14. Virion structure general composition and symmetry Flashcards
Are viruses alive
No. They are unable to carry out metabolic reactions outside a host cell
What are definition and properties of a virus
Viruses are filterable agents that are obligate intracellular parasites. They can’t make energy or proteins independently from a host cell.
They have a naked capsid or envelope morphology
Can viral genome be both RNA and DNA
No, only one or the other
Do virus components replicate by division
No
What are the consequences of viral properties
Aside for not being living things, they need to be infectious to endure in nature. They mist be able to use host cell to produce their components and they must encode any required processes not provided by the cell. Viral components must self-assemble
Where can viruses exist
Extracellularly or intracellularly
What happens to viruses when they are extracellular
They are inactive because they can’t reproduce outside a host cell
What happens to viruses when they are intracellular
They commandeer host cells and use them to synthesize viral components from which mature progeny viruses are assembled and eventually released
Who can viruses infect
-Eukaryotes( animals, plant, fungi)
-Bacteria and Archaea
-Other viruses( viriophages)
On what properties can we do virus classification
-Genome type
-Virion structure
-Replication strategy
What’s virus classification by genome type
-Type of nucleic acid(DNA or RNA)
-Double helix or single helix and polarity
-Unsegmented or segmented
-Linear or circular
–If the genome is diploid( retroviridae)
What’s virus classification by virion structure
-Capsid symmetry: icosahedral, helical, complex
-Number of capsomeres
-Presence or absence of envelope
What’s classification of viruses by replication strategy
It’s shown by Baltimore classification system where:
1. dsDNA->mRNA
2.ssDNA->dsDNA->mRNA
3.dsRNA->mRNA
4.(+)ssRNA->(-)ssRNA->mRNA
5.(-)ssRNA->mRNA
6.(+)ssRNA->DNA/RNA hybrid->dsDNA->mRNA
7.dsDNA->ssRNA->mRNA
In what taxa can viruses be divided in
-Orders(-virales)
-Families(-viridae)
-Subfamilies(-virinae)
-Genus(-virus)
-Species
-Strains
-Types
What characteristics do viruses of the same specie share
They infect same host or a tissue, cause the same pathology and have same genomic map
What about genotypes of viruses?
Genotypes posses nucleotide differences that do not result in antigenic differences
What about serotypes of viruses
Serotypes differ in the presence of specific epitopes- are neutralized by different antibodies
What are viral strains
Viral isolates that have been well defined; like laboratory strains
How is mature virus particle called
Virion; it’s infectious( able to infect a cell)
What do the simplest virions consist of
-nucleocapsid(composed of nucleic acid, either DNA or RNA)
-protein ccoat-capsin
What does capsid do
It surrounds viral nucleic acid, thereby protecting the genome and often aids in its transfer between host cells
How can viruses be classified based on the envelope
.Enveloped viruses-virions covered by a lipid membrane
-Nonenveloped or naked viruses- viruses lacking the lipid membrane
What’s missing from the viruses
Ribosomes( for protein synthesis ) and mechanism for generating ATP. Cytoplasm is absent, and there is not enough enzymes (if any at all) to sustain cellular processes
What is virion structure defined by
Capsid symmetry and presence or absence of envelope
How do nonenveloped viruses construct a capsid
From many copies of one protein and a few minor proteins. Each subunit is named protomer and thousands of protomers self-assemble to form the capsid
How do enveloped viruses form a capsid
They require both nucleocapsid proteins and additional proteins to anchor the membrane. Some viruses use noncapsid proteins as scaffolding upon which the capsids are assembled
What are helical capsids and give example
They are capsids shaped like hollow tubes with protein walls. One of the examples is tobacco mosaic virus
How does TMV assemble its helical capsid
Self-arrangement of TMV protomers into helical arrangement produces a rigid tube. Capsid encloses a genome, which is wound in a spiral and lies within a groove formed by the protein subunits
What kind of nucleic acid does helical capsid contain
RNA
What are other examples, except TMV of helical viruses
Rabies virus, Ebola, influenza
What is icosahedral
Regular polyhedron with 20 equilateral triangular faces and 12 vertices
Characteristics of icosahedral capsid
It’s the most efficient way to enclose a space. They are structured from ring- or knob-shaped assemblages of 5 or 6 protomers, the assemblages are called caspomers
What are capsomers composed of 5 and 6 protomers called
Pentons and hexons( hexamers)
Are hexomeras in every icosahedral capsid
No, but pentamers are
With what does number of exons vary
With size of a virus
Give example of viruses based on number of exons
0- Parvovirus B19
20- Polioviruses
60- Papillomaviruses
150- Epstein and Barr viruses
240- Adenoviruses
What are examples of viruses with complex symmetry
Poxviruses and large bacteriophages
What are characteristics of poxviruses
They are among the largest of the animal viruses and can be seen with the light microscope. they have complex internal structure with an ovoid to brick shaped exterior
What is virion morphology of poxvirus
Its double-stranded DNA genome is associated with proteins and contained in the core, a central structure shaped like biconcave disk and surrounded by a membrane. Two lateral bodies lie between the core and the virion’s outer envelope and contain viral enzymes
What’s characteristic of virions of some phages
Virions of T2,T4 and T6 phages8even phages) that infect Escherichia coli are said to have binal symmetry because they have a head that resembles an icosahedron and a helical tail
What’s different about the head of T-even phages
Icosahedral head is elongated by one or two rows of hexamers in the middle and contains the DNA genome
What is characteristic for the tail of T- even phages
Tail is composed of a collar joining it to the head, a central hollow tube, a sheath surrounding a tube and a complex baseplate
What’s characteristic for a baseplate of T-even phages
Baseplate is hexagonal and has a pin and a jointed long tail fiber and a short tail fiber at each corner
What are examples of icosahedral and helical viruses with and without envelope
Icosahedral without envelope: poliovirus ,adenovirus, hepatitis A virus
Helical without envelope: TMV; no human virus is found here
Icosahedral w envelope: herpesvirus
Helical w envelope: rabies virus, flu virus
