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Is protein or DNA the genetic material?
BIOSCI 101: MicrobiologyCampbell 16.4
The bacterial genome
The genome of most bacteria is a double-stranded circular DNA molecule
Replication of the bacterial chromosome beginsat a single origin of replication
The DNA of most daughter cells is identical tothe parent because the cells divide by an
BIOSCI 101: Microbiology
asexua process ca e inary ission
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a bacterial
chromosome
BIOSCI 101: MicrobiologyCampbell 18.14 (7th Edition)
Bacterial celldivision
BIOSCI 101: MicrobiologyCampbell 12.12
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Sources of genetic variation
Spontaneous mutations are rare but inbacteria, short generation times mean thatthey are a significant source of genetic diversity
Mutations that result in greater fitness conferselective advantages that drive the evolution ofbacterial populations
An additional source of diversity comes fromthe ability of bacteria to exchange geneticmaterial ie horizontal gene transfer
BIOSCI 101: Microbiology
genetic information is called genetic
recombination The recombination process involves specialised
enzymes as well as components of the DNAreplication and repair machinery
Genetic recombination Recombination involves breakage and rejoining
of DNA strands
In omo ogous recom ination, rea s in c ose yrelated DNA molecules are rejoined to formcrossovers
Nonhomologous recombination is much rarer specific proteins recognise nucleotide
BIOSCI 101: Microbiology
sites (eg integration of the genome in theE. colibacterial chromosome)
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Homologous recombination
BIOSCI 101: MicrobiologyFig 14.01; Clark, Molecular Biology -Understanding the genetic revolution
Evidence for genetic recombination
BIOSCI 101: MicrobiologyCampbell 18.15 (7th Edition)
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Transformation
Transformation is the uptake of free DNA fromthe environment
The source of DNA in Griffiths experiments with
the heat-killed virulent bacteria
The gene that conferred pathogenicity encoded acell coat (capsule) that protected the bacteriafrom the immune system of the mouse
Avirulent bacteria that took up the closely relatedDNA of the virulent strain and ac uired a co of
BIOSCI 101: Microbiology
the cell coat gene by homologous recombinationbecame virulent (ie killed their host)
Bacteria that can take up DNA from theenvironment are said to be competent- this canoccur naturally or be induced by chemicaltreatments
DNA can transform bacteria
BIOSCI 101: MicrobiologyCampbell 16.2
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Transduction
Transduction is the transfer of bacterial DNAfrom one cell to another via a bacteriophage
During replication of a lytic phage, the host
of nucleotides for new phage genomes
Occasionally a large host DNA fragment is stillpresent in the cell as the phage genomes arebeing packaged into head particles and can beincluded instead of the phage genome
After l sis of the infected cell the defective
BIOSCI 101: Microbiology
phage particle can adsorb to another cell andinject its DNA
The bacterial genes transferred by such aphage particle may replace the homologousregion of the recipient bacterial chromosome
Transduction (contd) Transduction that results in random transfer of
bacterial genes is known as generalisedtransduction
lsyogenic (temperate) phages because itrequires incorporation of the phage DNA intothe bacterial chromosome
Bacterial genes adjacent to the prophageintegration site can accidentally be included asthe prophage is excised from the bacterial
BIOSCI 101: Microbiology
chromosome All phage particles released from such a cell
would carry bacterial genes that could betransferred efficiently to a new host cell
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Transduction of bacteria byphage
BIOSCI 101: MicrobiologyCampbell 27.11
Transduction of bacteria by
phage (contd)
BIOSCI 101: MicrobiologyCampbell 27.11 contd
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Conjugation
Conjugation is plasmid-mediated transfer ofDNA from one bacterial cell to another
Plasmids are small, circular DNA molecules
bacteria in addition to the bacterialchromosome
Plasmids encode genes responsible for theirown replication and maintenance
F (fertility) plasmids encode ~ 25 genes,mostl re uired for the roduction of sex ili
BIOSCI 101: Microbiology
and transfer of DNA
