Morphology of Viruses Flashcards
Smallest Viruses
Porcine Circovirus type-1 (17nm)
Parvovirus (18nm)
Largest Viruses
Pandoravirus (400nm)
Poxvirus (200x300nm)
Pleomorphism
Ability of some viruses to alter their shape or size
Most viruses are _______ or __________ shaped.
Most viruses are rod or spherical shaped.
Common methods used to deterime the morphology of viruses:
Electron Microscopy (EM)
Cryo-Electron Microscopy (Cryo-EM)
X-Ray Crystallographic Method
Nuclear Magnetic Resonance (NMR)
_____________ method requires negative staining with electron dense material such as Uranyl Acetate or Phosphotungstate.
Electron Microscopy (EM) method requires negative staining with electron dense material such as Uranyl Acetate or Phosphotungstate.
Resolution range for electron microscopy
50-75 angstroms
________________ method allows the observation of biological specimens in thier native environment (not fixed or stained in any way)
Cryo-Electron Microscopy (Cryo-EM) method allows the observation of biological specimens in thier native environment (not fixed or stained in any way)
Resolution range for Cryo-Electron Microscopy
3.3 to 20 angstroms
Explain the X-Ray Crystallographic method process
- Virus or viral protein is crystalized
- X-ray
- Deflection mesasured to form electron density map
- Protein model created
- Reconstruction of virus structure
Capsid
Protein shell of a virus that encases/ envelopes the viral nucleic acid or genome
The capsid is made up of ______ held together by covalent bonds
The capsid is made up of capsomeres held together by covalent bonds
Viruses have one capsid, except for _____________ which have a double layered capsid
Viruses have one capsid, except for Reoviruses which have a double layered capsid
Capsomere
Basic subunit protein in the capsid of a virus
Nucleocapsid
Capsid + Virus Nucleic Acid/Genome
Common capsid symmetries
Helical Symmetry
Cubic/ Icosahedral Symmetry
Complex
Incomplete virions cannot form ____________ symmetry.
Incomplete virions cannot form helical symmetry.
Helical nucleocapsids of animals are enclosed by ____________
Helical nucleocapsids of animals are enclosed by lipoprotein envelope
Naked helical nucleocapsids are common among _______ viruses.
Naked helical nucleocapsids are common among plant viruses.
Icosahedral viruses always have ________ corners
Icosahedral viruses always have 12 corners
Two types of capsomers present in icosahedral capsid
Pentagonal Capsomers
Hexagonal Capsomers
Pentons make up the (vertices/facets)
Pentons make up the (vertices/facets)
Hexons make up the (vertices/facets)
Hexons make up the (vertices/facets)
Triangulation Number (T-Number)
Describes the relation between the number of pentagons and hexagons of the icosahedron.
The (larger/smaller) the T-Number the more hexagons are present relative to the pentagons.
The (larger/smaller) the T-Number the more hexagons are present relative to the pentagons.
T- Number Equation
T = H2 + h * k + k2
________ has a T = 1, simplest icosahedron
Parvovirus has a T = 1, simplest icosahedron
Functions of viral capsid
- Structural symmetry
- Encase and protect viral nucleic acid
- Facilitate attachment of the virus to specific receptors on the susceptible host cells
- Interact with host cell membrane to form envelope
- Uncoat genome
- Transport viral genome
- Recognize and package nucleic acid genome
Viral envelope
Lipid bilayer with embedded proteins, facilitates virus entry into host cells and may also help the virus to adapt fast and evade host immune system
Explain how a virus acquires its envelope
Acquired by budding of viral nucleocapsid through a cellular membrane
Two kinds of viral proteins found in the envelope
Glycoprotein
Matrix Protein
Glycoprotein
Anchored in the lipid bilayer by hydrophobic bonds, spikes seen on the virus surface
External Glycoprotein
Anchored in the envelope by a single transmembrane domain and a shorter internal tail
Antigens of the virus and involved in hemagglutination, receptor binding, antigenicity and membrane fusion
Channel Proteins
Hydrophobic proteins that form a protein lined channel through the envelope
Alter permeability of the membrane and important in modifying the internal envrionment of the virus
Fusion proteins
Channel proteins that facilitate attachment and entrance of viral contents to enter host cell
Two kinds of fusion proteins
pH Independent
pH Dependent
HIV and Measles are examples of pH (independent/dependent) fusion proteins.
