Viruses, viroids and prions Flashcards
(14 cards)
Describe the features of a virus
- can replicate and evolve
- do not grow
- lack genes needed for energy production and protein synthesis
- contain DNA or RNA but not usually both
- insensitive to changes in the environment
- can only reproduce inside a living cell –> obligate parasites
- found to affect organisms from all domains of life
- usually very small
Describe the general structure of a virus
- surrounded by a protein coat called the capsid –> made of subunits called capsomeres
- contain nucleic acid
- virus enzymes may be present
- contain glycoprotein spikes on the surface
What is the difference between a naked virus and an enveloped virus?
- in a naked virus, the outer layer is the capsid
- in an enveloped virus, the protein layer is surrounded by an envelope consisting of a lipid bilayer with proteins projecting from the surface
How do enveloped viruses acquire their membrane?
As the virus leaves the cell it picks up the glycoprotein spikes along with a portion of cell membrane bilayer
List the stages in the life cycle of a virus
- entry and uncoating
- replication
- Transcription and manufacture of capsid proteins
- Self-assembly of new virus particles and their exit from the cell
List some possible features of viruses with RNA genomes
- viruses with double stranded RNA genomes (rare)
- positive strand ssRNA viruses –> single stranded RNA genome, genome can act directly as mRNA
- negative strand ssRNA viruses –> single stranded RNA genome, genome is complementary to mRNA
- retroviruses –> replicate via a DNA intermediate
Describe the pathways during replication of a (+)ss RNA virus
One pathway:
1. translation (RNA-dependent RNA polymerase)
2. Capsid proteins
Second pathway:
1. Viral RNA (+) strand
2. RNA-dependent RNA polymerase synthesis
3. RNA complementary to viral genome (-) strand
4. RNA-dependent RNA polymerase uses (-) strand as template to synthesise new copies of virus genome
5. Assembly of new virus particles
Briefly describe the replication of HIV
Contains the enzyme reverse transcriptase. They make a DNA copy of their genome which is integrated into a host chromosome
What are viroids?
Infectious agents without protein
What are prions?
- infectious agents without DNA or RNA
- cause the transmissible spongiform encephalopathies (TSEs) –> a group of related diseases leading to fatal and untreatable neurological deterioration
How are TSEs characterised and give two examples
- very extensive neuronal death
- in most cases, spongiform degeneration (vacuolation) of the brain
- are much smaller than viruses
- very heat resistant
- very radiation resistant
1. Scrapie - affects sheep and goats
2. Bovine spongifrom encephalopathy (BSE)
List 3 TSEs that affect humans
- Creutzfeld-jakob disease –> arises in middle an old age and is usually sporadic
- Variant Cruetzfeld-jakob disease –> affects younger people
- Fatal familial insomnia –> a genetic disorder that can be transmitted by injection of affected brain tissue into healthy lab animals
Describe the action of prions
- believed to be mis-folded versions of a normal cellular protein (PrP)
- once introduced into an unaffected brain, prions cause normally folded prion protein to mis-fold in turn
- mutations in the gene encoding the prion protein may make it more likely to mis-fold spontaneously, accounting for inherited transmissible spongiform encephalopathies
Describe the features of a mis-folded prion protein
- protease resistant
- low a helix content
- high B sheet content
- insoluble
- aggregates to form fibres
