Download Bacteria and Viruses Bacterial Cells Bacterial Genome Bacterial

10/30/2014
Bacteria and Viruses
Date __________
BACTERIA
Bacterial Cells
• Prokaryotic
• No membrane-bound
organelles
• Cell wall with
peptidoglycan
• Much larger than
viruses, but smaller
than eukaryotes
Bacterial Reproduction
• Called binary fission
• Two identical daughter
cells formed, but not by
mitosis
Bacterial Genome
• Double-stranded,
circular DNA
– 1/1000 as much as
eukaryotes
– 100x more than viruses
• Densely packed in
nucleoid region
Bacterial Reproduction and
Genetic Diversity
• Bacteria change and evolve by…
– Mutations: occur often due to high replication rate; if
one mutation occurs, all future offspring will have it
– Transformation: cell absorbs DNA from its
environment that may have originated from another
ruptured cell
– Transduction: insertion of DNA by a virus
– Conjugation: two live bacteria connect using external
pili and pass a plasmid between the two of them
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Plasmid
Treating Bacterial Infections
• Small, circular, self-replicating segment of DNA
in bacteria
• Not necessary for survival, but can provide
benefits like antibiotic resistance
• May have originated as a virus
• Two types
• Much easier than viruses
• Antibacterial agents can attack the bacteria
without affecting our cells
• Antibiotics may target bacterial…
– Cell walls
– Protein synthesis
– DNA synthesis
– Ribosomes
– And so on…
– R plasmid: antibiotic resistance
– F plasmid: allows bacteria to pass DNA using pili
What are Viruses?
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We’re not sure!
Pieces of nucleic acid wrapped in a protein coat
No ribosomes or organelles
Cannot live independently
– Have no machinery for reproduction or metabolism
• They aren’t usually considered alive because…
VIRUSES
– They aren’t made of cells
– They can’t reproduce on their own
– They don’t grow or develop
Viral Body Plans
• Inner core: genetic material is DNA
or RNA, various enzymes
• Covering: coat is protein , some
have envelope
– Envelope is composed of
glycoproteins that bind to specific
receptors on the host cell surface
– Sometimes the envelope is formed
from the host’s membrane as the
viral capsids exit
Viral Genome
Helical virus
Polyhedral virus
• May be DNA or
RNA
• Can be single or
double-stranded
• Usually short and
circular
Complex virus
(bacteriophage)
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Examples of Viruses
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Cold (rhino virus)
Flu (influenza)
West Nile
HIV
Chicken pox
Smallpox
Ebola
Herpes
HPV
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Rabies
Polio
Meningitis (viral)
Measles
Mumps
Tobacco mosaic virus
• Also affect plants,
bacteria, and archaea –
extremely widespread
Viral Reproduction
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Uses host to replicate
Occurs extremely rapidly
DNA viruses use DNA replication
RNA viruses use transcription
There are two “phases” of viral reproduction
– Lytic cycle
– Lysogenic cycle
Lytic Cycle
• Host copies and assembles new viruses
• New viruses burst free from the cell
• Makes a virus virulent
Lysogenic Cycle
• Virus incorporates itself into genome
• Gets copied along with the host genome
• Can alter the function of the host
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Retroviruses
• RNA viruses like HIV must first make DNA from RNA
to enter lysogenic cycle
• Reverse transcriptase accomplishes this so DNA can
be inserted
• DNA inserts into host DNA; proteins are assembled,
and virus is released
• Hard to make vaccines because RNA
mutates easily and each virus can be
slightly different from the others
• Drugs like AZT fight this action
How do vaccines work?
• Think of it like this:
– You get sick with a virus and your body develops immunity
– The virus isn’t GONE, but your immune system suppresses it
• Vaccines are made by growing a weakened or killed form
of the virus
– This is injected into a person’s body, where it doesn’t make
them sick
– However, there are particles that the immune system can react
to, and it does – but you don’t get sick because it’s not alive
– You have immunity to it as if you were actually infected
• SO…if you get a flu shot and get sick…guess what? It
wasn’t with the type of flu you got the shot for.
Where Did they Come From?
• We don’t know!
• 3 main hypotheses, each with some support:
– Cells that lost other genes and structures
necessary for independent survival via evolution
– Evolved from plasmids or transposons (jumping
genes)
• Transposons, or “jumping genes”, can be copied/cut
and pasted throughout the genome. They can change
genes or introduce pattern repetition in the genome.
– Co-evolved with the first forms of life
Very Difficult to Fight Viruses
• There’s nothing to attack!
• Almost everything they use belongs to the
host
• Only mechanisms are vaccines to prevent
viruses from entering cells and preventing
reverse transcription
Viral Evolution
• Viral evolution occurs very
quickly because they reproduce rapidly
• Very little proofreading of errors (especially
RNA viruses)
• 2 viruses in the same host can combine
• Where did they come from?
– We believe that viruses are ancient, possibly as
old as life itself
– They likely don’t share a common ancestor – there
are millions of them!
Why Should We Care About Viruses?
• Effects on human health
– Contagious disease pathogens must directly contact a new host. They
may cause…
• Epidemics (incidence rate exceeds expected number), pandemics
(worldwide epidemic), sporadic (“singular”) cases, and endemic
(maintained in a population) cases
– Viruses may be responsible for up to 8% of our normal DNA
• Role in ecosystems
– Viruses kill bacteria (estimated to kill about 20% of the biomass of the
ocean every day)
– Recycle nutrients and materials, prevent overgrowth of bacteria
• Important role in genetics and evolution
– Viruses carry genes from one individual to another and are very
important in increasing genetic diversity
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Emerging Viruses
• Viral strains can jump species, such as avian
flu
• Emerging pathogens include the Ebola virus,
bird flu, and SARS
• There are drug-resistant strains
• These could be mutations of known viruses,
exposed to new, developing areas with new
hosts
• Hosts and pathogens are always coevolving
Not-Quite-Viruses
• Viroids
– Smaller than viruses – just strands or circles of RNA
– Cause many plant diseases
– Horizontal transmission via damaged cell walls or vertical
transmission when inherited from parent
• Prions (proteinaceous infectious particle)
– Small proteins that are folded incorrectly; simple have to
TOUCH another protein to convert it to a prion
– Linked to human genetic diseases like Creutzfeldt-Jakob
disease (CJD) and animal diseases like bovine spongiform
encephalopathy (mad cow disease)
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