Bact Genetics Lec5

7/29/2019 Bact Genetics Lec5

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Bacterial genetics lec 5


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Bacterial genetics

Genetics: Science dealing with heredity - constancyand change in physiologic functions that formproperties of organism

Unit- Gene DNA segment carrying in its nucleotidesequence information for specific physiologic orbiochemical property.

Bacterial Genetic material single circular DNA

E.Coli mol. Wt 2 x 109. 5 x 106 base pairs-code for

2000 proteinsAverage Mol. Wt 50,000

Mycoplasma mol. Wt 5 x 108,

Human3x109 base pairs - encode 100000 proteins


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Bacterial genetics

Bacteria - Haploid: single chromosome, single copyof gene. mutation loss of trait

Human - Diploid: pair of chromosomes. copy of

gene (allele) dominant other recessivePhenotype: collective physiologic or structuralproperties of cell.

Genotype: sequence of DNA within a gene/organization of gene.

Phenotype variation: because of genes capacity to

permit growth under conditions of selection,

resistance gene grow in presence of Penicillin.


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Bacterial genetics

Restriction enzymes cleave DNA at specific sites

DNA Restriction fragments, if introduced in plasmids amplified many times. PCR Polymerase chainreaction amplified DNA segment placed nearpromotors expression of protein technology of

genetic engineeringGenetic Code: written as sequence of bases ofnucleotides. Read as group of 3 nucleotides sequencecalled Codon . Each codon amino acid

Gene: series of codon read in series from startingpoint to end point at other end. Written in 5/ - 3/direction. nucleotides sequence corresponds to aminoacid sequence in protein written in direction from N to

C terminus


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Bacterial genetics

Mutations: permanent change in DNAsequence or gene leading to altered phenotype

i.e. insertion of different amino acid or loss of

encoded protein. Mutant organism identified by

difference in appearance, physiology or growthproperties compared to normal. Mutation results

from 3 types of molecular changes.

Base substitution

Frame shift

Transposons or insertion sequences


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Bacterial genetics

Base substitution: insertion of single base in placeof another. During replication due to error or

chemical modification of base. Mutation can beMissense: insertion of different amino acid

Nonsense: generate termination codon stopprotein synthesis prematurely. Loss of protein

functionFrame shift: insertion or deletion of single base pair change of reading frame of entire subsequentsequence insertion of wrong amino acid inactive protein. Generally followed by stop codon.

Acridine dyes

Point mutation: involve single base pair

Transposons or insertion sequences: integratedinto DNA large scale changes in genes effectingfunction


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Mutations

Spontaneous: occur as result of normal

cellular operations or random interactionwith environment. Can occur in allorganisms in spite of tendency to repair.

Induced: occur due to exposure tomutagens

Mutagens: substances or factors whichcause mutation

Physical

Biological

Chemical


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mutagens

Physical: X ray: DNA damage in 3 ways. Breaks

covalent bond that holds ribose-PO4 chain,

produce free radicals attacking bases, alters

electron in base changing H bonding

UV light: cross linkage of adjacent pyrimidinebases to form dimers inability to replicate

Biological: Viruses: Bacteriophages inserted in

chromosome


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mutagens

Chemical: act by changing physical or chemicalstructure of DNA.

Nitrous acid- alter existing base which form H bondwith wrong base

Bromouracil: act as base analogue insertedinstead of thymine, binds with guanine G-C pair

instead of A-T.

Iodoxuridine: act as thymidine base analogue Acridine dyes

Alkylating agents

Benzpyrene: cause frame shift mutation


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Mutations

Conditional lethal mutation

Expressed only under certain conditions.

Temperature sensitive organism

mutationamino acid change replicate

in low temp 32o C not at 37oC.

Experimental influenza vaccine


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Transfer of DNA

Occurs within bacterial cells or between bacterial cellsTransfer of DNAwithin bacterial cells:

Transposons

Programmed rearrangement

Transposon / jumping genes/ mobile sequences:genetic elements of several Kbp of DNA includinginformation for migration from one locus to other,within or between DNA of bacteria, plasmid orbacteriophage. Replicate as part of chromosomewhere inserted. Detection or genetic exploitation doneby selecting information they carry.

Transposition: process by which sequences arecopied & inserted to new site in genome.


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Transfer of DNASimple Transposon/ insertion sequences: genes

for transposase protein (mediate excision &integration) in centre & inverted repeats (IR) oneither sides. Molecular parasites, causemutation or inactivate genome where inserted.

Complex Transposon: 4 domains, IR on eithersides, gene for transposase, repressor gene (regulate transposae synthesis and gene productof 4th domain, gene for enzyme mediatingresistance, toxins, other enzymes

Programmed rearrangement: movement of genefrom one silent site to an active site where it isexpressed. Encode variants of antigen. Helpescape immune response. N.gonorrhoea,B.recurrentis


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Transfer of DNA

Transfer of DNA between cells: 3 methods

ConjugationTransduction

Transformation

Conjugation:

mating of 2 cells, DNA transfer from donor torecepient. Process controlled by F plasmid (fertility Ffactor). It has genes for required proteins eg pilin forsex pilus.

F+ attaches to receptor on F-Reeling in of pilus cells drawn together

Cleavage of F factor

Transfer of one strand of DNA across conjugal bridge

Complementory strand synthesized- doudle stranded


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Conjugation


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Hfr cells: Fplasmid of some F+ cells integrated

into DNA acquire ability of transferring

chromosome into another cells. Single DNA

strand having a piece of F plasmid in front andremaining behind. Complete DNA transfer in 100

mins breakage of attachment only portion of

DNA transferred. Genes near leading piece

usually. Variable genes transferred because ofvariable attachment of plasmid in DNA. DNA

recombine with recepient DNA.


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High frequency recombination


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Transfer of DNA between cells

Transduction

DNA transfer by bacteriophage. During viral replicationin bacteria, a piece of DNA may be inserted in viralcoat. When it infects other bacteria, DNA integratesinto bacterial DNA. The infected bacteria acquires new

trait lysogenic conversion. It may convert nonpathogen to pathogen.

Generalised Transduction: virus carries segment fromany part of DNA after fragmentation. frequency of 1in

1000 particlesSpecialized Transduction: integrated viral DNA isexcised and carries with it adjacent part of bacterialDNA. Genes are specific because phages integrate at

specific sites


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Transduction


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Transfer of DNA between cells

Transformation: transfer of DNA fromone cell to other by itself. 2 methods

Dying bacteria release DNA which is taken

up b recipient cells.

Lab introduction of DNA extracted from

bacteria into another bacteria

Transfection: introduction of purified DNA

into eukaryotic cell. Used in genetic

engineering


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Recombination

Process of integration of introduced DNA

into host cell. 2 types

Homologous: two DNA pieces have

extensive homologous regions which pair

upand exchange pieces by processes of

breakage and reunion

Non homologous: little if any homology is

necessary

Bact Genetics Lec5

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