1 - Intro to Viruses Flashcards
Viruses
- Inert outside living host
- Cause infection
- Most do not cause disease in humans
- We carry viral genomes as part of our own genetic material (endogenous retroviruses)
Four traits viruses are defined by
- Virions
- Capsomeres
- Assembly
- Capacity to evolve
Virion
Infectious extracellular stage of obligate intracellular viruses`
Capsomeres
- All viruses encode atleast one
- Cover and protect the nucleic acid in a virion
Assembly
Cells infected by virus synthesise the component parts of virion, and then the parts spontaneously assemble into new virions
Capacity to evolve
Because the minimum composition of viruses is nucleic acids and capsomeres, viral populations can change through typical evolutionary processes (e.g. selective pressures)
Obligate intracellular parasites
- Require host cells to multiply
- No machinery for protein synthesis and energy production
Capsid
- The protein coat of capsomeres that surrounds the nucleic acids
- May be spherical or helical
How are virions separated
- Into two classes based on whether they have an external layer consisting of a proteinaceous lipid bilayer
- Enveloped virions have a lipid bilayer
- Naked viruses do not
Spherical capsids
- Icosahedrons (approximate the volume of a sphere yet are constructed from
repeated subunits) - Spherical capsids are somewhat rigid, helical capsids can be rigid or flexible
Bacteriophages
Combine the icosahedral and helical elements, so that their heads, full of nucleic acids, are icosahedral but their tails are helical assemblages of specialized tail proteins.
Nucleocapsid
The nucleic acid genome and capsomeres are so intimately associated
Example of virus that lack clearly defined capsids
Poxviruses
Most successful vaccine
HPV (via virus like particles)
Examples of RNA viruses
- Coronaviruses
- Flaviviruses
- Retroviruses
Examples of DNA viruses
- Adenoviruses
- Herpesviruses
- Poxviruses
Baltimore classification
Viruses can be classified according to the ways they synthesise and use mRNA
Central dogma and importance of mRNA
- DNA –> RNA –> Protein
- Cellular genes are encoded in dsDNA from which mRNAs are produced
to direct the synthesis of protein. - Therefore all viruses must direct the synthesis of mRNA to produce proteins
Quasispecies
Upon infection with an RNA virus viral replication leads to a mutant spectrum of related genomes (quasispecies)
Size of viruses
- Many viruses are less than 200 nm (10^-9 m) in diameter (spherical
viruses) or length (helical viruses) - Too small to be seen by light microscopy
Animal cells in culture
- Form confluent carpets (cells introduced into culture attach to and spread out until cells are touching one another without overlapping)
- Contact inhibition prevents further population increase
Example of virus induced cytopathic effects (CPE)
- Cause infected cells to round up and detach from the tissue flask as they die
- Syncytia (large, multinucleate cells)
- Cause abnormal internal structures visible when the host cells are stained (e.g. inclusion bodies)
-Transformed cancerous cells grown on top of each other instead of responding to contact inhibition
Inclusion bodies
Sites of viral gene expression, genome replication, or assembly
Methods of culturing viruses
- In animal and human cells
- In embryonated chicken eggs
First step of SARS-CoV-2 cell entry
Binding of spike protein to receptor (ACE2)
What are classical taxonomic categories based on
- Disease they cause
- Symptoms of disease they cause
- Structure
- Phenotype
- Evolutionary relatedness