R (resistance) plasmids encode genes thatconfer antibiotic-resistance and can also betransferred between cells by conjugation
Bacteria carrying plasmidscontain extra genetic information
BIOSCI 101: MicrobiologyFigs 16.10 and 18.14; Clark, Molecular Biology -Understanding the genetic revolution
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Bacterial conjugation1 m
Sex pilus
BIOSCI 101: MicrobiologyCampbell 27.12
Conjugation (contd)
Cells carrying an F plasmid are described as F+
and those that do not are F-
When F+ cells divide, the offspring usually arealso F+
During conjugation the F+ cell acts as a donor,transferring a parental strand of F factor DNAacross the mating bridge to an F- (recipient) cell
Each parental strand of F factor DNA then acts asa template for synthesis of the second strand in
BIOSCI 101: Microbiology
The original donor is still F+ and the recipient isnow also F+
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Transfer of F plasmid by
conjugation
BIOSCI 101: MicrobiologyCampbell 27.13a
http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter13/animation_quiz_3.html
Mechanisms of gene transfer (contd)
F plasmids can also integrate into the bacterialchromosome at a number of different sites
Integration is reversible in a population of cellsthere will be some containing F as a separateplasmid (F+) and some containing an integratedcopy (Hfr)
Hfr indicates high frequency recombination
BIOSCI 101: Microbiology
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Creation of an Hfr cell
BIOSCI 101: MicrobiologyCampbell 18.18b (7th Edition)
Conjugation (contd)
An Hfr cell can act as a DNA donor duringconjugation
DNA replication is initiated at a specific site in the Ffactor DNA (called the origin of transfer)
As the F factor DNA moves into the F- recipient,adjacent chromosomal genes are also transferred
It is very rare for the entire chromosome plus F DNAto be transferred before conjugation is disrupted
The Hfr donor cell retains its integrated copy of F
BIOSCI 101: Microbiology
In the F- recipient, homologous recombination mayoccur between the recipient chromosome and theintroduced donor chromosomal DNA
DNA that is not integrated is degraded by nucleases
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An Hfr cell can transfer genes
from one bacterium to another
BIOSCI 101: MicrobiologyCampbell 27.13b
http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter13/animation_quiz_4.html
Transposable genetic elements
Further genetic variation arises when DNA moveswithin a single cell within the chromosome,between chromosome and plasmid, or betweenlasmids
This process is called transposition and it requiressite-specific recombination between thetransposing DNA and the site where it integrates
Some transposable elements move around thegenome and others remain at their original site andinsert copies in new locations
BIOSCI 101: Microbiology
Insertion sequences are the simplest transposableelements they contain a transposase geneflanked by inverted repeat sequences
The transposase binds to the inverted repeats andthe target site, catalysing cutting and resealing ofthe DNA
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Transposons contain DNA required for transpositionas well as extra genes eg antibiotic resistance
Transposable genetic elements(contd)
Retrotransposons are found in eukaryotic genomes.They encode a reverse transcriptase gene andmove around the genome via an RNA intermediate
Viruses may be transposons (or plasmids) that
BIOSCI 101: Microbiology
move from cell to cell
Transposable elements move
around the genome
BIOSCI 101: MicrobiologyCampbell 18.19 (7th Edition)
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Movement of transposableelements (transposons)
TransposonNew copy of
transposon
Transposon
is copied
DNA of
genome
Insertion
BIOSCI 101: MicrobiologyCampbell 21.9
Mobile transposon
Test question 1
A) the proteins on its surface and that of the host.B) whether the viral nucleic acid is DNA or RNA.C) the proteins in the cytoplasm of the host cell.D) the enzymes produced by the virus before it infectsthe cell.E) the enzymes carried by the virus.
BIOSCI 101: Cellular and Molecular Biology
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Test question 2
The genes for sex pilus construction and for tetracyclineresistance are located to ether on the same lasmidwithin a particular bacterium. Tetracycline resistancecould spread rapidly from this bacterium to others bythe process of:A) transformationB) mutationC) transductionD) meiosis
BIOSCI 101: Cellular and Molecular Biology