Viruses: Structure & Properties
|What are Koch's postulates?|| To prove that an agent causes disease it must:|
Be found in lesions.
Isolate in pure culture.
Inoculate pure culture & cause disease.
Recover again from lesions of host.
|What is the method for indirect demonstration of viruses?|| Grind infected tissue into fluid suspension.|
Filter to retain bacteria/fungi and protozoa.
Inoculate filtrate into to test animals/plants.
Watch for disease.
Repeat and pass in series.
(Show particulate nature by ultracentrifugation).
|What are the technical advances for the demonstration of viruses?|| Whole animals/plants/bacteria (in vivo).|
Chick embryos (in vivo) -
Used to produce influenza vaccine
Discovery of viral genetic material (DNA/RNA).
Tissue culture -
Used for vaccines and diagnostic reagents.
Electron microscopy -
Defines morphology by negative staining or thin section.
Various molecular techniques.
|What is tissue culture?|| Single cells grow as a monolayer on a flat surface (glass/plastic).|
Cells are covered by a medium containing nutrients + antibiotics + pH indicator.
|What is the structure of a virus?|| Virion - Infectious virus particle consisting of nucleic acid.|
Capsid - Composed of capsomeres and has either icosohedral or helical symmetry.
|What are the functions of viral proteins?|| Structural - |
Capsid proteins, viral surface proteins.
Enzymes concerned with various aspects of the replication cycle.
|How was hepatitis C virus discovered?||Ultracentrifugation of plasma from chimps.|
Extraction of nucleic acid from pellet.
Cloning of the bacterial virus (bacteriophage).
Grow bacteriophage clones in bacteria.
Screen clones for virus protein with human anti-serum.
DNA from the only +ve clone sequenced.
Use sequenced DNA as probe for virus which turns out to be an RNA virus.
|Bacteriophage.|| A virus that is parasitic in bacteria.|
It uses the bacterium's machinery and energy to produce more phage until the bacterium is destroyed and phage is released to invade surrounding bacteria.
|How do you classify viruses?|| Viruses are classified in families depending on:|
(DNA/RNA, ss/ds, +/- sense RNA, polycistronic or segmented).
Strategy of replication.
Morphology (icosohedral/helical/complex, +/- envelope).
|What are submicroscopic infectious agents?||Viruses, viroids and prions.|
|What is a viroid?||A single stranded RNA molecule that has no surrounding capsids (protein coat) and only infect plants.|
|What is a prion?||A small proteinaceous infectious particle which is resistant to most procedures that modify nucleic acids.|
|What are the general properties of viruses?|| Infect animals, insects, plants, bacteria.|
Size - submicroscopic (20-300 nm diameter).
Classified by morphology and behaviour.
DNA or RNA + protein coat ± envelope.
Extracellular infectious phase.
Obligatory intracellular replication.
Viral genes subvert cell biosynthesis to replicate.