chapter 7

virus

too small to be seen with first microscopes

cause - unknown for years

Louis Pasteur proposed this term

virus job

can infect every type of cell

not living, not made of cells

require host for replication/metabolism

viewing viruses

special stains + electron microscope

negative stains (background) - outlines shape

positive stains (virus itself) - internal details

shadow-casting - surface structures and details

viral components

external coating (capsid + envelope)

core (DNA or RNA)

capsid

external coating made of capsomers (made of proteins)

helical - rod-shaped

polyhedral - many sides

envelope

external coating in 13 of 20 families of animal viruses

made of phospholipids

have surface proteins (spikes) for binding to host

no envelope = naked virus

nucleocapsid

capsid and nucleic acid together

virion

fully formed virus (mature viral particle) able to establish infection in host cell

bacteriophage

complex virus that infects bacteria

other substances in virus particle

potential enzymes for functions in host cell

polymerases to synthesize DNA/RNA

replicases to copy RNA

viral multiplication cycle

adsorption

penetration

synthesis

assembly

release

adsorption

attaches to host cell by specific binding of spikes to cell receptors on membrane

exact fit needed so viruses have limited host range

penetration

engulfs into a vesicle

envelope is uncoated → freeing viral RNA into cell cytoplasm

synthesis

under control of virus, host synthesizes basic components of new viruses

DNA - replicate and assemble in host’s nucleus
RNA - replicate and assemble in cytoplasm

assembly

viral spike proteins insert into membrane for viral envelope

nucleocapsid formed from RNA and capsomers

release

enveloped viruses leave membrane (budding or exocytosis) carrying spikes

entire length - 8-36 hrs

ready to infect another cell