Microbial GeneticsMicrobial GeneticsGenetic transfer and Genetic transfer and

recombinationrecombination

Genetic recombinationGenetic recombination Genetic recombination through sexual Genetic recombination through sexual

reproduction is an important means of variation reproduction is an important means of variation in eukaryotesin eukaryotes

Prokaryotes do not have an equivalent process Prokaryotes do not have an equivalent process of sexual reproductionof sexual reproduction

However, prokaryotes do have mechanisms by However, prokaryotes do have mechanisms by which DNA can be transferred between strains which DNA can be transferred between strains of the same species, or even between different of the same species, or even between different speciesspecies

Contributes to a population’s genetic diversityContributes to a population’s genetic diversity

Gene transferGene transfer

Vertical gene transferVertical gene transfer Occurs during reproduction. Transfer of genes Occurs during reproduction. Transfer of genes

from an organism to its offspringfrom an organism to its offspring Horizontal gene transferHorizontal gene transfer

Transfer of genes from one organism to Transfer of genes from one organism to another within the same generationanother within the same generation

Horizontal gene transferHorizontal gene transfer Involves one way transfer from a Involves one way transfer from a donordonor cell to a cell to a

recipient recipient cell cell A recipient cell that incorporates DNA from the A recipient cell that incorporates DNA from the

donor is called a donor is called a recombinantrecombinant Genes are transferred naturally between Genes are transferred naturally between

bacteria by three mechanismsbacteria by three mechanisms Transformation:Transformation: DNA is transferred as “naked” DNA DNA is transferred as “naked” DNA Conjugation:Conjugation: DNA is transferred between bacteria that DNA is transferred between bacteria that

are in contact with each other are in contact with each other Transduction:Transduction: DNA is transferred by a bacterial virus DNA is transferred by a bacterial virus

(bacteriophage)(bacteriophage)

Homologous recombinationHomologous recombination DNA introduced into bacteria usually does not DNA introduced into bacteria usually does not

have a mechanism to replicate itselfhave a mechanism to replicate itself It relies on integration into the genome of the It relies on integration into the genome of the

host bacterium in order to survive and be passed host bacterium in order to survive and be passed onon

Host genome

Incoming DNA

gene A

gene A gene Z gene C

gene Cgene B

gene A gene Cgene ZRecombinantgenome

Griffith’s experiment, 1928Griffith’s experiment, 1928Streptococcus pneumoniaeStreptococcus pneumoniae

““Naked” DNANaked” DNA

When a bacterial cell When a bacterial cell lyses, it releases its DNA lyses, it releases its DNA into the environmentinto the environment

CENSORED!

TransformationTransformation DNA is transferred as DNA is transferred as

naked DNAnaked DNA DNA breaks into pieces DNA breaks into pieces

on cell lysison cell lysis DNA is taken up by the DNA is taken up by the

recipient cellrecipient cell A region of the recipient A region of the recipient

DNA is replaced by the DNA is replaced by the donor DNA donor DNA (recombination)(recombination)

Unrecombined DNA is Unrecombined DNA is degradeddegraded

Transformation occurs naturally in very few Transformation occurs naturally in very few genera of bacteriagenera of bacteria BacillusBacillus, , HaemophilusHaemophilus, , NeisseriaNeisseria, ,

AcinetobacterAcinetobacter, and some strains of , and some strains of StreptococcusStreptococcus and and Staphylococcus.Staphylococcus.

The recipient cell must be in a physiological The recipient cell must be in a physiological state to take up DNA.state to take up DNA.

Changes in the bacterial cell wall make it Changes in the bacterial cell wall make it permeable to large DNA moleculespermeable to large DNA molecules Some bacteria, which are not normally can be Some bacteria, which are not normally can be

made so in laboratorymade so in laboratory

ConjugationConjugation

DNA transfer between two bacteria that DNA transfer between two bacteria that are in contact with one anotherare in contact with one another

Contact between donor and recipient cells Contact between donor and recipient cells is initiated by sex piliis initiated by sex pili

DNA is transfer through a conjugation DNA is transfer through a conjugation bridge or open pore between donor and bridge or open pore between donor and recipient cellrecipient cell

