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Chapter 20
Viruses, Bacteria, and Archaea
Viruses, Viroids, and Prions
20.1
Virus
• Non-cellular parasitic agent consisting of an outer capsid and
inner core of nucleic acid.
• 2500 species identified
• Have some characteristics of living organisms
• DNA or RNA
• Evolve and Replicate
• BUT can only replicate using host cells machinery
• No metabolism
• Do not respond to stimuli
• Do not fossilize
Structure of Virus
• Categorized by:
1. Size and shape (thread-like or polyhedral)
2. Type of nucleic acid and single or double stranded
3. Presence or absence of outer envelope
• ALL have capsid and inner core (DNA or RNA)
• Range from 3-100 genes
• IF capsid is outermost structure- NAKED VIRUS
• IF surrounded by outer membranous envelope- ENVELOPED VIRUS
Parasitic Nature
• Cannot replicate outside living cell
• BUT has genetic material
• Must infect living cell
SO:
Virus “hijacks” cells protein synthesis machinery to replicate the
nucleic acid and other parts of the virus
*Some infected cells are killed or damaged by virus
Ex: Pneumonia or bronchitis
Host Specificity
• Particular virus is only capable of reproducing in a specific
organism
• rabies-animals
• Can even be tissue specific
• Hepatitis-liver
• HIV- certain blood cells
• Determined by structure (naked or spiked envelope)
• Lock and key manner with host surface
• Some medicines mess w this manner to prevent further infection
Reproduction of Viruses
1. Attachment- virus surface molecules and host cell receptors
2. Penetration- Host cell engulfs virus OR Virus injects genome
3. Biosynthesis- viral components synthesized using hosts stuff
4. Maturation- assembled into new viruses
5. Release- new virus exits though lysis or budding in order to infect
new host cells
Reproduction of Bacteriophages
• Bacteriophages- viruses that parasite bacteria
• Lytic- 5 steps occur immediately, cell lysed and the end by lysozyme
• Bacterial cell dies, 100s of new viruses released
• Lysogenic- not immediate, latent (not actively replicating)
• Attachment
• Penetration
Integration
• Then, biosynthesis, maturation, release
• Integration- viral DNA becomes incorporated in Bacterial DNA
Reproduction of Bacteriophages
• Lysogenic cells- carry copy of genome in the future induce
into lytic cycle by environmental factors
• EX: bacteria that causes strep throat is carrying prophage,
then it becomes scarlet fever
Reproduction of Animal Viruses
• Introduce genetic material differently
• Some envelope viruses- attach, fusion of spikedenvelope
• Many naked and other enveloped viruses- endocytosis
• Enters, uncoated, then lytic or lysogenic cycle happens
• Some viruses enter directly into lytic cycle causing
rapid and severe destruction of host cells.
• HIV, however, enters lysogenic and is inactive for many
years before AIDS symptoms show up.
Reproduction of Animal Viruses
• Viral Release• some release by budding, picks up its envelope from
plasma membrane
• Naked viruses released by lysis of host cell
Retroviruses
• Animal viruses with RNA genomes that have to be converted
to DNA within host cell by an enzyme called reverse
transcriptase
• Before it can integrate into host genome, RNA  DNA
RNA
cDNA
DNA
Retroviruses
• AIDs delayed by treatment with antiretroviral drugs
• AZT- reverse transcriptase inhibitors
• Acyclovior- inhibits replication of HIV viral DNA
Emerging Viruses
• Use to not be a problem,
• AIDs, West Nile, HPS, SARS, Bird Flu,
• Important to get flu shot every year.
• HIV also rapidly emerging. That’s why we cant find a cure
Viroids
• 1000 different viruses cause disease in plants
• 12 not viruses but viroids (naked strand of RNA)
Prions
• Proteins that normally exist in an animal but have different
conformation or structures
• Fatal brain disease
• TSEs are neurodegenerative diseases that destroy nerve tissue in
brain
• Untreatable and fatal
• Mad cow disease (bovine spongiform encephalopathy) (BSE)
Prokaryotes
20.2
Prokaryotes
• Includes bacteria and
archaea, which are fully
functioning, living, singlecelled organisms.
• Today, we know that
bacteria are plentiful in air,
water, and soil.
Structure of Prokaryotes
• Surrounded by plasma membrane and a cell wall situated
outside the membrane.
• Cell wall prevents cell from bursting or collapsing
• Some have additional cover layer of polysaccharides called a
glycocalyx
• Well organized- capsule
• Loosely organized- slime layer
Structure of Prokaryotes
• Some move by means of
a flagella
• Filament is inserted into
a hook anchored by a
basal body.
• 360- degree rotation
Structure of Prokaryotes
• Many adhere to surfaces by means of fimbriae
• Short, bristle-like fibers extending from the surface.
