Lecture 29: Viruses

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Lecture 29 Viruses
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Lecture outline 11/11/05

• Types of viruses
• Bacteriophage
• Lytic and lysogenic life cycles
• DNA viruses
• RNA viruses
• Influenza
• HIV
• Prions
• Mad cow disease

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Figure 18.4 Viral structure
Capsomereof capsid
Membranousenvelope
RNA
Capsomere
DNA
Head
Capsid
Tail sheath
DNA
RNA
Tail fiber
Glycoprotein
Glycoprotein
80 ? 225 nm
18 ? 250 mm
7090 nm (diameter)
80200 nm (diameter)
50 nm
20 nm
50 nm
50 nm
(d) Bacteriophage T4
(a) Tobacco mosaic virus
(b) Adenoviruses
(c) Influenza viruses
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Viral reproductive cycle
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A capsid is the protein shell that encloses the
viral genome
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Viral Envelopes are derived from the membrane of
the host cell
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Bacteriophage

• Viruses of bacteria have been studied for decades
• T1, T2, T4
• virulent
• Lambda
• temperate

See the animation
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The lytic cycle of T4
Attachment. binds to specificreceptor sites on
cell surface.
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Entry of phage DNA and degradation of host DNA.
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Release (lysis)
Phage assembly
Synthesis of viral genomes and proteins.
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Assembly of phage capsid
Head
Tail fibers
Tails
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The lytic and lysogenic cycles of phage ?
This is a temperate phage
Attachment and injection of DNA.
Phage DNA
Many cell divisions produce a large population
of bacteria infected with the prophage.
Phage DNA circularizes
Phage
Occasionally, a prophage exits the bacterial
chromosome, initiating a lytic cycle.
Lysogenic cycle
Lytic cycle
Replicates with host DNA
Certain factors determine whether
Lysis and release
or
Prophage
Integrated into host chromosome.
New phage particles synthesized
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Classes of Animal Viruses
Genome Type Viral coat Examples
ds DNA No Yes Herpes, chickenpox Smallpox
ss DNA no Parvovirus

dsRNA no Tick fever
ss RNA (serves as mRNA) no yes Rhinovirus SARS
ssRNA (template) yes Influenza Ebola
ssRNA (retrovirus) yes HIV
DNA Viruses
RNA Viruses
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Smallpox
nmhm.washingtondc.museum
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Influenza
One of the few viruses with genome in segments (8)
H5N1
Spikes of hemagglutanin And neuraminidase
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The reproductive cycle of an enveloped RNA virus
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Why are flu vaccines so hard to make?

• Flu strains are highly variable
• Recombination among the viral gene segments
• RNA polymerase has high mutation rate
• Now have some antiviral drugs (e.g. Tamiflu)
• blocks the neuramidase enzyme so virus isnt
  released from cell

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HIV
www.who.int/hiv/facts/en/
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The structure of HIV, the retrovirus that causes
AIDS
Only 9 genes in HIV Viral coat
proteins Reverse transcriptase Integrase Protea
se
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HIV reproduction
Reverse transcriptase synthesizes DNA from
RNA template .
HIV
Membrane of white blood cell
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HOST CELL
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Reverse transcriptase
Makes second DNA strand.
Viral RNA
RNA-DNAhybrid
Incorporated into host chromosome.
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0.25 µm
HIV entering a cell
DNA
NUCLEUS
Provirus
ChromosomalDNA
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New viral RNA is transcribed.
RNA genomefor the nextviral generation
mRNA
New viral proteins are produced.
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New HIV leaving a cell
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Reverse transcriptase is a special DNA polymerase

• 1. Copies DNA from an RNA template
• 2. Removes RNA template

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AZT
thymidine
azt

• Azidothymidine
• a modified thymidine
• The first anti-retroviral drug
• Stops DNA synthesis because it does not have a
  3OH
• Originally developed as an anti cancer drug, but
  too many side effects

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Protease inhibitors- another class of drugs for
HIV
Inhibitor in active site
Protein in active site
HIV initially produces one long polypeptide.
Protease is necessary to cut the polypeptide
into individual enzymes
www.chemistry.wustl.edu/edudev/LabTutorials/HIV/
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Prions are infectious mis-folded proteins
Starts a slow chain reaction, causing regular
proteins to assume the new shape
Altered PRP proteins in nerve cells cause Mad Cow
Disease