(17) Viruses Power Point CP Used
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Transcript of (17) Viruses Power Point CP Used
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Intro Questions
1. What are viruses?
2. What is special about viruses?
3. What are some common viruses?
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Chapter 19 Viruses
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What Is a Virus?
* particles ofnucleic acid, protein, and occasionally lipid
(DNA or RNA core surrounded by a protein coat)
capsid a viruss outer protein coat (allows the virus to enterthe host cell like a spy)
* reproduce only inside living cells
* using the resources of the host cell to produce
more viruses
Section 19-3
Section Outline
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How do viruses infect their hosts?
*viruses are usually host specific and onlywork on one type of cell in the host (see pages
484-485 in text)
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T4 BacteriophageTobacco Mosaic
VirusInfluenza
VirusHead
Tail
sheath
DNA
Tail
fiber
RNACapsid
Surfaceproteins Membrane
envelope
RNA
Capsid
proteins
Section 19-3
Figure 19-13 Virus Structures
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Types of Viruses
Depends upon whether it has DNA orRNA,double stranded or single stranded
1. double-stranded DNA ex: Chicken Pox
2. single-stranded DNA ex: Roseola3. double-stranded RNA ex: Reovirus,plant viruses
4. single-stranded RNA ex: HIV, Ebola
Called a DNA-like or RNA-like virusdepending on the nucleic acid found
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Viral Replication
Viruses- obligate intracellular parasites must have ahost cell!
Viruses lack the enzymes for metabolism and dont haveribosomes
Viruses can not make their own proteins
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Viral reproductive cycle - Overview
Virus uses hosts machinery
to replicate viral genome
Hosts machinery used to
make capsid proteins
Use transcription and
translation.
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5 Basic Steps of Virus Replication
1. Attachment Virus attaches to host
2. Penetration Injects DNA or RNA
3. Replication and Synthesis hijacks host machinery tomake new DNA/RNA
4. Assembly puts viral DNA/RNA into capsids,
envelopes
5. Release New Viruses Lyses (break out of) cell,
releasing more viruses to infect new cells
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*2 types of pathways to infection:
1. lytic cycle
(quick takeover, destroys some cells inorganism)
2. lysogenic cycle
(delayed takeover, reproduces itself and
waits)
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Lytic InfectionQuick like the common cold
1. Virus attaches to host cell2. Virus enters host cell (injects genome into host)
3. Makes copies of itself and new protein coats by
hijacking host cells metabolic machinery
4. Causes the host cell to burst (lyse) and die5. Host cells death releases the new virus
particles which can infect other cells
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Bacteriophage enzyme lyses the
bacteriums cell wall, releasing
new bacteriophage particles that
can attack other cells.
Bacteriophage proteins
and nucleic acids
assemble intocomplete
bacteriophage particlesBacteriophage takes over
bacteriums metabolism,
causing synthesis of new
bacteriophage proteins and
nucleic acids
Bacteriophage injects
DNA into bacterium
Bacteriophage attaches to
bacteriums cell wall
Bacteriophage
Bacteriophage DNA
Bacteriophage protein
Bacteriophage
protein coat
Bacteriophage DNA
Bacterial
chromosome
Lytic Cycle
Section 19-3
Figure 19-14 The Lytic Cycle
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1. Virus attaches to host cell2. Virus enters host cell
3. Joins its DNA to the host DNA (this viral DNA nowembedded in hosts DNA is called a prophage)
4.Each time host cell DNA replicates, viral DNA is also
replicated; thus, viral DNA will be inside EVERY cellthat was produced after the viral DNA was inserted
5. When organisms immune system is low, viral DNAtakes over the cells machinery and makes copies ofitself and new protein coats by hijacking host cells
metabolic machinery6. Causes the host cell to burst (lyse) and die
7. Host cells death releases the new virus particles whichcan infect other cells
Lysogenic Cycle Virus Hides
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LysogenicCycle
Slower!
Section 19-3
Figure 19-14 The Lytic vs. LysogenicCycle
Go toSection:
Viral genome hides out until conditions
are right then lyses (exits) the cell
Viral genome
entering host
incorporated as a
prophage in to
host genome
This way, viral
genome is
replicated with eachhost cell division;
more viruses
released
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Animal Viruses
Many animal viruseshave a membrane like
envelope present.
This envelope is a
lipid bilayer with
glycoproteins for
attachment.
Viral envelope comes
from the host cells
plasma membrane, sohost cell may not be
killed upon viral exit
Ex: Herpes
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Viruses and Disease
Viral diseases cannot be cured once caught must suffer through symptoms until body can fightoff infection
a. Most symptoms are caused by the toxinsreleased when host cell lyses (bursts)
b. Prevention is best medicine:vaccines to stimulate immunity
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So why arent all cells wiped out?
Natural Selection favors mutations with receptor sites
that viruses cannot attach to
Cellular enzymes of cell may break down virus before it
can replicate and take over. Restriction Nucleases actscissors for foreign DNA
BUT, Natural Selection also favors viral mutants that
are resistant to destruction
So virus-host relationship is in constant evolutionary
combat Red Queen Hypothesis
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Prions
-- Protein Infectious Particles
-- Contain no DNA or RNA, yetcause disease like mad cow
Ex. Mad cow,
Jacob-Crutzfeld
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Are Viruses Alive?NO!
They do not have all the characteristics of life(cellular structure, ability to reproduce independently),so they are not alive.
They can do what they do because they share thegenetic code with living things and affect living
things, but they are not considered to be alive.