• Ex: Neisseria gonorrhoeae attach to host cell and cause
gonorrhea, and STD
Structure of Prokaryotes
• Do not have a nucleus
• Do have a nucleoid region where single chromosome where
DNA is found.
• Also have plasmids (rings of DNA)- genes for antibiotic
resistance, production of toxins, and degradation of
chemicals.
Reproduction of Prokaryotes
• Mitosis- formation of spindle apparatus, does NOT
occur in prokaryotes.
• Reproduce asexually by binary fission.
• Single circular chromosome replicates, then two
copies separate as cell enlarges eventually leading to
two cells.
• Takes about 12 minutes
Reproduction of Prokaryotes
In eukaryotes, genetic recombination occurs as a result of
sexual reproduction.
Prokaryotic gene recombination:
1. Conjugation- two bacteria are temp. linked together and
pass DNA.
2. Transformation- cell picks up free pieces of DNA secreted
by others or released by dead cells.
3. Transduction- bacteriophages carry portions of DNA form
one to the other.
Exit Slip
1. Explain the difference between a eukaryote nucleus
and a prokaryote nucleoid.
2. Explain 3 ways how prokaryotes can recombine their
DNA without sexual reproduction.
Bacteria
20.3
Characteristics
• Most protected by cell wall composed of
peptidoglycan
1. Gram-positive- thick peptidoglycan cell wall
2. Gram-negative- thin peptidoglycan and second
plasma membrane, often blocks antibiotics
Gram Stain
Shapes
• Spirilli
• Cocci
• bacilli
Growth Arrangement
• Staph- clusters
• Strept- chains
• Diplo- pairs
Metabolism
• Aerobic- requires oxygen to grow.
• Facultative anaerobes- can grow in both.
• Obligate anaerobes- cannot grow in oxygen.
• Ex: botulism, tetanus
• Intestines or deep puncture wounds
Autotrophic Bacteria
Photoautotrophs- photosynthetic, use solar energy to reduce
CO2 to organic compounds
2 types:
1) anoxygenic photosynthesis- does not produce oxygen
Ex: green sulfur bacteria
2) oxygenic photosynthesis- produces oxygen
Ex: cyanobacteria
Autotrophic Bacteria
Chemoautotrophs- chemosynthetic, oxidize inorganic
compounds such as hydrogen gas, hydrogen sulfide,
and ammonia to reduce CO2 to organic compounds.
Heterotrophic Bacteria
Chemoheterotrophs- obtain carbon and energy in the form of organic
nutrients others produced.
Ex: feed on tissues and fluids of living host
• One type called saprotrophs serve as decomposers that break down
organic matter from dead organisms.
Chemoheterotrophs are used to make acetones, butter, cheese, silk,
coffee and some antibiotics.
Symbiotic Relationships
• 2 different species living together, 3 types
1. Mutualism- both benefit
• Live in intestines and release K and B12
2. Commensalism- one benefits, other is unaffected
• One usually modifies environment so other benefits
• Obligate anaerobes live in intestines only because E. coli
uses up oxygen
Symbiotic Relationships
3. Parasitism- one benefits, harming the other
• Pathogens
• They aren’t actually doing the harming, the toxins
they release are
• Ex: tetanus- releases toxin that travels throughout
body
Symbiotic Relationships
• Endospores- dried seed, deadliest pathogens produce
these
• Some of the cytoplasm and copy of chromosome
becomes dehydrated
• Can survive harsh environments, deserts, boiling temp.,
extreme UV rays, and can survive for long periods of time.
• Anthrax endospores from 1,300 years ago can still cause
serious infection
• Botulism from canned food
• Not means of reproduction but survival.
Antibiotics
• 2 classes:
1. Inhibit protein synthesis
• Erythromycin and tetracyclines affect ribosomes
2. Inhibit cell wall biosynthesis
• Penicillin, ampicillin
Antibiotics
• Heavily prescribed to treat infection, OFTEN when not
needed.
• Bacteria is becoming increasingly resistant to antibiotics
• WHY?
• They can even pass resistance to others by conjugation,
transformation, and transduction
• Ex: staphylococcus aureus was only 3% resistant to penicillin
when it came out. Now its over 90% resistant.
Cyanobacteria
• Gram- negative with unusual traits.
• Photosynthesize
• “blue-green algae”
• Now we know it is not an algae but a prokaryote
• Mask color of chlorophyll with other pigments to make it different
colors.
• No means of locomotion
How can something survive if it cannot move?
Cyanobacteria
• Can photosynthesize and fix atmospheric N2 so no need to move for
food.
• Can serve as food for some heterotrophs
• fresh and marine waters, soils, moist surfaces, hot springs, harsh
environments
• If care is not taken when disposing of wastes, phosphates build up
and drain into lakes and ponds, resulting in a “bloom” of this
organism.
• Light cannot penetrate to lower levels
• When cyanobacteria dies, the decomposer uses up all oxygen, killing
the fish