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)

YEAH!

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.

H1N1(the “Spanish flu” strain)

H5N1the “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