Mediated by a plasmid, called an F-factor Mediated by a plasmid, called an F-factor (fertility factor) or a conjugative plasmid(fertility factor) or a conjugative plasmid

PlasmidsPlasmids Small, circular molecules of Small, circular molecules of

DNADNA Replicate independently of Replicate independently of

the chromosomethe chromosome Usually dispensable for Usually dispensable for

growth, but under some growth, but under some conditions provide a conditions provide a selective advantage such selective advantage such as antibiotic resistance or a as antibiotic resistance or a unique metabolic pathwayunique metabolic pathway

Conjugative plasmids:Conjugative plasmids: carry carry genes for conjugation genes for conjugation including sex piliincluding sex pili

Mechanism of conjugationMechanism of conjugation

Donor contacts Donor contacts recipient, attaches recipient, attaches using sex pilususing sex pilus

F-factor initiates F-factor initiates transfer of a copy of transfer of a copy of itselfitself

Recipient is Recipient is converted to a new converted to a new donor celldonor cell

Importance of plasmidsImportance of plasmids Providing a selective advantageProviding a selective advantage

Some Some PseudomonasPseudomonas sp. have plasmid encoded sp. have plasmid encoded enzymes to degrade petroleum allowing them to live enzymes to degrade petroleum allowing them to live in fuel tanks or fuel spillsin fuel tanks or fuel spills

Enhancing pathogenicityEnhancing pathogenicity The The E. coliE. coli strain causing infant or traveler’s diarrhea strain causing infant or traveler’s diarrhea

carries plasmids for toxin production and bacterial carries plasmids for toxin production and bacterial attachment.attachment.

Antibiotic resistanceAntibiotic resistance Many antibiotic resistance genes are carried on Many antibiotic resistance genes are carried on

plasmids which can be rapidly transferred to other plasmids which can be rapidly transferred to other bacteria, resulting in widespread resistance to bacteria, resulting in widespread resistance to antibiotics and strains that resistant to multiple antibiotics and strains that resistant to multiple antibiotics such as methicillin-resistant antibiotics such as methicillin-resistant StaphylococcusStaphylococcus aureusaureus or Golden Staph or Golden Staph

TransductionTransduction

Mediated by a bacterial virus Mediated by a bacterial virus (bacteriophage or phage)(bacteriophage or phage)

DNA from the donor is transferred to the DNA from the donor is transferred to the recipient inside the phage particlerecipient inside the phage particle

Two types of transductionTwo types of transduction GeneralizedGeneralized SpecializedSpecialized

Generalized transductionGeneralized transduction Donor cell is infected with Donor cell is infected with

a phagea phage Donor DNA is Donor DNA is

incorporated into the incorporated into the phage (transducing phage (transducing phage)phage)

The donor cell lysesThe donor cell lyses The transducing phage The transducing phage

infects the recipient cell infects the recipient cell and injects the donor and injects the donor DNADNA

DNA integrated into the DNA integrated into the genomegenome

TransposonsTransposons Segments of DNA that can Segments of DNA that can

move from one region of move from one region of DNA to another and DNA to another and integrate through non-integrate through non-homologous recombinationhomologous recombination

Contain information for their Contain information for their own transpositionown transposition Transposase enzyme for Transposase enzyme for

cutting and resealing DNA cutting and resealing DNA Short terminal repeats which Short terminal repeats which

the transposase recognizes the transposase recognizes as recombination sitesas recombination sites

Insertion sequences are the simplest transposonsInsertion sequences are the simplest transposons Complex transposons carry other genes e.g., antibiotic Complex transposons carry other genes e.g., antibiotic

resistance genesresistance genes

Natural history of a transposonNatural history of a transposon

plasmid

chromosometransposon

Genomes of bacteria are elasticGenomes of bacteria are elastic

Bacterial genomes are often receiving Bacterial genomes are often receiving genetic information from other bacteria genetic information from other bacteria through genetic transfer and through genetic transfer and recombinationrecombination

In the same way that mutations can be In the same way that mutations can be beneficial, neutral or harmful, so is the beneficial, neutral or harmful, so is the recombination of incoming DNArecombination of incoming DNA

New gene combinations are maintained if New gene combinations are maintained if they provide the organism with a selective they provide the organism with a selective advantage advantage