Chapter 27A: Bacteria And Archaea

Chapter 27A:Bacteria and Archaea1. Extracellular Prokaryotic Structures2. Intracellular Prokaryotic Structures3. Genetic Diversity Prokaryotes

1. Extracellular Prokaryotic Structures

Prokaryotic Cell Shapecoccibacillispirilla spherical prokaryotes arereferred to as cocci(singular coccus) rod-shaped prokaryotesare referred to as bacilli(singular bacillus)Spherical3 µm1 µm1 µm rod-shaped cells can alsobe slightly curved (vibrio)or spiral (spirillum) or highlycoiled (spirochete)Rod-shapedSpiralvibriospirochete

Bacterial Cell Wall Structure(a) Gram-positive bacteria(b) Gram-negativebacteriaCarbohydrate portionof membranePeptidoglycan traps crystal violet,which masks the safranin dye.CellwallOutermembranePeptidoglycan layerPlasma membraneCrystal violet is easily rinsed away, revealingthe red safranin dye.

Gram StainingA Gram stain is a very common stain to distinguish 2 bacterial types:Gram negative123*4 does NOT retain crystal violet,only the safraninGram-negativebacteriaGram positiveGram-positivebacteria retains crystal violet due tothick peptidoglycan layer10 µm* key stepA Gram stain of 2 distinct Bacterial Species

CapsulesA polysaccharide or protein layer called a capsule coversmany prokaryotes. mediates adhesion andthe formation of biofilmsBacterialcell wall protects the cell fromdessication (drying out)and phagocytosis (beingconsumed by cells of theimmune system)BacterialcapsuleTonsilcell200 nm

Bacterial Flagella consist of a basal body,hook & filamentFlagellumFilamentHookMotorCell wallPlasmamembraneRodPeptidoglycanlayer20 nm basal bodyanchors flagellumin the membraneand cell wall, androtates the hook& filament topropel bacterium

FimbriaeSome prokaryotes have fimbriae (aka “pili”), which allow themto stick to their substrate or other individuals in a colony.Fimbriae1 µm

Sex PilusSex pili are longer than fimbriae and allow prokaryotes toexchange DNA through a process called conjugation.Sex pilus1 µm

2. Intracellular Prokaryotic Structures

EndosporesSome Gram-positive bacteria form metabolically inactive endosporeswhich can remain viable in harsh conditions for centuries.Endospore inactive, dormant cellsenclosed in a highlyresistant spore coatCoat remain dormant untilconditions improve0.3 µm very resistant to heating,freezing, dessication anddamaging radiation (e.g. UV)

Infoldings of the Cell MembraneSome prokaryotes have highly folded membranes to increasethe surface area for processes such as cellular respiration andphotosynthesis.1 µm0.2 µm such membrane infoldingsare not considered to betrue organelles such asthose found in eukaryotesRespiratorymembraneThylakoidmembranes(a) Aerobic prokaryote(b) Photosynthetic prokaryote

Prokaryotic ChromosomeProkaryotes typically have 1 circular DNA molecule that isthe chromosome. the prokaryotic chromosome islocated in a region of the cellcalled the nucleoid

PlasmidsSome bacteria have 1 or more small, extrachromosomal,non-essential circular DNA molecules called plasmids.ChromosomePlasmids generally contain genesthat confer some sort of advantagefor survival and reproduction:Plasmids protection from toxic substances(antibiotic resistance)1 µm toxins to kill competitors, enhancedisease gene transfer by conjugation

3. Genetic Diversity in Prokaryotes

Sources of Genetic Diversity in Prokaryotes3 general factors contribute to prokaryotic diversity:MUTATION changes in DNA sequencesRAPID REPRODUCTION some prokaryotes can reproduce every 20 minutesHORIZONTAL GENE TRANSFER transfer of DNA from one cell to another

Horizontal vs Vertical Gene TransferVertical transfer to thenext generationHorizontal (or lateral) transfer within thesame generation

Mechanisms of Prokaryotic Gene TransferBacteria can acquire DNA (i.e., new genes) in 3 basic ways:1)Transformation – the uptake and retention of externalDNA molecules2)Conjugation – direct transfer of DNA from one bacteriumto another “regular” conjugation HFR conjugation3) Transduction – the transfer of DNA between bacteria by a virus

TransformationUnder the right conditions, bacteriacan “take in” external DNA fragments(or plasmids) by transformation. DNA binding proteins transfer externalDNA across cell wall and membrane recombination with the chromosomalDNA can then occur

Bacterial ConjugationF plasmidBacterialchromosomeF cellF cell(donor)MatingbridgeF cell(recipient)1 One strand ofF cell plasmidDNA breaks atarrowhead.Bacterialchromosome2 Broken strandpeels off andenters F cell.F cell3 Donor andrecipient cellssynthesizecomplementaryDNA strands.Conjugation and transfer of an F plasmid4 Recipientcell is now arecombinantF cell.

Hfr ConjugationHfr cell(donor)A A A A A F factorA F cell(recipient)1 An Hfr cellforms amating bridgewith an F cell.A 2 A single strandof the F factorbreaks andbegins to movethrough thebridge.A A 3 Crossing overcan result inexchange ofhomologousgenes.A RecombinantF bacterium4 Enzymesdegrade andDNA notincorporated.Recipient cellis now arecombinantF cell.Conjugation and transfer of part of an Hfr bacterial chromosome,resulting in recombination

Phage DNA1 Phage infects bacterialdonor cell with A and B alleles.A B Donor cell2 Phage DNA is replicatedand proteins synthesized.A B 3 Fragment of DNA with A allele is packaged withina phage capsid.A 4 Phage with A alleleinfects bacterial recipientcell.5 Incorporation of phageDNA creates recombinantcell with genotype A B .TransductionCrossingoverA A B RecombinantcellRecipient cellA B A bacteriophage virus cantransfer DNA fragments fromone host cell to anotherfollowed by recombination: requires a virus to bepackaged with bacterialDNA “by mistake” infection of another cellby such a virus facilitatesthe gene transfer followedby recombination

Chapter 27A: Bacteria and Archaea 1. Extracellular Prokaryotic Structures 2. Intracellular Prokaryotic Structures 3. Genetic Diversity Prokaryotes. 1. Extracellular Prokaryotic Structures. Spherical Rod-shaped Spir