chapt28 lecture

7/27/2019 Chapt28 Lecture

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Prokaryotes

Chapter 28


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The First Cells

Microfossils are fossilized forms of

microscopic life

-Oldest are 3.5 billion years old


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The First Cells

Stromatolites are mats of cyanobacterial

cells that trap mineral deposits

-Oldest are 2.7 billion years old


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The First Cells

Isotopic analysis of carbon-12 in fossils

suggests that carbon fixation was active

as much as 3.8 BYA

Biomarkers are organic molecules of

biological origin

-Lipids were found in ancient rocks

-This indicates that cyanobacteria are

at least 2.7 billion years old


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Prokaryotic Diversity

Prokaryotes are the oldest, and structurally

simplest forms of life

Prokaryotes are ubiquitous

Less than 10% of species are known

Bacteria (also called eubacteria)

Archaea (formerly called archaebacteria)-Many archaeans are extremophiles


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Prokaryotic Features

Unicellularity

-Most are single-celled

-Some can form complex biofilmsCell size

-Most are less than 1 mm in diameter

Chromosome-Single circular double-stranded DNA

-Found in the nucleoid


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Internal compartmentalization

-No membrane-bounded organelles

Flagella-Simple in structure; spin like propellers

Cell division

-Most divide by binary fissionGenetic recombination

-Occurs through horizontal gene transfer


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Metabolic diversity

-Two types of photosynthesis

-Oxygenic = Produces oxygen-Anoxygenic = Nonoxygen producing

- E.g: Sulfur and sulfate

-Chemolithotrophic prokaryotes derive

energy from inorganic molecules


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Bacteria vs. Archaea

Cell wall

-Bacteria have peptidoglycan

-Archaea lack peptidoglycan

DNA replication

-Archaeal DNA replication is more similar tothat of eukaryotes

Gene Expression

-Archaeal transcription and translation aremore similar to those of eukaryotes


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1. Amino acid sequences of key proteins

2. Percent guanine-cytosine content

3. Nucleic acid hybridization4. Ribosomal RNA sequencing

5. Whole-genome sequencing

Molecular Classification


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Based on these molecular data, several

prokaryotic groupings have been proposed

-Bergeys Manual of Systematic

Bacteriology

-Contains about 7,000 bacterial and

archaeal species

The three-domain (Woese) system of

phylogeny is based on rRNA sequences



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Most prokaryotes have one of 3 basic shapes

-Bacillus = Rod-shaped

-Coccus = Spherical-Spirillum = Helical-shaped

Prokaryotic Shapes


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Maintains shape and protects the cell from

swelling and rupturing

Consists ofpeptidoglycan

-Polysaccharides cross-linked with peptides

Archaea do not possess peptidoglycan

-Some have pseudopeptidoglycan

Cell wall is the basis of the Gram stain

The Bacterial Cell Wall


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Two main types

-Gram-positive bacteria

-Thick peptidoglycan-Teichoic and lipoteichoic acids

-Gram-negative bacteria

-Thin peptidoglycan-Have an outer membrane

-Contains lipopolysaccharide



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Bacterial Appendages

Pili

-Short, hairlike structures

-Found in Gram-negative bacteria-Aid in attachment and conjugation

Flagella

-Long, helical structures-Composed of the protein flagellin

-Involved in locomotion


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Bacterial Appendages


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Internal Structure

Nucleoid region

-Contains the single, circular chromosome

-May also contain plasmidsRibosomes

-Smaller than those of eukaryotes and differ

in protein and RNA content-Targeted by antibacterial antibiotics


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Prokaryotic Genetics

Prokaryotes do not reproduce sexually

However, they undergo horizontal genetransfer, which is of three types

-Conjugation = Cell-to-cell contact

-Transduction = By bacteriophages-Transformation = From the environment


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Conjugation

In E. coli, conjugation is based on

the presence of the F plasmid

F+ cells contain the plasmid

F- cells do not

The F+ cell produce an F pilus

that connects it to an F- cell


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Conjugation

The F plasmid can integrate into the bacterial

chromosome

-Hfr cell (high frequency of recombination)

