Lec 2: Structure Flashcards
Viral components: (3)
- ) nucleic acids
- ) capsid
- ) envelope (if enveloped)
(+)RNA –>
(-)RNA –>
(+)RNA –> can be directly used to synthesize protein
-)RNA –> compliment to (+
viroid =
small, self-replicating ss circular RNA molecule w/o protein coat
viroids are the smallest
infectious pathogens known
ALL known viroids…
infect higher plants
viroids often transmitted (2)
mechanically
& through pollen and seed
genome is packages in a protein structure known as a
capsid
many viruses also have a
which…
lipid component
which are generally present @ the surface of the virion forming an envelope (which also contains proteins with roles in aiding entry into host cells)
a few viruses form
protective protein occlusion bodies around their virions
Function of Viral Capsids: (2)
- ) protect viral genome
2. ) aid in entry of virus to a host cell
Naked virus structure:
capsid + genome = virion
Other viruses structure:
nucleocapsid =
(outer layer protein sometimes) + lipid envelope + capsid + genome = virion
nucleocapsid = capsid + genome
5 types of viral structure:
Icosahedral Enveloped Icosahedral Helical Enveloped Helical Complex
Virus genomes: (7)
*note:
ss, linear DNA
ds, linear DNA
ss, circular DNA
ds, circular DNA
ss, linear RNA
ds, linear RNA
ss, circular RNA
*note: NO ds circular RNA viral genome discovereed yet
How are virus genomes packed?
- Some viruses:
- Many viruses:
- Some capsid proteins:
- Some viruses: capsid protein coats the genome
- Many viruses: nucleic acids bind inner part of capsid protein non-covalently (anchors DNA/RNA w/in capsid to avoid free-floating)
- Some capsid proteins: have regions such as zinc finger that binds to nucleic acids
Human genomes =
Viral genomes =
chromosomes
segmented
most viruses consist of
but some…
a single molecule of nucleic acid
…but some have genes encoded in 2 or more nucleic acid molecules = segmented
segmented genomes are more common in
RNA viruses than DNA ones
some viruses package genome segments into…
while others package genome segments into…
one virion (ex: influenza virus) separate virions (ex: brome mosaic virus)
Brome Mosaic Virus contains…
3 types of particles that are physically & virtually indistinguishable:
- ) most dense (RNA1)
- ) least dense (RNA2)
- ) middle density (RNA3 & RNA4)
Why is segmentation favorable? (3)
but…
1.) allows virus to survive environmental pressure
2.) more potential for gene recombination
3.) faster evolution
…but for those who package genome into separate virion, at least one of each has to infect the same cell
(Capsid proteins)
Simple Virus =
Complex Virus =
= 1 protein type for capsid
= multiple protein types for capsid
capsid proteins are…
asymmetrical, but are organized to form symmetrical structures
many _____ viruses have helical symmetry
ssRNA
Helical symmetry:
RNA is coiled in the form of a helix and many copies of the same protein species are arranged around the coil forming an elongated structure
helically symmetrical viruses can have rigid or flexible rods depending on
if strong or weak bonds are present between the protein molecules
the length of the capsid is determined by
the length of the nucleic acid
the symmetry of a capsid is based on
how you look @ it
to construct icosahedrons, you need
different types of proteins
to modify icosahedrons, you need
to add more proteins
when constructing icosahedrons…
so…
it is impossible for all molecules to be arranged perfectly symmetrically with equivalent bonds,
…so there is the Quasi-Equivalence Theory
Quasi-Equivalence Theory
the molecules of an icosahedron don’t interact exactly equivalently with e/o, but NEARLY equivalently
General Bacteriophage Structure:
Icosahedral Head: stores DNA
Tail: transfers DNA
Tail Fibers: distinguish which bacteria is the host
all tailed bacteriophages have _____ genome type
dsDNA
many viruses have a _____ ________ component
lipid membrane component
virus lipid membrane is typically associated with
virus protein
virus lipid membrane + virus protein = ________, which….
envelope, which encloses the nucleocapsid (nucleic acid + capsid)
most viruses get their membrane from _____, and this makes them
some viruses get their membrane from _____, and this makes them
budding out of cells = external
assembly step = internal
virus membranes can be either
internal or external on the virion
many animal viruses are
enveloped
most of the proteins associated with the membranes of enveloped viruses are
integral membrane proteins
most virion membranes are derived from
host cell membranes that undergo modification before incorporation into virions
some viruses are able to replicate in
more than one kind of host
progeny virions are released from cells; the lipid composition of the envelope may
reflect that of the host cell
envelopes are obtained by
budding through a cellular membrane (except poxviruses)
envelopes allow the possibility of
exiting the cell w/o killing it
envelopes contain at least one
virally coded protein (for host attachment)
loss of envelope =
loss of surface protein = loss/reduction of infectivity
(envelope glycoproteins)
are =
function =
found =
are = proteins modified by attachment to a sugar function = recognition found = on cell surface (firmly embedded in the envelope bilayer)
envelope glycoproteins are firmly embedded
in the envelope bilayer
envelope glycoproteins can form…
used for…
spikes or other structures on the outside of the virion used for attachment to host cell
enveloped viruses are susceptible to
acid, detergents, etc
occlusion bodies mostly found in
insect cell viruses
occlusion bodies =
crystalline protein matrix (extra layer of protection) surrounding the nucleocapsid
occlusion bodies are produced in the
nucleus or cytoplasm of infected cells
occlusion bodies confer
resistance to adverse environmental conditions on the virus
occlusion bodies are dissolved by
the alkaline pH of the insect gut, resulting in the release of infectious virus particles
virus genome can only be…
but virions can have…
one type
different types of nucleic acids to help in replication
DNA viruses contain…
& function…
virus RNA (short RNA sequences covalently attached to DNA) & function as primers for DNA synth & remain attached to the genome in the mature virion
other cell molecules incorporated into virions:
tRNA
proteins
polyamines
cations
polyamines & cations function in virions =
neutralize the negative charges on the virus genome
