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