MaterialDNA& RNA
DNA=deoxyribonucleic acid
RNA=ribonucleic acidBasic building blocks:
Nucleotides
Phosphate groupPentose sugar
Nitrogenous base
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Structure of DNA
Double stranded (double helix)
Chains of nucleotides
5 to 3 (strands are anti-parallel)Complimentary base pairing(nitrogenous groups)
A-TG-C
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DNA Structure
Phosphate-P
Sugar-blue
Bases-ATGC
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The bacterial chromosome
The bacterial nucleoid is formed of a singlechromosome that is supercoiled in the cytoplasm.
Each chromosomal strand is formed of regularlyalternating phosphate and sugar groups.
Nitrogenous bases are attached to sugar groups.
The 2 strands are held together by hydrogen bondsbetween complementary bases (A-T) and (C-G)
The average length of bacterial chromosome is 40005000 kbp (I mm long).
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DNA Replication
Bacteria have a closed, circular DNAThe Bacterial chromosome replicates by semi-conservative method of DNA replication:
-the 2 strands separate
- each strand act as a template to synthesize acomplementary strand by the action of polymeraseenzyme.
The bacterial chromosome follows the same rules ofgene expression and protein synthesis as higher cells
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Genetic Code
DNA: triplet code
mRNA: codon (complimentary totriplet code of DNA)
tRNA: anticodon(complimentaryto codon)
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Protein Synthesis
DNA------ mRNA------ proteintranscription translation
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Plasmids
Plasmids are extra-chromosomal, circular, double strandedDNA molecule dispersed in the cytoplasm.
They are much smaller than chromosome.
Plasmids are capable of replicating independently of thebacterial chromosome.
Most plasmids encode properties that are not essential forgrowth, replication or survival of bacterial cells
Functions exhibited by plasmids:
1- Plasmids with F factor mediate the process of conjugation.
2- Some carry genes for antibiotic resistance.3- Virulence plasmids code for exotoxins, adhesion or invasion factors
4- Bacteriocin production: a bactericidal substance produced by somebacteria & are active against other strains.
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Transposable genetic elements
These are non-replicating DNA segments that are capable ofinserting themselves into other DNA molecules.
They are also capable of mediating their transfer from onelocation to another on the same chromosome or betweenchromosome and plasmids.
The transferof these elements (transposition) occursinfrequentlyand in a random manner.
Insertion of a transposable element into a geneleads todisruptionor inactivation of this gene.
Examples of Transposable genetic elements:1- Transposons (may encode antibiotic resistance).
2- Pathogenicity island: encodes a variety of virulencecharacters .
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Genotypic variation:
changes that occur as a result of change in thegenetic constitution. It is:-Irreversible-Heritable
Genotypic variations occur through:
Mutation
Gene transfer
a- Transformation
b- Transduction
c- Conjugation
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1-MutationDefinition:
change in the nucleotide base sequence of thebacterial genome which may lead to change inphenotypic character. It can be:
Harmful
Lethal
Helpful
Silent
Mutations That arise in bacterial populations:
Induced:byX-ray, chemicals, UV light(once103/104)cell division
Spontaneousoccur as an error during DNAreplication (once106/107) cell division
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Wild-Type (normal) A,B,C,D,E,F,G
Inversion A,B,F,E,D,C,G
Duplication A,B,C,B,C,D,E,F,G
Deletion A,B,C,E,F
Substitution A,B,X,D,E,F
Insertion A,B,X,C,D,E,F
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Mutation Type
Frameshift (deletion)
(leu) (ser) (arg)
Normal AAT AGT GCC
(leu) (val) (pro)
Mutant AATAGTG CCA
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Mutation Type
Frameshift (insertion)
(leu) (ser) (arg)Normal AAT AGT GCC
(leu) (glut) (cyst)
Mutant AAT CAGTGCC
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2. Gene Transfer in Bacteria
Genetic transfer-results in genetic variationThree ways:
Transformation: genes are transferred by up take offree DNA from the surrounding medium (dying bacteria
release naked DNA)
Conjugation: plasmids are transferred from onebacteria to another via a sex pilus
Transduction: DNA is transferred from one bacteria toanother by a bacteriophage (virus)
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Transformation Recombination by transformation requires homology or
competence between the DNA's involved (donor andrecipient.
This type of recombination is called homologous orgeneral recombination.
Only DNA from closely related bacteria would be expectedto successfully transform, although in rare instances genetransfer between distantly related bacteria has been shownto occur.
Artificial competence can be induced by treating the
recipient bacteria with Calcium chloride for enabling theuptake of DNA.
Recombination by transformation is used in vaccinespreparationand in gene therapy.
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Transformation
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Transduction
Description:Gene transfer from a donor to arecipient by way of a bacteriophage
Role of the bacteriophageThe lytic cycle
Lysogeny
Generalized transduction (occur with lytic cycleSpecialized transduction (occur with lysogeny)
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Generalized Transduction
Release of phage
Phage replication and degradation of host DNA
Assembly of phages particles
Infection of recipient
Infection of Donor
Potentially any donor gene can be transferred
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3. Conjugation
Definition:Gene transfer from adonor to a recipient by direct
physical contact between cells
Mating types in bacteriaDonor
F factor (Fertility factor) F (sex) pilus
Recipient
Lacks an F factor
Donor
Recipient
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Conjugation (in E. coli)
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Conjugation continued
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