Virus

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Virus

• What do you know about virus?

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Virus characteristics

• Small
• Only view with an electron microscope

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LE 18-2
Virus
Bacterium
Animal cell
Animal cell nucleus
0.25 µm
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Virus characteristics

• Not alive
• Obligate intracellular parasites. Infect
  cells-turn into a virus factory.
• Computer virus analogy

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Virus Characteristics

• Contain only protein and nucleic acid (either DNA
  or RNA)
• Nucleic acid can be SS or DS-makes up virus core
• Core surrounded by a protein capsid. Shape of
  capsid gives the virus its characteristic shape.
  Some virus have envelope (animal virus
  only-produced by budding)
• Some virus have enzymes.

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LE 18-4a
Capsomere of capsid
RNA
18 ? 250 mm
20 nm
Tobacco mosaic virus
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LE 18-4b
Capsomere
DNA
Glycoprotein
7090 nm (diameter)
50 nm
Adenoviruses
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LE 18-4d
Head
DNA
Tail sheath
Tail fiber
80 ? 225 nm
50 nm
Bacteriophage T4
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Figure 18.02x2 Phages
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LE 18-4c
Membranous envelope
Capsid
RNA
Glycoprotein
80200 nm (diameter)
50 nm
Influenza viruses
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LE 18-9
Viral envelope
Glycoprotein
Capsid
RNA (two identical strands)
Reverse transcriptase
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Table 18.1 Classes of Animal Viruses, Grouped by
Type of Nucleic Acid
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Virus characteristics

• Virus are host specific
• A. animal virus
• B. plant virus
• C. bacterial virus (phage)
• Sometimes a virus can mutate so it can infect a
  new host (avian flu)

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Figure 18-01
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Virus characteristics

• Virus cause diseases
• Rabies
• Influenza
• Colds
• Etc.

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Figure 18-03
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LE 18-11
The SARS-causing agent is a coronarvirus like
this one (colorized TEM), so named for the
corona of glyco-protein spikes protruding form
the envelope.
Young ballet students in Hong Kong wear face
masks to protect themselves from the virus
causing SARS.
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Virus Life cycle

• Simplified virus life cycle (lytic). Virulent
  virus. What kind of cell can be infected by a
  virus (any-plant, animal, bacteria) Typical
  cycle-20-30 minutes.
• A. Attachment (specific-received e-mail)
• B. Entry (dont open that e-mail)
• C. Synthesis
• D. Assembly (spontaneous)
• E. Release

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LE 18-5
VIRUS
Entry into cell and uncoating of DNA
DNA
Capsid
Transcription
Replication
HOST CELL
Viral DNA
mRNA
Viral DNA
Capsid proteins
Self-assembly of new virus particles and their
exit from cell
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Enveloped Virus life cycle
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LE 18-8
Capsid
Capsid and viral genome enter cell
RNA
HOST CELL
Envelope (with glycoproteins)
Viral genome (RNA)
Template
mRNA
Capsid proteins
ER
Glyco- proteins
Copy of genome (RNA)
New virus
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LE 18-10
Membrane of white blood cell
HIV
HOST CELL
Reverse transcription
Viral RNA
RNA-DNA hybrid
0.25 µm
HIV entering a cell
DNA
NUCLEUS
Provirus
Chromosomal DNA
RNA genome for the next viral generation
mRNA
New HIV leaving a cell
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Treatment of viral diseases

• Infectious cycle becomes an uncontrolled chain
  reaction. How can you deal with it?
• A. Antibiotics? No
• B. Immune system. Cant respond quick enough
  first time you see virus (immunization). Colds,
  flu.
• C. Antiviral drugs-AIDS
• D. Restriction enzymes-if youre a bacteria.

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Why do virus infections cause disease symptoms?

• Damage to tissue-cell destruction (some cells
  regenerate (epithelium of upper respiratory
  tract), some cant (nerves-polio)
• Cause infected cells to produce toxins (infected
  cell becomes transformed)
• Viral cancer-oncogenes. Hep B-liver cancer
  Herpes-Burkitts lymphoma papovavirus-cervical
  cancer
• Immune system-inflammation, lymphokines

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Emerging virus

• Evolve and infect individuals who are only
  resistant to ancestral virus (influenza)
• Spread from species to species. Hanta virus-deer
  mice to humans.
• Spread from small populations to larger. AIDS
• Environmental disturbances can increase
  emergence. Roads into tropical rainforest,
  global warming, airplanes)

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Triple Re-assorted H1N1Influenza virus
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Lysogenic life cycle (temperate virus)

• Characteristics of the Lysogenic life cycle
• Steps Provirus inserts into infected cells
  genome. Repressor gene usually represses viral
  gene expression. Induction.
• Differences in lytic and lysogenic life cycle

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LE 18-7
Phage DNA
The phage attaches to a host cell and injects its
DNA.
Daughter cell with prophage
Many cell divisions produce a large population
of bacteria infected with the prophage.
Phage DNA circularizes
Phage
Bacterial chromosome
Occasionally, a prophage exits the bacterial
chromosome, initiating a lytic cycle.
Lytic cycle
Lysogenic cycle
Certain factors determine whether
The bacterium reproduces normally, copying the
prophage and transmitting it to daughter cells.
The cell lyses, releasing phages.
Lytic cycle is induced
Lysogenic cycle is entered
or
Prophage
Phage DNA integrates into the bacterial
chromosomes, becoming a prophage.
New phage DNA and proteins are synthesized and
assembled into phages.
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Consequences of Lysogeny

• none-(viral genetic info lies dormant)
• Transformation-makes cell cancerous (viral gene
  is an oncogene)
• Induction-M.S.? Herpes-

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Prions

• Infectious proteins
• Proteins are misfolded-cause other proteins to be
  misfolded
• Misfolded proteins impair cellular function
• Mad cow disease

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LE 18-13
Original prion
Prion
Many prions
New prion
Normal protein
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Origin of Virus?

• There are three main hypotheses regarding the
  origins of viruses
• a. The progressive, or escape, hypothesis states
  that viruses arose from genetic elements that
  gained the ability to move between cells
• b.The regressive, or reduction, hypothesis
  asserts that viruses are remnants of cellular
  organisms
• c. The virus-first hypothesis states that viruses
  coevolved with their current cellular hosts.

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Progressive hypothesis

• Genetic material moved from cell to cell via
  injured cell surfaces?
• Evolution of capsid proteins may have facilitated
  infection of undamaged cells.
• Candidates for sources of viral genomes?
• a. plasmids?
• b. Transposons? (Mobile genetic elements)

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Mirror image rna sequences

• Often associated with rna virus
• AAAAAAUUUUUXXXXXAAAAAAUUUUUU
• RNA folds back on itself
• DS RNA recognized by a system of enzymes and
  proteins (the COP)
• The COP then destroys an DS m-rna with these
  mirror image sequences
• RNai

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Fig. 18-13
Hairpin
miRNA
Hydrogen bond
Dicer
miRNA
miRNA- protein complex
3?
5?
(a) Primary miRNA transcript
Translation blocked
mRNA degraded
(b) Generation and function of miRNAs
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RNAi Video

• http//www.nature.com/nrg/multimedia/rnai/animatio
  n/index.html