6 / Viruses, viroids, & prions

The smallest microbes

Dmitri Ivanovski, Martinus Beijerinick, & Freidrich Loeffler & Paul Frosch - discovery of viruses

1. tried to find cause of tobacco disease; extract of diseased plant → filter w/ tiny holes (bacteria can’t pass) → agent causing tobacco disease fit through

2. developed idea of virus; confirmed Ivanovski’s results

3. showed similar agent caused foot & mouth disease

Reed, Stanley, & Ruska - major discoveries

1. demos viruses can cause disease in humans & be transmitted by vectors

2. isolates & purifies virus 1st time → helped determine what they’re made of

3. uses electron microscope & visualizes viruses for first time

Major virus properties

• very small (mostly < 0.3 μm)

• obligate intracellular parasites

  • no growth/reproduction outside host cells, no metabolic system of their own, totally dependent on cells building blocks to multiply (not alive)

• infect all forms of life

• largely just protein shells (capsid) w/ genome - designed for delivery to new cells

Virus properties detailed

All viruses contain:

1. genome (nucleic acid)

• DNA or RNA

• single or double stranded (or partially)

• lineaar or circular

• can be segmented (genome divided into many pieces)

• single-stranded RNA genomes can have diff forms: positive (+) and negative (-) senses

  • (+): looks like mRNA, can be translated into protein by ribosomes (mRNA genome)

  • (-): genome needs to be converted into (+) before being translated