Section 1: Introduction to Virology Flashcards
Viral history
Greece/Mesopotamia
-the Iliad, Homer describes Hector as rabid
-1000 BC, Mesopotamian law dictates how to be responsible for Rabid dogs
-Egyptian hieroglyphs depict withered legs, typical of poliomyelitis
-evidence of smallpox being endemic in the Ganges River basin- 500 BC (later to decimate the native peoples of the Americas- 1400 BC)
- Agriculture and the domestication of animals changed the ways of humans and viruses interact
Variolation
-inoculation, of healthy individuals with material from smallpox pustules into a scratch was widespread in China and India- 1000 AD
Vaccination
-vacca = cow
-Edward Jenner (1790s) observed that milkmaids were protected against smallpox
-extracts from cowpox lesions protected individuals against smallpox
-Louis Pasteur coined the term vaccination in the 1800s in honor of Jenner
-Cowpox virus and vaccinia virus are not the same, at some point during the history of passing cowpox from animal to human vaccinia appeared,,, and no one definitely knows how
tobacco mosaic disease (TMV)
-first virus was discovered in 1892, by Ivanovsky
-Beijerinck discovered the agent causing the disease was ultra filterable (smaller than bacteria, virus = poison)
Bacteriophage
- phage = eating
-discovered by Twort in 1915 and named for their ability to lyse bacteria on agar plates
Properties of Viruses
- Infectious obligate intracellular parasites
- Genome is either DNA or RNA
- Viral genome is replicated and directs synthesis, by cellular systems, of other viral components
- Progeny virus particles, Virions are formed by de novo assembly from synthesized components in host
- A progeny virion assembled during the infectious cycle is the vehicle of transmission of the viral genome to the next host
Lytic cycle
-infection resulting in lysis
lysogenic (lysogeny)
-infection results in the integration of viral DNA into host genome (Prophage)
-longer run time
viroids
-infectious agents of plants, a single molecule of RNA
prions
-infectious protein molecules
Viral classification
-all life must produce mRNA to thus create proteins, this led to the modern classification of viruses
The Baltimore classification
-Based on the genetic system of each virus and the relationship between viral genome and mRNA
-Positive strand or Sense (+)
-Negative strand or Sense (-)
-single-stranded (ss) nucleic molecule
-double-stranded (ds) nucleic molecule
Positive strand or Sense (+)
immediately translatable information
-makes sense
Negative strand or Sense (-)
-the complement to a positive sense DNA or RNA molecule
Viral genetic content
DNA viruses contain ssDNA and dsDNA
-mostly are dsDNA
-mostly are not simple linear molecules
RNA viruses are mostly ssRNA molecules
-can be -, +, or ambisense
-most viruses appear rod-shaped or spherical under electron microscopy, this is due to the economy of viral construction of the plasmid
Where is viral genetic material contained
- capsid (contains the material within itself)
- nucleocapsid
Capsid symmetry
-capsids are constructed from a small number of protein subunits by using helical symmetry (rod-shaped) or the symmetry of platonic polyhedra (tetrahedron, icosahedron, etc, spherical
helical symmetry
-imagine a two-dimensional lattice and rolling it into a cylindrical structure
icosahedral symmetry
-most efficient way of forming a spherical shell to maximize internal volume (the simplest case is an icosahedral symmetry with 20 triangular faces)
complex particles
-form complex capsids with structures that are poorly understood (ex: retroviridae)
Enveloped viruses
-enveloped viruses have a lipid membrane acquired from the host that surrounds the capsid, naked do not have an envelope
-easy to get rid of
Class I
dsDNA
-viruses contain dsDNA as their genome, their mRNA directly transcribed from the DNA template
ex: herpes simplex (herpesvirus)
Class II
ssDNA
dsDNA
-viruses have ssDNA as their genome, they convert DNA to double stranded form before RNA is transcribed
ex: parvovirus
Class III
dsRNA
- their mRNA is transcribed from the RNA genome
ex: rotavirus (childhood gastroenteritis)
Class IV
(+)ssRNA
(-)ssRNA
- the positive polarity means their genomic RNA can serve directly as mRNA
-negative polarity (complement of positive strand genomic RNA) are formed from intermediates and serve as a template for the production of RNA with positive polarity
ex: picornavirus
Class VI
(-)ssRNA-RT
DNA/RNA
dsDNA
-viruses have diploid ssRNA genomes that need to be convert with reverse transcriptase to dsDNA, this is then put in the nucleus of host cell, then mRNA can be produced by transcription of viral DNA
ex: Human immunodeficiency virus (HIV)
Class V
(-)ssRNA
-viruses contain a ssRNA genome with negative polarity, so their sequence is complementary to mRNA. So the negative stranded genome can be converted directly to mRNA.
ex: rabies (rhabdovirus)
Class VII
dsDNA-RT
-viruses have partial dsDNA genomes and make ssRNA intermediates that act as mRNA, but are also converted back into dsDNA genomes by reverse transcriptase, necessary for genome replication.
- ex: Hepatitis B virus (hepadnavirus)
Naked particles
-naked particles are harder to get rid of (intestinal viruses), they just contain the viral capsid so they can survive a very long time (have to bleach it to dissolve it)
