Intro and virus structure Flashcards
What is a virus?
Very small, infectious, obligate intracellular parasite.
What is the escape or progressive hypothesis?
Mobile genetic elements, pieces of genetic material capable of moving within a genome, gained the ability to exit one cell and enter another.
What is the evidence for the escape or progressive hypothesis?
- 42% of the human genome is composed of retrotransposons that move within the genome via an RNA intermediate and then integrate at another site – a bit like a retrovirus.
- Some retrotransposons possess a reverse transcriptase and integrase.
- Acquisition of structural protein which allowed them to enter and exit a cell.
What is the reduction or regressive hypothesis?
Viruses are degenerate life forms that have lost many functions and retained only genetic information required for a parasitic way of life.
What is the evidence for the reduction or regressive hypothesis?
- Chlamydia and Rickettsia evolved from free-living ancestors.
- Large viruses depend less on the host for replication than other viruses.
- Mimivirus genome had genes which may be relics of a previously complete translation system.
What is the Virus- first hypothesis?
Independent entities that evolved in parallel to cellular organisms from the self-replicating molecules that existed in the primitive, prebiotic RNA world.
What is the evidence for the virus-first hypothesis?
- First replicating molecules consisted of RNA not DNA
- Nucleus in eukaryotic cells may have arisen by an endosymbiotic event in which a complex enveloped DNA virus became a permanent resident of an merging Eukaryotic cell.
- Viruses such as Mimi virus have relics of a protein synthesis machinery, inc tRNA genes and replicate independently of host genome
What are the defining features of a virus?
- Obligate intracellular parasites.
- Package their genome in a protein particle which is used to transfer genome from host to host.
- Genome has all material required to initiate and complete a replication cycle within a host.
- All viral genomes must establish themselves within a host to maintain their survival.
- All viruses must make mRNA that can be translated by host ribosomes.
Definition of: Virion, Envelope/viral membrane, Capsid/coat protein, Nucleocapsid, Capsomere, Protomer.
Virion-infectious viral particle.
Envelope-host derived lipid bilayer
Capsid-shell surrounding the nucleic acid
Nucleocapsid-protein assembly packaged within the vision- DNA/RNA
Capsomere-surface structure seen by EM
Protomer-structural unit from which capsids or nucleocapsids are built
Explain the metastable structure of virions
They are not inert but need to be stable to act as a protective container for the genome.
Once in the host cell the vision must be able to open up and release its genomic content.
The metastable structure is generated by non covalent bonding of protein subunits.
What symmetry do rod shaped and spherical viruses have?
Rod shaped- helical symmetry
Spheres- Icosahedral symmetry
Examples of viruses with helical symmetry
Paramyxoviruses
Orthomyxoviruses
Rhabdoviruses
Assemble of helical symmetry of an Orthomyxovirus
Has multiple ribonucleic proteins which organise each RNA into a helical structure.
This is further folded into a compact circle by viral P proteins which bind to specific sequences at the 5’ and 3’ ends of each (-) strand RNA molecule.
Icosahedral symmetry features
An Icosohedron has 20 triangular faces and 12 vertices
5:3:2 axes of rotational symmetry
Composed of a minimum of 60 identical subunits
How do you make a virus bigger?
Increase triangulation value and increase the number of subunits
