Viral Structure, Genome & Classification Flashcards
Virion
A complete, fully developed, infectious viral particle
Capsid-composition and function
Composed of protein subunits called capsomeres
Functions:
1. Protection of the genome (packing)
2. Delivery of the genome (naked)
Capside binds host cell receptors
-Direct Penetration: sink-in membrane genome alone enters the cell
-Endocytosis: host cell endocytoses the entire virus uncoating( by drop the pH) of capsid, releasing genome
Naked and enveloped virus
Most bacterial and plant viruses are naked
Most animal viruses are enveloped
What is the morphology of helical viruses?
Hollow cylindrical capsid
Helical viruses have a spiral or rod shape.
What determines the length and width of helical capsid viruses?
Length is determined by the length of nucleic acid
Width is determined by size and packaging of capsomeres
What is the structure of icosahedral capsid viruses?
Spherical and rigid, with a closed shell enclosing the nucleic acid
Have many sides
How are the faces and segments of icosahedral viruses arranged?
Identical triangular faces, edges, vertices
This arrangement is the most efficient, requiring the fewest capsomeres
5/3/2 identical segments
What are the protein assembly types in icosahedral capsid viruses?
Pentamers and/or hexamers
Which types of viruses are usually complex viruses?
Particularly bacterial viruses, bacteriophages, and complex viruses
Complex viruses have more complicated structures compared to helical and icosahedral viruses.
Fill in the blank: Helical capsid viruses have a _______ shape.
spiral/rod
True or False: Most animal viruses have helical capsids.
False
Most animal viruses have icosahedral capsids.
What type of nucleic acid can helical capsid viruses package?
ssRNA
This is due to the rigidity of double-stranded nucleic acid.
Morphology of capsids
Helical viruses
Icosahedral viruses
Complex viruses
What kind of genome can helical viruses have
Helical capsids can only package ssRNA due to the rigidity of double-stranded nucleic acid.
what genome can icosahedral capsid packed
Icosahedral capsids can package ss/ds RNA or DNA.
Which types of viruses usually have icosahedral capsids
Most animal viruses have icosahedral capsids.
Complex viruses
-The capsid symmetry is neither icosahedral or helical
-Particularly bacterial viruses, bacteriophages, are complex viruses
• Capsid is polyhedral and then the tail sheath is helical
-Poxviruses – overall shape: brick-shaped
No clearly defined identifiable capsids but have several cleats around the nucleic acid
Enveloped virions
- envelop protein and glycoproteins often play role in host recognition
- envelop provides some protection from the immune system
- enveloped viruses are more fragile than naked viruses
• Lipid bilayer sensitive to changes in the environment (pH, temp.)
• easily disrupted by physical & chemical agent and can dry out easily
Delivery of genome from enveloped virions
First attach to receptors on host cell
-Endocytosis: Engulfing of the virus by the host cell, uncoating, release genome
(ex. Herpesviruses)
-Membrane fusion: Viral envelope and host membrane fuse, releasing capsid, uncoating, release genome
Enveloped virions leave the host
Acquire host cell membrane during viral replication or release
- Budding
- Burst the host cell
Virus particles (virions) are metastable
• Stable: Must protect the genome
Symmetrical arrangement , max contact
• Unstable: must come apart quickly upon infection, undergo structural changes to release viral genome in infected cell
No chemically bond, can fall apart when needed
proof: DNA is a kind of viral genome
DNA virus that infects bacteria
Hershey-Chase experiment with phage T2
35S: radiolabelled proteins
32P: radiolabelled DNA
radioactivity enters the cell, proof DNA is inherited
RNA is a kind of viral genome
RNA virus that infectes tobacco leaves
Fraenkel-Conrat’s and Singer’s work with TMV
Proof that RNA is the genetic material of TMV
Based on the creation of hybrid viruses
TMV A—>RNA A +protein B —> Type A progeny
(Degradation) (Infection)
TMV B—>RNA B +protein A —> Type B progeny
Composition of viral genome
dsDNA/ssDNA
dsRNA/ssRNA
Single strand viral genome can be positive sense or negative sense
Shape of virus genome
Linear
Circular
Segmented
Coding capacity of viral genomes
4 to hundreds
Positive vs. Negative sense RNA and DNA
mRNA: (ribosome ready) is always the positive (+) strand
RNA and DNA complements of (+) strands are (-) strands
(-)sense viral RNA cannot be directly translated
Not all (+)RNA is mRNA= not all (+) sense RNA is translated
Eukaryotic DNA viruses
Mostly double-stranded (ds) DNA genomes
- Different structure than host cell chromosomes
DNA replication usually takes place in the nucleus
- Except poxviruses, which replicate in the cytoplasm
Eukaryotic RNA viruses
Mostly ss RNA genomes
RNA replication usually takes place in the cytoplasm
-With some exceptions like retroviruses, which require an intra-nuclear step
RNA viruses challenge the dogma of molecular biology
Encode or carry enzymes that can :
-reverse transcribe RNA into DNA using reverse transcriptase
-synthesize RNA from an RNA template using RNA dependent RNA polymerase (RdRp)
Polymerases
DNA polymerases: synthesize DNA
-DNA-dependent DNA polymerase: DNA template
Ex.DNA polymerase alpha
-RNA-dependent DNA polymerase: RNA template
Ex. Viral reverse transcriptase
RNA polymerases: synthesize RNA
-DNA-dependent RNA polymerase: DNA template
Ex. RNA polymerase II
-RNA-dependent RNA polymerase: RNA template
Ex. Viral RdRp
RdRp=RNA transcriptase=RNA replicase
Viral classification
- Based on viral attributes
-nucleic acid: DNA/RNA, ss/ds,(+)/(-) sense, linear, circular, non-segmented or segmented
-symmetry of capsid: icosahedral, helical, complex
-enveloped or not enveloped
-host range
-size - ICTV classification
International Committee on Taxonomy of Viruses
Order name: end in “virales” ex.Amarillovirales
Family name: end in “viridae” ex.Flaviviridae
Genus names: end in “virus” ex.Hepacivius
Species names: descriptive common names are used
Subspecies are designated by a number or letter
Ex. HSV-1, Hepatitis C virus
3.Baltimore’s System of virus classification
-devised by David Baltimore to classify animal viruses
-based on 3 criteria: DNA/RNA; replication method; expression method (the way to make mRNA for expression of the viral proteins)
