T4 bacteriophage infecting an E. coli cell

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T4 bacteriophage infecting an E. coli cell
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Science as a Process

• Research into tobacco mosaic disease led to the
  conclusion that the pathogen was smaller than a
  bacterial cell
• The pathogen was named virus
• Characteristics of viruses
• Smaller than bacteria
• Not cellular
• Composed of nucleic acid and protein
• Obligate intracellular parasites

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Comparing the size of a virus, a bacterium, and
an animal cell
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Infection by tobacco mosaic virus
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Figure 18.4 Viral structure
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Capsids and Envelopes

• Capsid

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Capsids and Envelopes

• Capsid protein coat that surrounds the viral
  genome
• viral envelope

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Capsids and Envelopes

• Capsid protein coat that surrounds the viral
  genome
• viral envelope derived from host cell or
  nuclear membranes, it helps the virus invade

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Viral Genome

• Double stranded DNA
• Single Stranded DNA
• Double stranded RNA
• Single stranded RNA
• A virus has only one of these types of nucleic
  acids

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Viral Replication

• What are the possible patterns of viral
  replication?
• DNA --gt DNA
• RNA --gt RNA, where viral genes code for viral RNA
  and proteins (class IV and V)
• RNA --gt DNA --gt RNA where viral gene uses
  reverse transcriptase to create a provirus in
  the nucleus that does not leave host cellviral
  RNA and protein is also made
• (class VI)

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Bacterial Viruses

• Which scientists used bacteriophages to prove
  that DNA was the hereditary material?
• Hershey and Chase
• What are the two mechanisms of phage infection?
• Lytic and Lysogenic cycles (of DNA viruses)

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Lytic Cycle

• Virulent phage example T4 phage
• Steps
• Attachment
• Entry of phage DNA and degradation of host DNA
• Synthesis of viral genome and protein
• Assembly
• Release (host cell dies while releasing
  100-200 phages)

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Lysogenic Cycle

• Temperate phage example?lambda phage
• Steps
• Entry
• Integration of viral DNA into bacterial
  chromosome creating a prophage
• Bacterium reproduces normally copying prophage
  and transmitting it to daughter cells
• Under certain environmental conditions, a
  switchover to lytic cycle is triggered
• Other prophage genes may alter hosts phenotype
  and have medical significance
• Ex. bacteria causing diphtheria is harmless
  unless infected by a phagephage experiences a
  lysogenic cycle and prophage causes host cell to
  make a toxin that causes illness!

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Bacterial Defense

• What defense do bacteria have against phage
  infection?
• Restriction enzymes (a.k.a. ? restriction
  endonucleases)
• What do restriction enzymes do?
• They cut up DNA. The bacterial DNA is modified to
  protect it from the restriction endonucleases.

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Animal Viruses

• What is the viral envelope?
• An outer membrane (outside of the capsid) that
  helps the virus to invade the animal cell.
• The invasion of the virus has the following
  stages ...

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1. Attachment 2. Entry 3. Uncoating 4. RNA
and protein synthesis 5. Assembly and release
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Herpes virus

• Consists of double stranded DNA
• Envelope derived from host cell nuclear envelope
  not from plasma membrane
• It, therefore, reproduces within the nucleus
• May integrate its DNA as a provirus (becoming
  like mini-chromosomes in nucleus)
• Tends to recur throughout lifetime of infected
  individual. Often triggered by environmental
  situations.

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RNA Viruses

• Different classes of RNA viruses single stranded
  range from class IV to class VI
• Class IV invades as mRNA, is ready for
  translation
• Class V RNA serves as template for mRNA
  synthesis
• Class VI Retrovirus ? RNA ? DNA (using enzyme
  reverse transcriptase)? RNA

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The structure of HIV, the retrovirus that causes
AIDS
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The reproductive cycle of HIV, a retrovirus
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Reasons for success of HIV

• Has an envelope
• Creates a provirus which stays in the nucleus of
  the host cell
• Is an RNA virushigh rate of mutation

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Viral Disease

• Some viruses have toxic components
• Some cause infected cells to release enzymes from
  lysosomes
• Recovery involves ability to repair damaged
  region of the body.

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Vaccines / Drugs

• What are vaccines and how do they work?
• Introduce body to harmless or weakened strain of
  the virus, so that your immune system learns to
  recognize the virus prior to invasion
• Few drugs around to fight viruses, most interfere
  with DNA, RNA or protein synthesis
• Often mimic nucleosides that would allow for
  nucleic acid synthesis
• Ex. AZT ? HIV replication
• acyclovor ? herpes

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Emerging Viruses

• HIV, Ebola, SARS, West Nile Virus, Influenza,
  Hantavirus
• From where do these viruses emerge?
• From mutated versions of current viruses
• Jump from current host to new host
• Move from a previously isolated region of the
  world

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SARS (severe acute respiratory syndrome)
(a) Young ballet students in Hong Kong
wear face masks to protect themselves from
the virus causing SARS.
(b) The SARS-causing agent is a coronavirus
like this one (colorized TEM), so named for the
corona of glycoprotein spikes protruding
from the envelope.
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Viroids and Prions

• Viroids are naked circular RNA that infect plants
• Prions are proteins that infect cells (cause
  tangles of proteins in brain)
• Examples of prions seen in scrapies in sheep,
  mad-cow disease, and Creutzfeldt-Jakob disease
  (CJD) in humans
• Timeline of Mad Cow Disease Outbreaks
• How can a prion spread infection?
• Altered versions of proteins that can alter other
  proteins (altered protein is thought to be a
  result of a mutated gene)
• Or can be ingested by eating contaminated meats

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Figure 18.13 Model for how prions propagate
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Viral Evolution

• How did viruses evolve?
• Because viruses depend on cells for their own
  reproduction, they most likely evolved after the
  first cells appeared.
• Possible link to mobile genetic elements.
  (transposons, plasmids)
• Much debate about viral evolutionlots to learn
  about viruses!