Viruses and Prions

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Viruses and Prions

• Bacteriophage and animal virus replication

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• Look at animation on course web site

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Bacteriophage Multiplication

• Lytic Cycle
• Ends with Cell lysis
• Death of host cell
• Lysogenic Cycle
• Viral Genome integrates into host chromosome
• Prophage viral DNA
• Result of lysogeny
• Phage immunity host is immune to phage of the
  same type
• Phage conversion host receives new properties
  (C. diphtheriae)

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Lytic Cycle Bacteriophage

• Attachment
• Phage attaches by tail fibers to host cell
• Penetration
• Phage lysozyme opens cell wall
• DNA is injected into cell
• Biosynthesis
• Replicate viral DNA
• Make viral proteins
• Maturation
• Assemble new virus particles
• Release
• Phage lysozyme breaks cell wall
• New virions are released

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Lysogenic Cycle Bacteriophage

• Integration
• Viral DNA becomes prophage
• Cell divides like normal replicating prophage DNA
  while it reproduces

Figure 13.12
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Animal Virus Multiplication

• Attachment
• Penetration
• Uncoating
• Biosynthesis
• Maturation
• Putting the virus together
• Release

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Animal virus attachment/penetration

• Naked virus
• No envelope
• Binds to surface of host cell and injects its DNA
  (similar to bacteriophage)
• Envelope virus
• Membrane fusion
• Binds to receptors and envelope fuses with the
  host membrane
• Envelope virus
• Phagocytosis
• Virus particles taken in by endocytosis

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Animal Virus Multiplication

• Attachment
• Penetration
• Uncoating
• Biosynthesis
• Maturation
• Putting the virus together
• Release

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Biosynthesis and maturation

• Single stranded DNA
• DNA enters the nucleus
• First produce ds DNA in nucleus
• Replication make more DNA
• Transcription in nucleus make RNA
• Proteins made in the cytoplasm from mRNA
• Proteins transported to the nucleus
• Viruses assembled in the nucleus with single
  stranded DNA

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Biosynthesis and maturation

• Double stranded DNA
• DNA enters the nucleus
• Replication and transcription in the nucleus
• Proteins made in the cytoplasm from mRNA
• Proteins transported into the nucleus
• Assembly of virus in the nucleus with dsDNA

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Biosynthesis and maturation

• strand RNA (sense RNA)
• Entry and stay in the cytoplasm
• Make strand RNA with RNA dependent RNA
  polymerase
• - strand RNA serves as template for more strand
  RNA
• strand RNA serves as mRNA
• Makes mRNA in the cytoplasm
• Assembly in the cytoplasm
• Packaged with RNA dependent RNA polymerase

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Biosynthesis and maturation

• - Strand RNA (antisense RNA)
• Entry and stay in the cytoplasm
• Make strand RNA with RNA dependent RNA
  polymerase
• strand RNA serves as mRNA and makes viral
  proteins in the cytoplasm
• strand also serves as a template to make
  strand RNA
• Assembly of virus in the cytoplasm
• Packaged with RNA dependent RNA polymerase

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Biosynthesis and maturation

• Double stranded RNA
• Entry and stay in the cytoplasm
• strand RNA serves as mRNA and makes viral
  proteins in the cytoplasm
• Makes RNA dependent RNA polymerase
• strand also serves as a template to make
  strand RNA
• - strand serves as a template to make strand
  RNA
• Assembly of virus in the cytoplasm
• Packaged with RNA dependent RNA polymerase

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Biosynthesis and maturation

• Retroviruses
• strand RNA viruses
• Reverse transcriptase makes DNA from RNA in the
  cytoplasm
• Then DNA moves to the nucleus
• (negative) Single strand DNA enters the nucleus
  and makes double stranded DNA
• DNA makes mRNA in the nucleus
• RNA makes viral proteins in the cytoplasm
• Assembly in the cytoplasm
• Reverse transcriptase packaged with the virus

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Animal Virus Multiplication

• Attachment
• Penetration
• Uncoating
• Biosynthesis
• Maturation
• Putting the virus together
• Release

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Animal Virus Multiplication

• Release
• Naked viruses
• Lyse the host cell
• Kills the host cell
• Virus particles released can infect other host
  cells
• Enveloped viruses
• Merge with the host cell membrane
• Form an envelope around the virus
• The cell is not lysed and not killed
• Cell continues to produce more viruses

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Cancer

• Cancer caused by viruses often goes unrecognized
• Oncogenes
• Portion of a genome that, when activated by
  cancer-causing alterations in DNA, causes
  malignant tumors
• Oncogenic viruses
• 10 of cancers are virus induced
• Virus genetic material integrates in hosts
  chromosome and replicates causing host cells to
  go through a transformation
• Transformed cells usually have chromosomal
  abnormalities, increased growth and express T
  antigen on their surface

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Prions

• Infectious proteins
• A gene exists in host cell normally as PrPc
• When host is infected with PrPSc (abnormal prion
  protein) it transforms PrPc to PrPSc
• PrPSc aggregates in nerve cells and eventually
  impairs function and later kills those cells
• Requires dry heat to decontaminate

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Prions
Figure 13.22
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Prions

• Diseases usually transmitted via contact with
  nerve tissue
• Examples of diseases causes by PrPSc
• Scrapie in sheep
• Exact mechanism of transmission among sheep is
  unknown
• Bovine spongiform encephalitis in cows (BSE)
• Possibly transmitted to cows from feed that was
  fortified with bone marrow from sheep
• Diseases in Humans
• Kuru in old New Guinea tribes
• Cannibalistic ritualscontact with brain matter
• Creutzfeldt-Jakob disease (CJD)
• Known cases include
• Contamination during corneal transplants (CJD)
• Contamination from eating beef from cows with BSE
  (vCJD)