Bacterial Genetics

Front 1

How do bacteria pass on genetic information?

Back 1

1. bacterial population is clonal
2. cell division replication (asexual)

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clonal

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a group/colony that is genetically identical to a single common ancestor.

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Why is there variety in bacterial populations?

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1. Mutation
2. horizontal transfer of additional genetic information (by some means other than chromosome replication)

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Overview of mutation

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1. DNA >transcribed into mRNA > translated into proteins
2. Alteration in nucleotides result in amino acids sequence and thus proteins w/ no/altered function
3. Occurs when base pairs are added, deleted, or substituted

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Mutations

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spontaneous or induced

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Spontaneous Mutations

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occur in bacteria in the range of 1:10^6 (every one in 1 million) to 1:10^9 (every one in a billion) -->rare
Random
leads to selection

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Induced Mutations

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Outside source
1. by mutagenic agents such as ethidium bromide
2. by environmental conditions (UV radiation)

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mutagens

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mutagenic agents that induce mutations (ethidium bromide)

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hypermutators

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pathogens with defective DNA can repair genes, thus they have rapid mutations that lead to resistance to many antimicrobials

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Selection

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extremely powerful in organisms that multiply exponentially; happens by mutation, and thus one org. is selected by the environment and makes progeny and its genes are passed down.

-a small number of surviving mutants can quickly grow to a large number

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Bacterial variety

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1. mutation
2. integration of exogenous DNA

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recombinants

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bacteria/cells that integrate DNA received through mutation or exogenous DNA into their own DNA

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Aquisition of exogenous DNA

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DNA that enters cells exogenously is processed THE SAME regardless of acquisition

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exogenous DNA

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DNA received from outside the cell

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Steps of exogenous DNA acquisition

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1. recipient DNA is cleaved by DNAse
2. matching donor fragment is linked to received DNA via DNA ligase
3. DNAse then degrades replaced DNA (the old DNA)

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Means of acquiring exogenous DNA

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1. Transformation
2. Transduction
3. Conjugation

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'naked" DNA

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strictly just chromosomes (DNA) fragments, not inside an organism (necessarily)

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Transformation

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Griffth1920; streptococcus pnumoniae
1. uptake of naked DNA
2. double stranded DNA binds to cell surface receptor
3. one strand is degraded, the other enters competent cell

*DNA binds to homologous region (the identical region on the recipient's DNA) meaning the naked DNA and bacteria cell are close relatives

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competence

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a unique physiological state found only in a few genera of bacteria allowing for exogenous DNA to enter

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bacteriophages (phage)

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Viruses that use bacteria cells as hosts

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Transduction

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Bacterial DNA is transferred to a second bacterium via a virus vector (phage)

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Steps of transduction

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DNA from bacteria #1 is taken up by virus (phage) and then virus infects bacteria #2 with bacteria #1 DNA in subsequent contact

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Transducted DNA

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-this DNA may have a little or a lot of genes that it transfers

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What makes the two bacteria involved in transduction closely related?

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1. there is homologous recombination
2. similar/identical phage cell receptors needed

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Intentions of bacteriophage in transduction

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this viruses intentions are to have its viral DNA (nucleic acids) replicated by bacteria

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Stages of phage infections

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1. attachment of virus to host bacterium
2. penetration of cell membrane by viral genome
3. viral gene (DNA) replication, usually with host genes shut down
4. packaging of newly produced virons (virus particles) possibly including parts of host DNA
5. Phages decision to enter lytic cycle, lysogenic cyle, or lysogenic conversion

after this is when genes can be transduced to another bacterium

the transduced bacteria genes may be active in new bacterium

usually happens first before transduction to bacteria #2

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Lytic cylce

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host cell lysis after release of virons when virus DNA replicates

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Lysogenic cycle

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host grows with viral genes being replicated but having no effect;
*possible lysis of cell if there is inactivation of repressor proteins

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Lysogenic conversion

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when viral genes produced after the original viral DNA injection and bacterial replication, become active in the prophage

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prophage

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the latent form of a bacteriophage inside an infected lysogenic bacteria

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Transduction spreads bacterial genes. What makes this a problem in human health?

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1. responsible for the spread of virulence; turns benign bacteria into pathogens
2. innocuous viruses are being developed as vaccines vectors to prevent bacterial infections
3. phage may be used to destroy bacterial pathogens in vivo

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Problems for using phage in vivo

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1. specificity; have to have the right amounts
2. foreign nucleic acid; meaning it could carry an unexpected gene
3. immune system will recognize it as foreign regardless

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conjugation

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requires direct contact b/w bacteria
mediated by self-transmissible plasmid
-frequent in gram neg., also in gram +
-transfers large amounts of DNA from donor to recipient
*medically important transfers*

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conjugative plasmids

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circular piecie of extra-chromosomal DNA
ie. F plasmid/ R plasmid

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Escherichia coli, Shigella, Salmonella

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gram neg bacteria that use conjugative method of exogenic transfer

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F plasmid

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encodes for the F or sex pilus or mating bridge

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F+ / F-

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bacteria with F plasmid/ bacteria without F plasmid

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Sex Pilus/bridge/F

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brings cells together; attaches them

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Mechanism for conjugation

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1. Sex Pili brings the two cells together,
2. Single strand of DNA in F+ being transferred to F-
3. Strand in donor replicates
4. Strand in recipient replicates
5. Both cells are F+ and they separate

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Hfr

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high frequency recombinant
-when F+ plasmid combines with bacteria chromosome

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recombinant F- cell

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When conjugation occurs between Hfr cell and F-, and complete F+ plasmid is not passed

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R plasmid (R factor)

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Plasmid (gene) for resistance that is frequently passed

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antibiotic resistance genes

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1. multiple are passed together via conjugation
2. cause for enzymatic breakdown of antimicrobial agent
3. Alteration of target molecule on pathogen
4. reduced uptake or enhance removal of antimicrobial agent

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plasmids

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not all are self-transmissible, but a self-transmissible one is needed for conjugation

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plasmid properties

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1. circular and less in size than chromosomes
2. transfered by conjugation, transformation, and transduction
3. autonomous (independent) replication; 100 copies may be in 1 cell
4. variation in number of copies
5. frequency of presence in population increased by selection pressure

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Transposon

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1.small segment of DNA within the recombinant plasmid that can move from one segment of DNA to another
2. non-homologous replication
3. can jump to a plasmid
4. non autonomous replication
5. may have addl genes adding to resistance or toxin production
6. May inactivate genes after it inserts

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Pathogenicity Islands

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example of genomic islands
-cassette (chunk) of virulence associate genes
-horizontally transferred
-rapid phenotypic shifts
-structure makes it easy to insert other genes, thus why its so loaded
-mech. for insertion unknown

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genomic islands

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-cassette (chunk) of virulence associate genes
-horizontally transferred
-rapid phenotypic shifts
-structure makes it easy to insert other genes, thus why its so loaded
-mech. for insertion unknown
(genotia)

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Integrons

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-accumulates antibiotic resistance genes from other genes

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selection pressure

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an environmental/outside source that either kills an organism, thus its genes, or gives it an adaptive advantage (resistance)
*removing the pressure will lessen advantage, but it will still allow for increased resistance
*LOSS OF RESISTANCE IS SLOWER THAN DEVELOPING RESISTANCE

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Flexible gene pool

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1. 20 - 30% of bacteria genome is foreign.
2. comes from horizontal transfer
3. from prophage, mobile genetic elements, genomic islands
4. Can get pathogenic and non-pathogenic material from diff. bacteria