Chapter 13-Viruses

General Characteristics of all viruses Contain a single type of nucleic acid Contain a protein coat Obligate intracellular parasites Are viruses the only 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

Sizes of viruses

Polyhedral virus Capsid coat made of capsomeres Nucleic acid inside

Helical virus with an envelope The shape is a long rod Rabies and Ebola are helical viruses Influenzae virus is helical with an envelope

Bacteriophage: Complex virus

Bacterial viruses Known as bacteriophages or phages Two different life cycles 1.Lytic cycle-results in lysis of the cell 2.Lysogenic cycle-may result in lysis of the cell or the virus becomes a permanent part of the chromosome by integrating

Lytic Cycle

Growth curve of bacteriophage

Lysogenic Cycle

How can you study bacteriophages? Plaque assay -pour agar with bacteria and phage on top of an agar plate -“plaque” develops where virus infected bacterial cell -each plaque is counted as one virus

How do animal viruses differ from bacterial viruses? Attachment Replication of nucleic acid Penetration Uncoating

DNA Animal Viruses Adenoviridae –dsDNA, non-enveloped –First isolated in the adenoids –Cause upper respiratory infections

DNA Animal Viruses Poxviridae –dsDNA, enveloped –Cause small pox (variola)

DNA Animal Viruses Poxviridae –dsDNA, enveloped –small pox virus (variola)

DNA Animal Viruses Herpesviridae (dsDNA, enveloped virus) -simplex 1(cold sores) -simplex 2 (genital herpes) -chicken pox, shingles -epstein barr

Herpes simplex-1 HHV-1 causes fever blisters, HHV-2 genital herpes Symptoms: fluid filled skin lesions Treatment: Acyclovir

Varicella (chickenpox) and Herpes Zoster (Shingles) HHV-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

Hepadnaviridae dsDNA, enveloped Hepatitis B -passes through intermediate stage (RNA) -three particles in blood Dane filamentous sphericle -exposure through blood/body fluids

Hepatitis 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

RNA animal viruses Is there an enzyme in animal cells to replicate RNA? What does RNA polymerase do?

RNA animal viruses (+) single stranded RNA viruses –RNA serves as mRNA (-) single stranded RNA viruses –RNA does not code for proteins

Picornaviridae (+) ssRNA Poliovirus Virus ingested then travels throughout the body In some cases it impairs the upper motor neurons, less than 1% of all cases Vaccines –Salk vaccine (IPV) –Enhanced-inactivated polio (E-IPV) –Sabin vaccine

Cases of Poliomyelitis in US

Picornaviridae (+) ssRNA Rhinovirus -causes the common cold -100 or more serological types -virus grows best in the nose and conjunctiva

Picornaviridae (+) ssRNA Enterovirus responsible for 90% of viral gastroenteritis –Rotavirus Most common cause of viral gastroenteritis –Norwalk-like virus Responsible for local epidemics

Rotavirus Note the shape which gave it the name rota=wheel

Picornaviridae (+) ssRNA Hepatitis A - 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

Flaviviridae (+) ssRNA, enveloped Hepatitis C virus –Obtain from blood/body fluids –Incubation period averages 6 weeks –Hard to screen blood for the virus –85% of all cases become chronic

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

Pathology of rabies

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

Influenza virus

Retroviruses: convert RNA to DNA HIV, Hepatitis B

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

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

Prions: How do they replicate?

Can viruses cause cancer? Michael Bishop and Harold Varmus received the 1989 Nobel Prize for cancer- causing genes carried by a virus from animal cells Oncogenes:genes that can be transformed to cause cancer 10% of cancers have been found to be due to oncogenic viruses