4.0 Virology: structure, function and classification into RNA/DNA viruses

viruses

small obligate intracellular parasites which contain nucleic material surrounded by a protective virus coded protein coat

viruses are simple acellular and present with a distinct pattern of multiplication

viruses may be viewed as mobile genetic elements

viruses are dependent on a cellular host such as mammalian, plant or bacterial cells

for propagation with host cells supplying the complex metabolic and biosynthetic machinery required

viruses are specific to a particular cell type

with different viruses requiring different specialised cellular machinery

virion

complete virus particle

main function of a virion

to deliver its genome into the host cell so that the genome can be expressed (transcribed and translated) by the host cell

the viral genome often associated with basic proteins

is packaged inside a symmetric protein capsid

nucleoprotein

the nucleic acid associated protein

nuceleoprotein + genome

→ nucleocapsid

in elveloped viruses the nucleocapsid is surrounded by a lipid bilayer

derived from the modified host cell membrane and studded with an outer layer of virus envelope glycoproteins

viruses are inert outside the host cell

small virruses (polio/ tobacco mosaic virus) can be crystalized

as obligate intracellular parasites

they fully depend on the machinery of their host cells for replication

therefore the main function of a virus is to

deliver its genome into the host cell to allow its expression (transcription + translation) by host cells

virion

fully assembled virus

basic components of a virion

• nucleic acid

• capsid

• envelope

nucleic acid

single/ double stranded RNA/DNA

capsid protein coat

coded for by the viral genome which functions as a shell to protect the viral genome form nuclease

during infection the capsid protein coat…

…attaches the virion to specific receptors exposed on the protective host cell

envelope, not univeersal to all viruses

consists of a lipid bilayer that closely surrounds a shell of virus envoded membrane associated proteins

the exterior envelope bilayer is studded with virus coded glycosylated (trans) membrane proteins

therefore enveloped viruses often exhibit a fringe of glycoprotein spikes or knobs AKA poplomers

in viruses that acquire their envelope by budding through the plasma or another intracellular cell membrane

the lipid composition of the viral envelope closely reflects that of the particular host membrane

The outer capsid and the envelope proteins of viruses are glycosylated

this is important in determining the host range and antigenic composition of the virion.