Bacterial Genetics. Mutation. Gene Transfer. Lysogenic Conversion. Plasmids. Transposons

7/13/2019 Bacterial Genetics. Mutation. Gene Transfer. Lysogenic Conversion. Plasmids. Transposons.

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BACTERIAL GENETICSBACTERIAL GENETICS


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ProkaryotesProkaryotes

the genome of prokaryotes is not in a

separate compartment: it is located in thecytoplasm

prokaryotes contain no membrane-bound

organelles; their only membrane is themembrane that separates the cell from the

outside world

nearly all prokaryotes are unicellular


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NucleoidNucleoid

the nucleoid is an irregularly-shaped

region within the cell of prokaryoteswhere the genetic material is localized

the genome of prokaryotic organismsgenerally is a circular, double-stranded

piece of DNA, of which multiple copies

may exist


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NucleicNucleic acidsacids structurestructure

backbone polynucleotide strands

consists of sugar+base+phosphate sugar: ribose-RNA

2-deoxyribose-DNA 5 heterocyclic bases: holds N atom

adenine, guanine-purinesuracile, cytosine, thymine-pyrimidines


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http://en.wikipedia.org/wiki/File:DNA_chemical_structure.svg


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Hydrogen bonds between bases

2 A-T

3 C-G


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Primary-single strand

Secondary-double helix

Tertiary-folding of double helix


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DNA Replication

synthesis of new strands of DNA (precise

complementary copy) are established byDNA polymerases (Taq polymerase)

in bacteria starts at fixed points-oriC

elongation of strand by addition ofnucleotides to 3OH end (5 to 3 end direction)

starts with primers (oligonucleotides) replication fork


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replicationreplication
http://www.youtube.com/watch?v=teV62zrm2P0http://www.youtube.com/watch?v=teV62zrm2P0http://www.youtube.com/watch?v=teV62zrm2P0


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Bacterial genetic information is stored

in

ChromosomePlasmid


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BacterialBacterial chromosomechromosome

usually double stranded circular DNA(Borrelia-linear chromosome)

condensed into nucleoid (supercoiled loopdomains, histon-like proteins)

no nuclear membrane size from 0,56 x 106 bp Mycoplasma

genitalium

4,639 x 106 bp Escherichia coli replication starts from specific site-origin of

replication (OriC)


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Bacteria Chromosome(s)

Bacillus subtilis One circular 4,2 Mb

Brucella melitensis Two circular (2,1+1,2

Mb)E. coli One circular 4,6 Mb

Vibrio cholerae Two circular (2,9+1,1

Mb)


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BacterialBacterial

chromosomechromosome


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Bacterial genes

short (average 1000 nucleotides)

biosynthetic genes repetetive sequences


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catabolic genes loss of gene activity leads to loss of the

ability to catabolise (use) the compound

drug resistance and bacteriophage genes

functionally linked genes are grouped into

operons all the genes in a operon are transcribed

onto one large mRNA


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Replication of bacterial

chromosome replication starts from specific site-

origin of replication (OriC) andproceeds bidirectionally into terminus

region terC

50 kbp per min (1 miscorporation for

every 1010 nucleotides)


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Plasmids

important features of bacterial genome

no essential for life of bacteria

conjugative (mobilizable) x

nonconjugative (horizontal transfer) spread by partition between cells or by

conjugation between donor cell andrecipient cell

circular DNA 1.000-500.000 bp


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Phenotypic properties carried by

plasmids ANTIBIOTIC RESISTANCE-usually

enzymatic inactivation of the antibiotics VIRULENCE FACTORS

Toxins Adherence factors

PRODUCTION OF ANTI-MICROBIALAGENTS-antibiotic, bacteriocins


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METABOLIC PATHWAYS--

Pseudomonas sp. (catabolic activity fortoluene, salicylic acid)

Plasmid cloning-introduction of new

genes into bacteria


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Plasmids are selftransferable by

conjugation (encode the sex-pilusproteins and other transfer genes)

the transfer of genetic material betweenbacteria through direct cell-to-cell

contact

conjugation is the main transfer of

antibiotic resistance

ConjugationConjugation


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Transposons jumping genes

move from one site to another

important in moving of antibioticresistance genes between chromosome

and plasmids simple transposons only encode

information for transposition

compound transposons carry otherinformation (antibiotic resistance)

cause genomic instabil ity resulting ingene inactivation


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Retrotransposons: copy and paste

(RNA intermediate) DNA transposons: cut and paste (no

RNA intermediate)


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Pathogenicity islands

large tracts of chromosomal DNA (up to 4 x

106

bp) which have been obtained fromanother bacterial species by horizontal

transfer

usually contain the genes required forvirulence (toxins) and antibiotic resistance

typical examples are adherence factors,

toxins, iron uptake systems, invasion factors

and secretion systems


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usually absent from those non-

pathogenic organisms of the same orclosely related species

1. PAIs carry genes encoding one or more

virulence factors: adhesins, toxins, invasins,etc.

2. They are located on bacterial chromosome

or may be a part of a plasmid 3. They are flanked by direct repeats: The

sequence of bases at two ends are the same

5. PAIs carry functional genes, e.g. integrase,transposase, or part of insertion sequences


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Examples of microbes with PAI

E.coli:

EHEC-enterohemorrhagic E. coli (diarrhoeaand hemolytic-uremic syndrome after

colonization of the large intestine)

EPEC-enteropathogenic E. coli (watery

diarrhea after colonization of the small

intestine)

UPEC-uropathogenic E. coli


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Integrons

expression cassettes for antibiotic

resistance genes (the type and numbermay vary)

able to capture genes have not repeated sequences

have integrase gene


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Mutations

caused that bacteria are very variable

and highly adaptable mutation=change within DNA sequence

bacteria mutate 1000x more frequentlythan human cells


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Types of mutation

point mutation (affect one base pair)

insertion and deletion of larger DNAfragment (usually due to transposons)

can alter the function of a gene

amino acid substitution in protein

premature termination of protein

synthesis (nonsence) gene over-expression (changed

regulation)


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MRSA-mutation in PBP (penicillin

binding proteins) ESBL-point mutation in beta-lactamase


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Transfer of genetic information

Transformation

Conjugation Transduction


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Transformation

uptake of naked DNA

occurs in most bacteria and play role inchanging the phenotype (variation in

Neisseria gonorrhoeae)


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Conjugation

encoded by plasmids

major mechanism of antibioticresistance transfer


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Transduction

bacteriophage mediated

(bacteriophages are bacterial viruses) transduction results in phage

conversion


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Bacterial Genetics. Mutation. Gene Transfer. Lysogenic Conversion. Plasmids. Transposons