Lecture 9 Viruses, Viroids, Prions

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Lecture 9 Viruses, Viroids, Prions
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Viruses

• Parasites that are not cells or organisms
• Infect all forms of lifeBacteria, Archea,
  Eukarya
• Virus particle called virion
• Comprised of two parts
• 1. Nucleic Acid
• 2. Protein coat (capsid)
• Nucleocapsid- capsid with nucleic acid inside
• Each capsid made of identical protein subunits
  called capsomeres

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Virus Shapes
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Two basic types of virions

• Naked consist only of nucleic acid and capsid
• Enveloped virus consists of nucleic acid,
  capsid, and envelope

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Virus attachment to host cells

• All viruses must be able to attach to specific
  receptors on host cells
• Attachment spikes project from the capsid or
  envelope
• In viruses with tails- tail fibers attach the
  virus to the host cell

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Virus size

• Approximately 100 to 1000 fold smaller than the
  cells they infect
• Size generally from 10 nm to 500 nm
• The smallest viruses contain very little nucleic
  acid, perhaps as little as 10 genes

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Viral Genome

• Structure of viral genome is unusual
• Contain only single type of nucleic acid- DNA or
  RNA
• DNA may be linear or circular, either
  double-stranded or single-stranded
• RNA is usually single-stranded

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Overview- Replication Cycle of Viruses

• Can only multiply within living cells that are
  actively metabolizing
• Viruses lack cellular components necessary to
  harvest energy and synthesize proteins
• Viruses must use structures and enzymes of cells
  they infect to support their own reproduction-
  considered parasites

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Multiplication of Bacteriophages
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Bacteriophage

• Virus that infects bacteria
• Excellent model
• for other bacteria

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Two possible outcomes following viral infection
of a bacterial cell

• Lytic Infection viruses multiply inside the
  cells they invade
• Lysogenic Infection integrate viral DNA into
  host cell chromosome the virus DNA replicates as
  the bacterial chromosome replicates

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Replication of lytic phages

• Phage nucleic acid enters the bacterium and
  capsid remains outside
• Nucleic acid replicated along with phage proteins
• Many virions are formed
• Phages exit by bursting the cell
• Phages that go through this life cycle are called
  virulent
• Virulent has the ability to overcome host
  defenses and cause disease

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Lysogenic Infection

• The viral DNA is incorporated into the host
  genome
• Each time the host chromosomes replicated and
  split into new cells, so is the viral DNA
• Phage may excise itself from the chromosome and
  later and revert to lytic growth

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Host range of phages

• Host range number of different bacteria that a
  particular phage can infect
• Two factors determine the host range of a phage
• Phage must be able to attach to receptors on host
  cell surface
• The restriction modification system of the host
  cell

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Receptors on bacterial surface

• Receptors vary in chemical structure and location
• Receptors are usually on cell wall, although a
  few phages attach to pili or flagella
• Receptor sites can be modified, thereby creating
  a resistant cell
• Some temperate phages can alter the cell surface,
  an example of lysogenic conversion
• As a result receptor no longer available
• Thus, prophage protects its host and, in turn,
  is able to keep replicating inside of it

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Restriction Modification System

• In some bacteria to protect themselves from viral
  infection
• Bacterial cell makes restriction enzyme and
  methylating enzyme
• Methylating enzyme adds methyl group to bacterial
  DNA
• Bacteria now knows this is its own DNA
• Uses restriction enzyme to cut any DNA that is
  not methylated
• Cuts viral DNA- inactivating it

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Multiplication of Animal Viruses
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Multiplication of Animal Viruses

• Similar to bacteriophage replication
• Animal viruses attach to host plasma membrane via
  spikes on the capsid or envelope
• Animal viruses are usually taken into the
  cytoplasm as intact nucleocapsids
• Uncoating is the separation of the capsid from
  the genome

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Entry of Animal Viruses into their Host Cells
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Interactions of Animal Viruses with their hosts

• Acute Infections
• Latent Viral Infections
• Persistant Viral Infections

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Latent Viral Infections

• When acute infection followed by symptomless
  period and then reactivation of disease
• Symptoms of the initial and reactivated diseases
  may differ
• Provirus latent form of virus in which viral DNA
  incorporated into host DNA

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Examples of Latent Infections-HSV-1
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Examples of Latent Infections-varicella zoster
virus

• Initial infection of children- chicken pox
• Then can remain latent for years with no disease
  symptoms
• Can be reactivated and cause shingles
• Chicken pox and shingles- different diseases
  caused by same virus

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Persistent Viral Infections

• Occurs gradually over a long period
• Example Measles
• Several years after contracting measles can get
  Subacute sclerosing panenchaphalitis

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Retroviridae

• Family of viruses that carry their genetic
  information as ssRNA
• Have enzyme reverse transcriptase which forms a
  DNA copy that is then integrated into the host
  genome

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Viral-Induced Tumors

• Tumor- results from abnormal growth of cells
• Benign tumor- growth remains within defined
  region, not carried to other areas
• Malignant tumor- when abnormal growth spreads to
  other parts of body
• Tumor-causing viruses- Oncogenic Viruses

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Viral-Induced Tumors

• Proto-oncogenes- normally occurring genes
• They can be converted to oncogenes by
• radiation
• chemical carcinogens
• DNA damage
• Viruses
• Oncogenes then cause tumors

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Viriods

• Much smaller than viruses
• Just consist of small ssRNA molecule
• No protein coat
• Infect plants

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Prions

• Proteinaceous infectious agents
• Contain only protein, no nucleic acid
• Linked to number of fatal diseases in humans and
  animals
• Obligate intracellular parasite
• How does it replicate if no nucleic acid?
• Prion protein converts host protein to prion
  protein

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Prions

• Cannot be killed by UV light or nucleases, can be
  killed by proteases and heat
• Usually cannot be transmitted across species

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