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VIRUSES
video
what are they?
are they living
or nonliving?
“viruses lead a kind
of borrowed life”
- researches
history…
Adolf Mayer
• 1883
• disease of tobacco leaves (mosaic
color) could be transferred
rubbing diseased leaves onto
healthy ones
• cause invisible under a microscope
• maybe tiny bacteria?
Dimitri Ivankowsky
• a decade later
• filtered out bacteria
• disease still passed
• maybe bacteria too small (passed
through the filter) or produced
toxins?
Martinus Beijerinck
• infectious agent in the sap could
reproduce
• could NOT be cultivated
on nutrient media
• must be something much smaller
than a bacterium (hmmm… what
can it be?
Wendell Stanley
• 1935
• crystallized the infectious particle
• tobacco mosaic virus
structure of viruses
(nucleic acid enclosed in a protein coat,
some have a membranous envelope)
and they are not…
good for us
viral genome
viruses can posses…
•double stranded DNA
•single stranded DNA
•double stranded RNA
•single stranded RNA
usually a single linear
or circular molecule of
nucleic acid
capsids and envelopes
capsid
• Protein shell
• Protein subunits capsomeres
• Various shapes
envelopes
• only some viruses (e.g. influenza)
• derived from the host cells
• but also proteins and
glycoproteins of viral origin
Bacteriophages
(some of the most complex and studied viruses)
need a host to reproduce
(obligate intracellular parasites)
narrow or broad host range
in general…
• virus delivers its genome inside the host cell
• the host provides nucleotides, and all the
components needed to make viral protein
(enzymes, tRNA, ribosomes, ATP, etc…)
• the simplest type of viral reproductive cycle
ends with the exit of a large number of
viruses
simplified viral
reproduction
the lytic cycle
• ends with death of the host cell (lysis)
• virulent phages
how do bacteria fight back?
• mutations causing unrecognizable
receptor are selected for
• the viral (foreign) DNA is often cut
up by restriction enzymes
Sometimes phages
coexist with their hosts…
the lysogenic cycle
• does not destroy the host cell
• viral DNA is incorporated into the
host’s DNA (prophage)
• the viral DNA replicates every time
the cell replicates
• bacteriophages using both modes of
reproduction – temperate phages
various viral genes are being
expressed, changing the
phenotype of the host cell
(often leading to formation of more harmful bacteria)
An environmental
signal usually triggers
a switchover to the
lytic mode
reproductive cycles of animal viruses
• numerous variations, mostly dependent
on the type of the viral genome…
• DNA ?
• RNA?
• double stranded?
• single stranded?
viral envelopes
• used to enter the host cell
• mostly derived from the host’s
plasma membrane
RNA as viral genetic material
• mostly animals infecting viruses
• class IV – directly used as mRNA
• class V – RNA serves as a template for
mRNA
• class VI – retroviruses RNA  DNA
(reversed transcriptase)
e.g. HIV (like other retroviruses – envelope and two
molecules of single-stranded RNA)
HIV video
evolution of viruses
• because they depend on cells, most
likely evolved after first cells
• most likely from naked bits of
cellular nucleic acids that moved
from one cell to another
• two main candidates: plasmids and
transposons
the interesting case of mimivirus
• described in 1992
• at the time the largest known virus (over
400nm in diameter)
• genome 1.2 M bp
• 911 protein coding genes
• it blurs boundaries between viruses and
the smallest parasitic cellular organisms
viral diseases in animals
vaccine
- a harmless variant or derivate of
a pathogen that stimulates the IS
smallpox –
successfully
eradicated in
1979
(a vaccination initiative by the WHO)
emerging viruses
– appear suddenly or are new to medical scientists
• HIV
• Ebola virus
• West Nile virus
• SARS
how do they just show up?
(3 reasons)
• unusually high rate of mutation
• dissemination from a small
isolated human population
• spread of existing viruses from
other animals
“Spanish flu”
• 1918 – 1919 pandemic
• Influenza virus type A
• the source: most likely birds
• Infected about 500M people
(killed 50-100M)
Different strain of
influenza A are given
standardized names…
e.g.

(the “Spanish flu” strain)
the “avian flu” strain
viral diseases in plants
• more than 200 types
• $15B estimated annual loss in crop
destruction
• similar basic structure and
reproduction cycle as animal viruses
• horizontal transmission
• vertical transmission
viroids
• circular RNA molecules
• only a few hundreds of
nucleotides long
• do not encode proteins
• affects plants
• siRNA (RNA silencing)
Prions
(the scariest of all)
• infectious agent composed only of (misfolded)
protein
• causing TSEs – neurodegenerative disease of the
CNS (formation of amyloids)
• ability to misfold other proteins
• A long incubation period (5-20 years)
• Nearly impossible to denature
• Most known diseases include: BSE (aka mad cow
disease) and CJD in humans