Download Viruses

Chapter 23
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Agents that cause disease
Many microorganisms: bacteria, fungi,
protozoa
Bacteria are prokaryotes, but only a few are
pathogens; most are essential for life on
Earth
Viruses are all pathogens – they all cause
disease in something, although they don’t all
cause disease in us
Influenza virus
Ebola virus
Bacteriophage
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A nucleic acid core (either DNA or RNA but
not both)
A protein coat – capsid
Generally extremely small – much smaller
than bacteria
Acellular:
 No metabolic activities: cannot perform cellular
respiration or protein synthesis
 Cannot reproduce without the help of the host cell
they infect
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Most scientists do not consider them living
things
They are not classified in any domain or
kingdom
They are usually grouped according to the
type of nucleic acid and the presence or
absence of a capsid
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Escaped gene hypothesis:
 Most widely held idea
 They escaped from living cells – this explains why
each virus is only able to infect one particular type
of cell
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They evolved early in the history of life on
Earth, before the three domains separated
 This hypothesis has no formal name
 Viruses probably did not exist before their host
cells arose
Lytic cycle – the virus lyses the host cell by forcing it
to replicate viral particles
 Virulent – viruses that only reproduce this way
 Five steps:
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Attachment – attaches to specific receptors on the host
cell
2. Penetration – nucleic acid passes into the host cell
3. Replication and synthesis – host cell’s DNA is degraded
and the cell replicates viral nucleic acids and proteins
4. Assembly – newly synthesized viruses are put together
5. Release – lytic enzymes destroy the host cell’s
membrane and the new viruses are released
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Lysogenic cycle – the viral genome becomes
integrated into the host DNA
Temperate – these cells are not killed directly,
but instead replicate the viral nucleic acid as
they reproduce; these cells often exhibit new
properties
Four steps:
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4.
Attachment – attaches to specific receptors
Penetration – nucleic acid passes into host cell
Integration – viral nucleic acid is integrated
Replication – all nucleic acid is copied
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Viroid – very short strand of RNA without any
protein coat
 Cause a variety of plant diseases
 Hard to eradicate
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Prion – an infectious agent that is only
protein - consists of 208 amino acids
 ‘mad cow disease’