week 4 microbiall genetics 2013.ppt

8/10/2019 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


29/333-Conjugation Conjugation (in E coli)


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Conjugation (in E. coli)


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Conjugation continued


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week 4 microbiall genetics 2013.ppt

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