Viruses: Structure & Properties

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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.

Lipoprotein envelope.

What are the functions of viral proteins?

Structural -
Capsid proteins, viral surface proteins.

Non-structural -
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:

Genome
(DNA/RNA, ss/ds, +/- sense RNA, polycistronic or segmented).
Strategy of replication.
Morphology (icosohedral/helical/complex, +/- envelope).
Size.

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.

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