chapter 13-viruses. viroids, and prions. general characteristics of all viruses contain a single...
TRANSCRIPT
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Chapter 13-Viruses. Viroids, and Prions
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General Characteristics of all viruses
• Contain a single type of nucleic acid
• Contain a protein coat
• Obligate intracellular parasites
• Are viruses the only known obligate intracellular parasites?
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History began with the Tobacco Mosaic Virus (TMV)
• 1886 Aldolf Mayer showed that a virus was transmissable between plants
• 1892 Iwanowski tried to isolate it by filtering with porcelain filter
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Sizes of viruses
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Common Shapes
• Capsid coat made of capsomeres
• Nucleic acid inside
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Two different types of Viruses
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Relationship of virus with host cell
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Bacterial viruses
• Known as bacteriophages or phages
• Two different life cycles1. Lytic cycle (lytic or virulent phage)-results in
lysis of the cell
2. Lysogenic cycle (temperate or lysogenic phage)-may result in lysis of the cell or becomes a permanent part of the chromosome by integrating
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T4 phage replication
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Lambda Phage replication
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Lambda integrates into the chromosome
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Properties conferred by prophage
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Some phage are filamentous
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Replication of filamentous phage
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M13 is ssDNA…how does it replicate the ssDNA?
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How do bacteria protect themselves against phage?
• Prevent phage attachment
• Attacking foreign DNA with restriction enzymes, protecting native DNA with methylation
• CRISPR system degrades incoming viral nucleic acid
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CRISPR defense system against phage
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Methods to study bacteriophage
• Plaque Assay used to quantitate phage
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How do animal viruses differ from bacterial viruses?
• Attachment or entry into the cell
• Replication of viral nucleic acid (remember eukaryotic cells have a nucleus)
• Uncoating step is required by animal viruses
• Exit the host cell by budding or shedding
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Effects of animal virus on cells
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Entry of animal virus
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Replication strategies
• Watch the type of nucleic acid
• What enzymes are needed for the process?
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Release of enveloped viruses
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Acute viral infections
• Usually short in duration
• Host develops long lasting immunity
• Infection of the virus results in a productive infection…host cells die as a result of infection
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General Steps of Acute Viral infection
• Attachment• Entry into host cell• Targeting where it will reproduce• Uncoating of the capsid• Synthesis of proteins, replication of nucleic acid• Maturation• Cell lysis
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Can you identify some examples of viruses that produce an acute viral infection?
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Persistent infections
• Virus is continually present in the body, released by budding
• Three categories– Latent infections– Chronic infections– Slow infections
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Persistent: Latent Infections
• Persistent infection with symptomless period followed by reactivation of virus and symptoms
• Example of latent viruses are found in the family Herpesviridae – Herpes simplex virus -1– Herpes simplex virus -2
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Latent Viral infections
• All of these viruses are in the Herpesviridae family
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Herpesviridae Family
• Double stranded DNA (dsDNA), enveloped viruses
-herpes simplex virus type 1(cold sores)
-herpes simplex virus type 2 (genital herpes)
-Varicella-zoster virus (chicken pox, shingles)
-Epstein-Barr (infectious mono and Burkitt’s lymphoma)
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Herpes Simplex virus-1
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HSV-1 reactivation
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Herpes simplex-1
• HSV-1 causes fever blisters, HSV-2 genital herpes
• Symptoms: fluid filled skin lesions
• Treatment: Acyclovir
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Varicella (chickenpox) and Herpes Zoster (Shingles)
• HSV-3 causes chicken pox and latent activation known as shingles
• Acquired by respiratory route, 2 weeks later see vesicles on skin
• Vaccine established in 1995 for chickenpox
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Epstein Barr
• Causes infectious mononucleosis
• Acquire by saliva, incubation period is 4-7 weeks
• Identify by
-lobed lymphocytes
-heterophile antibodies
-fluorescent antibody tests
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Chronic infections
• Infectious virus present at all times
• Disease may be present or absent
• Examples are Hepatitis Type B and Type C viruses
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Type Hepadnaviridae family: Hepatitis B• dsDNA virus, enveloped• Hepatitis B
-passes through intermediate stage (RNA) for replication-three particles found in blood sample1. Dane2. filamentous3. sphericle-exposure through blood/body fluids
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Replication of Hepatitis B
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Hepatitis Type B
• Incubation period is ~12 weeks
• 10% of cases become chronic, mortality rate is less than 1%
• About 40% of the chronic cases die of liver cirrhosis
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Flaviviridae Family: Hepatitis Type C
• Hepatitis C virus– (+) ssRNA virus, enveloped– Obtain from blood/body fluids– Incubation period averages 6 weeks– Hard to screen blood for the virus– 85% of all cases become chronic
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What other types of Hepatitis viruses are known to infect humans?
• Hepatitis Type A– Found in the Picornaviridae family (+) ssRNA
-obtain through fecal-oral route, enters GI tract and multiplies
-incubation period is ~4 weeks
-symptoms include: anorexia, malaise, nausea, diarrhea, abdominal discomfort, fever, and chills lasting 2-21 days
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Slow Infections
• Infectious agent increases in amount over a long time during which there are no symptoms
• Examples are HIV found in the Retroviridae family
• Retroviruses use reverse transcriptase to replicate ssRNA
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Retroviridae-multiple strands of (-)RNA
• HIV
-infects Helper T cells
-requires the enzyme reverse transcriptase
-integrates as a provirus
-is released by budding, or lyses the cell
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HIV replication
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Viruses associated with cancers
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Viruses and tumors
• dsDNA viruses are most common to cause viral-induced tumors
• Cancer is result of integration of viral genes into the host chromosome
• Transforming genes are called oncogenes
• Examples: papillomavirus, herpesvirus
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Orthomyxoviridae-multiple strands of (-)RNA
• Influenza virus– Consists of 8 segments of RNA– Envelope has H spikes (hemagglutinin) and N
spikes (neuraminidase)– Incubation is 1-3 days– Symptoms include: chills, fever, headache,
muscle aches, may lead to cold-like symptoms
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Influenza virus
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If multiple forms infect one cell…reassortment can
occur
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Antigenic shift vs antigenic drift
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Ways to study viruses
• Since viruses grow in living cells….need a live cell to culture them– Cell culture/tissue culture– Embryonated chicken eggs
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Cell Culture
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Proteinaceous infectious particles: PRIONS
• 1982 Stanley Prusiner proposed that there were infectious proteins
• Caused the disease “scrapie” in sheep
• Caused the “mad-cow”disease in 1987
• Human forms suggest a genetic component
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Prions
• Contain no nucleic acid
• Abnormal protein promotes conformational change to normal protein
• Results in damage to neurons…transmissible spongiform encephalopahthies
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Brain with spongiform encephalopathy
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Infections caused by prions
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Mechanism of prion
replication
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Rhabdoviridae (-)ssRNA, enveloped
• Rabies virus
-enters the skin and multiplies in skeletal muscle and connective tissue
-virus travels along nerves to the CNS causing encephalitis
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Pathology of rabies