micro lect 2 2011 colour 2 slides per page

8/13/2019 Micro Lect 2 2011 Colour 2 Slides Per Page

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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



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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


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


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



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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


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



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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


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


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


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


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


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


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



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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


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


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


move from cell to cell

Transposable elements move

around the genome



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Movement of transposableelements (transposons)

TransposonNew copy of

transposon

Transposon

is copied

DNA of

genome

Insertion


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

micro lect 2 2011 colour 2 slides per page

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