virus particles Flashcards
define virus
sub microscopic, obligate intracellular parasite
why do virus particles have symmetry?
since they encode no metabolic enzymes, assembly relies on thermodynamics
what is a helical particle?
particle has rotating constant amplitude, usually larger viruses
what is an icosahedral particle?
2, 3 and 5-fold symmetry and always 60 subunits. usually small
how does redundancy make the viral capsid an effective barier?
there are many copies of protein subunits in a capsid; damage to one (or many) does not destroy the infectivity of the particle
why do viruses need more than one subunit?
the genetic code is triplet; viruses must have their genome in a format that can be read by the host, and therefore must use the triplet code, a larger particle is required to fit the genome
how and when did Fraenkel-conrat & williams show that there is no energy requirement for viral assembly?
in 1955 they showed purified TMV RNA and coat proteins incubated together form a virus particle
what forces drive viral assembly?
hydrophobic and electrostatic interactions
why are covalent bonds rarely used in virus particles?
too strong, must be possible to break the particle to release genetic information
briefly outline assembly of TMV particles
particle assembly initiated by association of coat protein molecules, one of these disks associate with the origin of assembly in genomic RNA. more disks added and vRNA is trapped in the middle of the disk
explain the structure of bacteriophage M13
major coat protein g8p is arranged helically and has charged regions. the particle contains five proteins, and has helical symmetry with predominantly electrostatic interactions.
why does infectivity drop as the size of DNA fragment cloned into M13 increases?
as the helices get longer, they are more prone to breakages; in the evironment, a virus is subjected to hydrodynamic shear forces
outline the structure of rhabdovirus particles
inner helical nucleocapsid surrounded by an outer lipid envelope and associated glycoproteins. has a matrix protein which links the central part of virus with its envelope. the envelope is hydrophobic and works to stabilise interactions with the membrane
why do icosahedral viruses formed from a single form of coat protein not exist?
- the virus would not be structurally secure
- this would not permit the genome size for a virus of this complexity
outline the structure of picornavirus
icosahedral structures with triangulation numbest T =3. there are three virus proteins (VP1, VP2 and VP3) which make up the surface of the coat. VP4 is present in each repeating unit in the capsid. all three proteins are based on a single structure - eight stranded antiparallel B-barrel
