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Chapter 13-Viruses. Viroids, and Prions. 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?. History began with the Tobacco Mosaic Virus (TMV).
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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?
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
Common Shapes • Capsid coat made of capsomeres • Nucleic acid inside
Bacterial viruses • Known as bacteriophages or phages • Two different life cycles • Lytic cycle (lytic or virulent phage)-results in lysis of the cell • Lysogenic cycle (temperate or lysogenic phage)-may result in lysis of the cell or becomes a permanent part of the chromosome by integrating
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
Methods to study bacteriophage • Plaque Assay used to quantitate phage
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
Replication strategies • Watch the type of nucleic acid • What enzymes are needed for the process?
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
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
Can you identify some examples of viruses that produce an acute viral infection?
Persistent infections • Virus is continually present in the body, released by budding • Three categories • Latent infections • Chronic infections • Slow infections
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
All of these viruses are in the Herpesviridae family Latent Viral infections
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)
Herpes simplex-1 • HSV-1 causes fever blisters, HSV-2 genital herpes • Symptoms: fluid filled skin lesions • Treatment: Acyclovir
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
Epstein Barr • Causes infectious mononucleosis • Acquire by saliva, incubation period is 4-7 weeks • Identify by -lobed lymphocytes -heterophile antibodies -fluorescent antibody tests
Chronic infections • Infectious virus present at all times • Disease may be present or absent • Examples are Hepatitis Type B and Type C viruses
Type Hepadnaviridae family: Hepatitis B • dsDNA virus, enveloped • Hepatitis B -passes through intermediate stage (RNA) for replication -three particles found in blood sample 1. Dane 2. filamentous 3. sphericle -exposure through blood/body fluids
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
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
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
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
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
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 Viruses and tumors
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