lecture 2 Flashcards
Smallest viruses
porcine circovirus type 1
largest viruses
- Poxvirus (200 nm diameter and 300 nm in length)
- in animals, humans, and birds
Pleomorphism
ability of virus to alter size or shape
Common methods to determine morphology of virus
- electron microscopy
- Cryo-Electron Microscopy (Cryo-EM)
- X-ray crystallography
- Nuclear Magnetic Resonsonance (NMR)
Capsid
- protein shell of virus that encases/envelopes the viral nucleic acid or genome
- capsid is made up of capsomeres
- most viruses have one capsid, except Reoviruses which are double layered
Capsomere
basic subunit protein in the capsid of a virus
Nucleocapsid
capsid+ viral nucleic acid (DNA or RNA)/ Genome
Capsid symmetry and virus architecture
- helical symmetry
- cubic/icosahedral symmetry
helical symmetry
- in all animal viruses, helical nucleocapsid is enclosed within a Lipoprotein Envelope
- naked helical nucleocapsids are common among plant viruses (eg. tabacco mosaic virus)
cubic/icosahedral symmetry
- two types of capsomeres present in icosahedral capsid
- the pentagonal capsomeres at the vertices (pentons) and hexagonal capsomeres at the facets (hexons)
- there are always 12 pentons, but number of hexons varies by virus group
- triangulation number
triangulation number (T-number)
- describes the relationship between the number of pentagons and hexagons in icosahedron
- calculated using the formula: T=h^2+h*k+k^2
true/false: icosahedron may be naked or enveloped
true
Parvovirus icosahedral symmetry
-T=1, simples, capsid consists of 60 copies of CP protein
Reoviridae icosahedral symmetry
- outer capsid= T=13 symmetry
- inner capsid is T=2 symmetry
Which type of virus we discussed has a capsid with a complex symmetry?
a. Reovirus
b. Herpes virus
c. Pox Virus
D. HIV
C. pox virus
functions of the viral capsid
- structural symmetry of virus particle
- encases and protects viral nucleic acid from enzymes (nucleases), chemical and physical conditions (pH and temp)
- receptor attachment proteins on viral capsid facilitates attachment of receptors to the susceptible host cells
- interaction with host cell membranes to form envelope
- uncoating of genome in host cell
- transport of viral genome to appropriate site
- packaging of nucleic acid genome
- determines antigenicity of the virus
enveloped viruses
- envelope is lipid bilayer with embedded proteins
- envelope acquired by budding of viral nucleocapsid through cellular membrane such as cytoplasmic membrane, golgi membrane, or nuclear membrane
- composed of glycoprotein and matrix protein
Types of Glycoprotein in Viral Envelope
- external glycoprotein
- channel proteins
External glycoprotein
usually major antigens of virus, function in hemagglutination, receptor binding, angtigenicity, and membrane fusion
channel proteins
- mostly hydrophobic
- form a protein channel through the envelope
- alters the permeability of the membrane (ion channel)
- important in modifying internal environment of the virus
Other types of proteins than glycoproteins in the viral envelope
- fusion proteins
- matrix proteins
Matrix protein
- provides link between internal nucleocapsid to the lipid membrane envelope
- crucial role in virus assembly
- stabilization of lipid envelope
- recognition site of nucleocapsid at the plasma membrane
- mediates encapsidation of RNA-nucleoprotein cores into the membrane envelope
Lipid bi-layer in virus envelope
- acquired from cell membrane of host cell such as cytoplasmic membrane, nuclear membrane, ER membrane, etc
- maintained only in aqueous or moist environments, sensitive to dessication, heat, and alteration of pH
- enveloped viruses can be inactivated by dissolution of lipid membrane with lipid solvents such as: ether, chloroform, sodium deoxycholate, detergents, etc
- enveloped viruses easier to sterilize than non-enveloped viruses, cannot survive for longer periods in environment
Which of the following is not a component of a virus?
A. nucleic acid
B. Capsid
C. Envelope
D. Capsule
D. capsule
The viral matrix protein is located between the viral nucleic acid (DNA/RNA) and the capsid
B. False
Mechanisms of genetic diversity of viruses
- antigenic drift (mutations)
- Antigenic shift:
- -recombinations
- -reassortment
Recombination (antigenic shift)
-intramolecular recombination involves the exchange of nucleopeptide sequences between different, but usually closely related, viruses during replication
reassortment (antigenic shift)
-most important mechanism for genetic diversity in viruses with segmented genome
Which of the following is not a mechanism of genetic diversity of viruses?
A. reassortment
B. point mutations
C. recombination
D. conjugation
D. conjugation
retroviral integrase
-enzyme produced by retrovirus (such as HIV) that enables its genetic material to be integrated into the DNA of the infected cell
reverse transcriptase
-enzyme used to generate complementary DNA (cDNA) from a RNA template
nucleic acid polymerases
-viral genome replication
Viral nonstructural proteins
-proteins may play roles within the infected cell during virus replication or act in regulation of virus replication or virus assembly
nonstructural proteins are seen in extracellular virions
T/F
false
incomplete virions
virion without nucleic acid (empty capsid)
defective virions
- virus that can’t replicate because it lacks a full complement/copy of viral genes
- defective viral particles result from mutations or errors in the production or assembly of virions
Psuedovirion
-contains non-viral genome within the viral capsid, such as host nucleic acid instead of viral nucleic acid
Psuedotypes
-when related viruses infect the same cell, the genome of one virus may be enclosed in the heterologous capsid of the second virus
An incomplete virion is composed of:
A. only nucleic acid (DNA/RNA)
B. only lipid envelope
C. only capsid
D. lipid envelope and nucleic acid, no capsid
C.
Which of the following is not a component of a virus?
A. nucleic acid
B. Capsid
C. envelope
D. capsule
D. capsule
