Structure and Chemical Composition Flashcards
What are the two smallest viruses?
porcine circovirus type I (17nm diameter) and parvovirus (18nm diameter)
What are the two largest viruses?
pandoravirus (400 nm diameter) and poxvirus (200 nm diameter and 300 nm length)
What is pleomorphism?
ability of a virus to alter shape or size
Which virus is bullet-shaped?
Rabies
Which virus is brick shaped?
poxvirus
Which virus is rod-shaped?
tobacco mosaic
Which virus is filament-shaped?
Ebola
Which virus is spherical?
rotavirus
electron microscopy
- need staining -> negative staining with electron dense material (uranyl acetate or phosphotungstate)
- resolution 50-75 angstroms
- detailed structural interpretation not always possible
cryo-electron microscopy
- can observe specimens in native environment
- doesn’t require fixing or staining
- -180C for liquid nitrogen, -269 for He
- resolution 3.3-20 angstroms
X-ray crystallography
- virus diffracts x-rays which are detected by a computer to produce an electron density map
- map used to create a protein model which is used to reconstruct the virus structure
What is a capsid
protein shell of a virus that encases/envelops the viral nucleic acid or genome
What is a capsid made of ?
capsomeres held together by non-covalent bonds
How many capsids do viruses have?
most have one, but reoviruses have a double layered capsid
What is a capsomere?
basic subunit protein in the capsid of a virus
What is a nucleocapsid?
capsid + virus nucleic acid (DNA or RNA)
helical symmetry
capsomeres and nucleic acid are wound together to form helical or spiral tube; incomplete virions cannot form
- in all animal viruses, helical nucleocapsid is enclosed within a lipoprotein envelope
- naked helical nucleocapsids common among plant viruses (e.g. tobacco mosaic)
How many corners, facets and edges does an icosahedron have?
12 corners
20 facets
30 edges
How many pentons and hexons are present in an icosahedral capsid?
always 12 pentons # hexons varies with virus group
Triangulation number
describes the relation between the number of pentagons and hexagons of the icosahedron; the larger the T number the more hexagons are present relative to pentagons
T number formula
T = (h^2) + (h)(k) + (k^2)
What is the simplest icosadedron?
parvovirus, T = 1
Reoviridae capsid symmetry
outer: T = 13
inner: T = 2
complex symmetry
virions are composed of several parts, each with separate shapes and symmetries (e.g. pox viruses)
functions of viral capsid
- responsible for the structural symmetry of the virus particle
- encases and protects the viral nucleic acid from enzymes, chemicals and physical conditions
- attachment of virus to specific receptors on susceptible host cells
- interaction with host cell membranes to form the envelope
- uncoating of the genome in host cell
- transport of viral genome to appropriate site
- facilitates specific recognition and packaging of nucleic acid genome
- contains antigenic sites that determine antigenicity of virus
What makes up an envelope?
lipid bilayer with embedded proteins
What is the function of an envelope?
facilitates virus entry into host cells and may also help the virus to adapt fast and evade the host immune system; cause persistent infections
How is the envelope acquired?
budding of viral nucleocapsid through a cellular membrane, such as cytoplasmic membrane, golgi membrane or nucleus membrane; budding only occurs at sites where virus specific proteins have been inserted into host cell membrane
What are the 2 kinds of viral proteins found in the envelope?
- glycoprotein
2. matrix protein
Which viral proteins are seen as spikes on the virus surface?
glycoproteins (transmembrane)
external glycoprotein
anchored in envelope by a single transmembrane domain and short internal tail; usually major antigens of the virus and involved in functions such as hemagglutination, receptor binding, antigenicity, and membrane fusion
channel proteins
type of glycoprotein which are mostly hydrophobic and form protein lined channels through envelope; alters permeability of membrane; important in modifying the internal environment of the virus
What are 2 examples of external glycoproteins in Influenza virus?
- Hemagglutinin (HA): binding, fusion, antigenic, hemagglutination
- Neuraminidase (NP): release progeny virus from host cell, antigenic
Are fusion proteins in HIV and measles pH dependent or pH independent?
pH independent
Are fusion proteins such as HA in influenza virus pH dependent or pH independent?
pH dependent (acidic)
virus envelope matrix protein
- link internal nucleocapsid to lipid membrane envelope
- allow stabilization of lipid envelope
- recognition site of nucleocapsid at plasma membrane and mediates encapsidation of RNA-nucleoprotein cores into the membrane envelope
3 ways viral proteins can interact with viral envelope proteins
- direct
- via matrix protein
- via multiprotein layer
T/F the lipid bilayer of envelope is sensitive to desiccation, heat, and alteration of pH
true
3 lipid solvents that can inactivate enveloped viruses:
- ether
- chloroform
- sodium deoxycholate, detergents etc.
T/F enveloped viruses are difficult to sterilize and can survive for longer periods in environment
F: enveloped viruses are easy to sterilize and cannot survive for longer periods in the environment when compared to non-enveloped viruses
positive sense viral RNA
similar to mRNA and can be immediately translated by host cell
negative sense viral RNA
complementary to mRNA and must be converted to positive sense RNA by an RNA polymerase before translation
monopartite
non-segmented genome
multipartite
segmented genome
outcomes of antigenic drift
- most cases minor changes
- virus may become resistant to antiviral drugs
- may cause change in antigenicity -> detrimental effect on efficacy of vaccines
recombination
exchange of nucleotide sequences between different, but closely related, viruses during replication
reassortment
*most important mechanism for high genetic diversity in viruses with segmented genome
lysins
hydrolytic enzymes produced by bacteriophages to cleave the host’s cell wall
retroviral integrase (IN)
enzyme produced by a retrovirus (such as HIV) that enables its genetic material to be integrated into the DNA of the infected cell
reverse transcriptase (RT)
enzyme used to generate complementary DNA (cDNA) from a RNA template
nucleic acid polymerases
viral genome replication
neuraminidases
enzymes that cleave glycosidic bonds; allows released of viruses from host cell
structural proteins
form the viral capsid; such as VP7 and VP4 proteins of rotaviruses
viral nonstructural proteins
encoded by viral genome and are produced in the organisms they infect but not packaged into virus particles; may play roles within infected cell during virus replication or act in regulation of virus replication or virus assembly
regulatory proteins
play indirect roles in the biological processes and activities of viruses
defective virions
virus that can’t replicate because it lacks a full complement/copy of viral genes; result from mutations or errors in the production or assembly of virions
Replication of defective virions occurs only in mixed infections with ____.
a helper virus -> supplement genetic deficiency and make defective viruses replicate progeny virions when they simultaneously infect host cell with defective viruses
defective interfering particle (DIP)
when defective viruses can’t replicate but can interfere with other congeneric mature virion entering the cells
pseudovirion
contains non-viral genome within the viral capsid; look like ordinary viral particles under electron microscope but don’t replicate
- can be used to inject foreign nucleic acid into a cell, such as delivery of DNA vaccines
pseudotypes
when related viruses infect the same cell, the genome of one virus may be enclosed in the heterologous capsid of the second virus
