Viruses, viroids, prions! Flashcards
(28 cards)
Give a simple definition of a virus
One or more nucleic acid molecules within a coat made of protein
Properties of a virus
Can replicate and evolve, but….
* Lack genes needed for energy production and (usually) protein synthesis
* Are insensitive to changes in their environment
* Can only reproduce inside a living cell (i.e. are obligate parasites)
* Do not grow
* Are found to affect organisms from all domains of life
* Are very small (usually)
* Contain DNA or RNA, not usually both
What kind of parasite is a virus?
Obligate parasite - cannot live without the host
What is the protein coat of a virus called? What are the subunits of the coat called?
The capsid, made up of capsomeres
What does it mean if a virus is enveloped as opposed to naked?
Enveloped - Surrounded by a lipid membrane acquired from the cell or nuclear membrane of an infected cell
What are the 4 basic steps of a generic life cycle of a virus with a DNA genome?
- Entry and uncoating
- Replication
- Transcription and manufacture of capsid proteins
- Self-assembly of new virus particles and their exit from the cell
What are the 4 kinds of viruses with RNA genomes? Give examples
Viruses with double stranded RNA genomes (rare)
Positive strand (+) ssRNA viruses: Single stranded RNA genome which acts directly as mRNA and uses RNA polymerase. E.g. SARS-Cov-2
* Negative strand (-) ssRNA viruses: Single stranded RNA genome. Genome is complementary to mRNA. E.g. Influenza, ebola, rabies
Retroviruses: Replicate via a DNA intermediate. E.g. H.I.V
What kind of virus is HIV?
Retrovirus
How do retroviruses replicate?
Via a DNA intermediate e.g HIV, using Reverse transcriptase
Make a DNA copy of their genome which is integrated into host chromosome
Define viroid and describe its method of action
Infectious agent without proteins - infection RNA
Circular, single stranded RNA molecule about 350 nucleotides in length with substantial internal base pairing.
The RNA replicates inside infected cells
Give an example of a viroid
Potato spindle tuber viroid
Peach latent mosaic viroid
Define prions
Infectious agents without DNA or RNA - like the opposite to viroids, which are infectious agents without proteins
What do prions cause?
Transmissible spongiform encephalopathies
(TSEs).
A group of related diseases leading to fatal and untreatable neurological deterioration
What are TSEs characterised by?
Very extensive neuronal death
In most cases, Spongiform degeneration
(vacuolation) of the brain
Name 2 animal TSEs
- Scrapie – affects sheep and goats
- Bovine spongiform encephalopathy (BSE)
Name 3 human TSEs
- Creutzfeld-Jakob Disease. – arises in middle and old age; most cases sporadic, a few transmitted (e.g. through treatment with contaminated growth hormone), some
cases inherited as a dominant genetic disorder. - Variant Creutzfeld-Jakob Disease – affects younger people, transmitted via BSE infected beef, now virtually eradicated.
- Fatal familial insomnia. – a dominant genetic disorder that can be transmitted by injection of affected brain tissue into healthy lab animals
What do viruses lack?
Genes for energy production and usually for protein synthesis
Prions compared to viruses?
Prions are:
Much smaller
Heat resistant
Radiation resistant
What is prion misfolding usually caused by?
Mutations
Prion + form (misfolded), compared to Prion - form (normal)?
Misfolded Prion+ is:
- heat resistant
- insoluble
- aggregates form fibres
- less alpha helices - more beta pleated sheets
+ form is the infectious form!!!!!!
Do viruses grow, and do they evolve?
They do not grow, they do evolve
How do enveloped viruses obtain their membrane?
By budding through the host cell or nuclear membrane during exit
What are the 3 major shapes of viral capsids?
Helical, icosahedral (20-sided), complex
What is a (+) ssRNA virus?
A virus whose RNA genome can act directly as mRNA (e.g. SARS-CoV-2)