HIV and Measles are examples of pH (independent/dependent) fusion proteins.
HA of the influenza virus is an example of pH (independent/dependent) fusion proteins.
HA of the influenza virus is an example of pH (independent/dependent) fusion proteins.
Matrix Protein
Proteins that lnk the internal nucleocapsid to the lipid membrane envelope
Recognition site of nucleocapsid at the plasma membrane and mediates the encapsidation of the RNA nucleoprotein cores into the membrane
________ proteins play a crucial role in virus assembly
Matrix proteins play a crucial role in virus assembly
(Enveloped/Non-Enveloped) viruses are easy to sterilze and cannot survive for long periods in the environment
(Enveloped/Non-Enveloped) viruses are easy to sterilze and cannot survive for long periods in the environment
(+/-) Sense RNA viruses have infectious genomes
(+/-) Sense RNA viruses have infectious genomes
+ Sense RNA Genome
Similar to mRNA and thus can be immediately translated by the host cell
- Sense RNA Genome
Complementary to mRNA and thus must be converted to + sense before translation
RNA Polymerase
Converts - Sense RNA to +Sense RNA
Antigenic Drift
Changes in the virus that arise from point mutations, may cause change in antigenicity
Processes of Antigenic Shift
Recombination
Reassortment
Recombination
Exchange of nucleotide sequences between different, but usually closely related, viruses during replication
Reassortment
Exchange of segmented genomic segments
Most important mechanism for high genetic diversity in viruses with segmented genome is _____________
Most important mechanism for high genetic diversity in viruses with segmented genome is Reassortment
Viral _________ constitute up to 50-70% of the virion
Viral protein constitute up to 50-70% of the virion
Two classes of virion proteins
Modified Proteins
Unmodified proteins
(Modified/Unmodified) viral proteins are obtained by post-translational proteolytic cleavage of polyprotein.
(Modified/Unmodified) viral proteins are obtained by post-translational proteolytic cleavage of polyprotein.
Lysins
Hydrolytic enzymes produced by bacteriophages to cleave the host cell wall
Retroviral Integrase (IN)
Enzyme produced by a retrovirus 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 an RNA template
Nucleic Acid Polymerase
Responsible for viral genome replication
Structural Viral Proteins
Proteins the form the viral capsid
Nonstructural Viral Proteins
Encoded by viral genome that are produced in the organisms they infect, but not packed into the virus particles
Play role within the infected cell during virus replication or act in regulation of virus replication or virus assembly
Regulatory Proteins
Broad category of proteins that play indirect roles in the biological processes and activities of viruses
Regulate the expression of viral genes or are involved in modifying host cell functions
Incomplete virions
Virion without nucleic acid
Effective Virions
Virus that cannot replicate because it lacks full complement/ copy of viral genes
Defective Viral Particles
Result from mutations or errors in the production or assembly of virions
Replications of defective virions occurs only in mixed infection with a ___________.
Replications of defective virions occurs only in mixed infection with a helper virus.
Helper Virus
Supplement the genetic deficiency and make defective viruses replicate progeny virions when they simultaneously infect host cell and defective viruses
Defective Interfering Particle (DIP)
Defective virus can not replicate, but can interfere other congeneric mature virion enter the cells
Pseudovirion
Contains non-viral genome within the viral capsid, look like ordinary viral particles but do not replicate
Pseudotypes
Related viruses infect the same cell, the genome of one virus may be enclosed in the heterologous capsid of the second
