The Prokaryotes

Chapter 16

Virus

Bacterium

Animalcell

Animal cell nucleus0.25 µm

• Unicellular• Prokaryotic cells have no nucleus

– DNA is a single, circular chromosome• DNA is in an unbound region called the nucleoid

– Prokaryotic cells lack membrane-bound organelles• Many bacteria also have plasmids

– smaller circular DNA molecules that can replicate independently of the chromosome

• Bacterial cells divide by binary fission• Genetic variation mainly by mutations

– Since bacteria can reproduce rapidly, new mutations quickly increase genetic diversity

• Cell Wall surrounds plasma membrane

Comparing Prokaryotic and Eukaryotic Cells

Bacterial Genome and Its Replication

• The bacterial chromosome is usually a circular DNA molecule with few associated proteins

• Many bacteria also have plasmids, smaller circular DNA molecules that can replicate independently of the chromosome

• Bacterial cells divide by binary fission

Origin ofreplication

Replication fork

Termination of replication

A typicalrod-shapedbacterium

A thin section through thebacterium Bacilluscoagulans (TEM)

0.5 µm

Pili

Nucleoid

Ribosomes

Plasmamembrane

Cell wall

Capsule

Flagella

Bacterialchromosome

Prokaryotic Classification• Domain Bacteria vs Archaea• Cell Wall composition

– Gram negative or gram positive

• Cell shape• Mode of nutrition• Molecular characteristics

– rRNA sequence comparisons

Composition of Cell Wall: Gram-positive vs Gram-negative bacteria

• Cell Wall surrounds plasma membrane

– Composed of lipids, carbohydrates and protein

• Peptidoglycan (mix of protein and carbohydrates)

– NO Cellulose (plants cells)

– Provides structure and support

• Structure of cell wall categorize bacteria based on ability to retain the Gram stain

• Series of two staining procedures– Crystal violet + iodine– Ethanol rinse– Safranin counterstain

Gram-Positive• Extra thick peptidoglycan layer allows cells to RETAIN

primary stain and thus appear BLUE

Bacillus anthracis

Gram Negative• Do NOT retain primary stain and thus take in the PINK counterstain

• Most PATHOGENIC bacteriaare Gram-Negative!!

Some Prokaryotes also may exhibit:• Capsules

– a sticky layer of polysaccharides or protein.

• shields pathogenic prokaryotes from attacks by a host’s immune system.

• Fimbriae – Hairlike projections enable

prokaryotes to stick to their substrate or each other

• Flagella– help prokaryotes move in their

environment

• Produce endospores– Allow bacteria to remain dormant

during harsh conditions

Additional Features of Prokaryotic Cells

Endospore

Flagella

Fimbriae

CapsuleTonsil cell

Bacterium

Roles of Bacteria in Ecosystems and Human Health• Ecosystems

– Decomposers• Recycle nutrients that would otherwise remain

unavailable in dead organisms and waste– Producers

• Important primary producers in oceans and lakes

• Human Health– Source of antibiotics– Aid in digestion and vitamin synthesis

• Disease

• Fermented foods– Yogurt– Cheese– Soy sauce

All organisms are almost constantly exposed to pathogenic bacteria. Most bacteria that cause illness do so by producing a poison.

• Exotoxins are proteins that bacterial cells secrete into their environment.

• Endotoxins are components of the outer membrane of gram-negative bacteria.

Some bacteria cause disease

© 2012 Pearson Education, Inc.

ExotoxinSecreted by cell

Staphylococcus aureus Salmonella enteritidis

EndotoxinComponent of gram-negative plasma membrane

1. find the bacterium in every case of the disease,

2. isolate the bacterium from a person who has the disease and grow it in pure culture,

3. show that the cultured bacterium causes the disease when transferred to a healthy subject, and

4. isolate the bacterium from the experimentally infected subject.

Koch’s postulates are used to prove that a bacterium causes a disease

© 2012 Pearson Education, Inc.

Mutation and Genetic Recombination as Sources of Genetic Variation

• Rapid reproduction, mutation, and genetic recombination contribute to the genetic diversity of bacteria

• Since bacteria can reproduce rapidly, new mutations quickly increase genetic diversity

• Additional genetic diversity arises by recombination of DNA from two different bacterial cells

• Three processes bring bacterial DNA from different individuals together:– Transformation– Transduction– Conjugation

Bacteria can transfer DNA in three ways

Bacteria use three mechanisms to move genes from cell to cell.1. Transformation is the uptake of DNA from the

surrounding environment.

2. Transduction is gene transfer by phages.

3. Conjugation is the transfer of DNA from a donor to a recipient bacterial cell through a cytoplasmic (mating) bridge.

Once new DNA gets into a bacterial cell, part of it may then integrate into the recipient’s chromosome.

© 2012 Pearson Education, Inc.

A+

Phage DNA

A+

Donorcell

B+

A+

B+

Crossingover

A+

A– B–

Recipientcell

A+ B–

Recombinant cell

Sex pilus

F plasmid Bacterial chromosome

F+ cell

Matingbridge

F+ cell

F+ cell

Bacterial chromosome

F– cell

Conjunction and transfer of an F plasmid from and F+ donor to an F– recipient

http://highered.mheducation.com/sites/dl/free/0072835125/126997/animation6.html

Antibiotics Target Unique Features of Prokaryotes

• 3 major target of antibiotics:– Enzymes required for cell wall synthesis– Bacterial ribosomes (interfere with protein synthesis)– DNA replication in bacteria

• All of these drugs inhibit or slow bacterial growth by inhibiting enzymes or cellular structures unique to bacteria

• Not effective against viruses

• Bacterial cells in a population resistant to antibiotic will continue to grow, selecting for antibiotic resistant strains

http://www.sumanasinc.com/scienceinfocus/antibiotics/antibiotics_fla.html

Evolution Connection:The Evolution of Antibiotic Resistance

– Many antibiotics disrupt cellular structures of invading microorganisms.

– Introduced in the 1940s, penicillin worked well against such infections.

– But over time, bacteria that were resistant to antibiotics, such as the MRSA strain, were favored.

– The widespread use and abuse of antibiotics continue to favor bacteria that resist antibiotics.

© 2013 Pearson Education, Inc.

Figure 4.23