The F plasmid can also excise itself byreversing the integration process


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Conjugation


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Conjugation

An inaccurate excision may occur

-F cell

Conjugation can occur between an F and an

F- cell

-The result is a partial diploid, ormerodiploid


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Transduction

Generalized transduction

-Occurs via accidents in the lyticcycle

-Viruses package bacterial DNA and

transfer it in a subsequent infection

-Virtually any gene can be transferred

Specialized transduction

-Occurs via accidents in the lysogeniccycle

-Imprecise excision of prophage DNA

-Only a few genes can be transferred


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Transduction


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Transformation

Natural transformation

-Occurs in many bacterial species, including

Streptococcus which was studied by Griffith

-DNA that is released from a dead cell is

picked up by another live cell

Artificial transformation

-Accomplished in the lab

-Used to transform E. colifor molecular

cloning


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Transformation


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Conjugative Plasmids

Conjugative plasmids may pick up additionalgenes

-R (resistance) plasmids

-Encode antibiotic resistance genes-Staphylococcus aureus

-Virulence plasmids

-Encode genes for pathogenic traits-Enterobacteriaceae

-E. coliO157:H7 strain


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Mutation

Mutations can arise spontaneously in bacteria-Also caused by radiation and chemicals

Mutations (and plasmids) can spread rapidlyin a population

-Negative consequences for humans

-For example:

-Methicillin-resistanceStaphylococcus aureus(MRSA)

-Vancomycin-resistantStaphylococcus aureus(VRSA)


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Prokaryotic Metabolism

Acquisition of Carbon

-Autotrophs =From inorganic CO2

-Heterotrophs =From organic molecules

Acquisition of Energy

-Chemolithotrophs =From inorganicchemicals

-Phototrophs =From sunlight


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Photoautotrophs

-CyanobacteriaChemolithoautotrophs

-Nitrifiers

Photoheterotrophs

-Purple and green nonsulfur bacteria

Chemoheterotrophs

-Majority of prokaryotes

-Use organic molecules for C and energy


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Type III secretion system

-Found in many Gram-negative bacteria

-Used to transfer virulence proteins directlyinto host cells

-Yersinia pestis Bubonic plague

-Pseudomonads Plant pathogens-Blights, soft rot, wilts


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Human Bacterial Disease

In the early 20th century, infectious diseases

killed 20% of children before the age of five

-Sanitation and antibiotics considerably

improved the situation

In recent years, however, many bacterial

diseases have appeared and reappeared


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Tuberculosis

-Mycobacterium tuberculosis

-A scourge forthousands of years

-Afflicts the

respiratory system-Mutidrug-resistant

(MDR) strains are

very alarming


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Dental caries (tooth decay)

-Plaque consists of bacterial biofilms

-Streptococcus ferments sugar to lactic acid-Tooth enamel degenerates

Peptic ulcers-Helicobacter pyloriis the main cause

-Treated with antibiotics


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Gonorrhea

-Neisseria gonorrhoeae

-Can pass from mom to baby via birth canal

-Can cause pelvic inflammatory disease (PID)

Chlamydia

-Chlamydia trachomatis-Silent STD

-Can cause PID and heart disease

Sexually transmitted diseases (STDs)


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Beneficial Prokaryotes

Prokaryotes are crucial to chemical cycles

-Decomposersrelease a dead organisms

atoms to the environment

-Photosynthesizers fix carbon into sugars

-Nitrogen fixers reduce N2 to NH3(ammonia)


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Prokaryotes may live in symbiotic

relationships with eukaryotes

-Mutualism = Both parties benefit

-Nitrogen-fixing bacteria on plant roots

-Cellulase-producing bacteria in animals

-Commensalism = One organism benefitsand the other is unaffected

-Parasitism = One organism benefits and

the other is harmed


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Bacteria are used in genetic engineering

-Biofactories that produce various

chemicals, including insulin and antibiotics

Bacteria are used for bioremediation

-Remove pollutants from water, air and soil-Exxon Valdezoil spill


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