2. Viruses and Prions Flashcards
How do prions spread?
- from cell to cell between individuals
- via contaminated food, hormone treatments, blood and surgical instruments
What kind of disease do prions cause?
neurodegenerative
- transmissible spongiform encephalopathies
How do normal proteins become infectious prions?
- protein undergoes rare conformational change to give abnormally folded prion form
- this abnormal form causes conversion of normal proteins in the host’s brain into misfolded prion form
- prions aggregate into amyloid fibrils (which disrupt brain cell function) causing neurodegenerative disorder
Why are viruses acellular?
- no cytosol
- no cytoplasmic membrane
Give some diseases caused by viruses
- common cold
- warts
- chickenpox
- polio
- rubella
- smallpox
- herpes
- mumps
Capsids are made of what?
- repeating protein subunits or capsomers
- a capsomer is composed of a single repeating protein to save on gene space
Togavirus, HIV and SARS-CoV-2 are what shape?
enveloped
2 features that can distinguish viruses
- nature of nucleic acid
- virion shape
Irrespective of the nature of viral nucleic acid, what must happen?
- must be converted to mRNA
- to allow synthesis of viral proteins
5 stages of viral replication
- attachment of virion to host cell
- entry of viral nucleic acid into host cell
- synthesis of viral nucleic acid and proteins
- assembly of new viruses with host cell
- release of new virions from host cell
Why does no immune response occur in Creutzfeldt-jakob disease?
- caused by prions
- the misfolded protein is the human’s own protein so won’t produce an immune response
4 main virion shapes seen in EM
- helical
- polyhedral
- complex
- enveloped
Shape of virus is determined by what?
capsid
3 types of Creutzfeldt-Jakob disease
- spontaneous (sporadic)
- inherited (familial)
- acquired (variant)
Where does viral proteinsynthesis occur?
on host cell ribosomes
Explain lytic replication
- replication cycle usually results in lysis and further death of host cell
- lysis allows newly synthesised virions to be released
How to synthesise mRNA from positive sense RNA virus?
used directly as mRNA
Viruses are small. What’s their size range?
20-200nm
Why can viruses not replicate themselves?
- don’t possess genes encoding enzymes required for nucleic acid replication
- don’t produce ribosomes for protein synthesis
How do helical virions form?
- capsomers bond together in spiral fashion
- form a tube around nucleic acid
- results in rod-shaped or filamentous virions
Stacking of … allows misfolded proteins to aggregate into …
beta sheets
amyloid fibrils
Which is the most common prion disease?
What frequency has it?
- sporadic CJD
- 1-2 per million
Misfolded proteins can contribute to non-infective tissue degenerative disorders e.g …
- Alzheimers
- Parkinsons
- Hungtindon’s
- atherosclerosis
- type 2 diabetes
Dental tissue is low/high risk for prion transmission
low
How do dentistry protocols avoid prion transmission?
- difficult to destroy with autoclaving and no reliable assay
- strict regulations on decontamination procedures and designation of single use instruments
Define ‘virus’
- small, acellular, infectious agents
- evolved to transfer nucleic acids from one cell to another
How do viruses replicate?
- utilise chemical and structural components of cells they infect
- they’re obligate intracellular parasites
What kind of microscopy is needed to see viruses?
electron
What’s the difference between virus and virion?
- viruses have an intracellular and extracellular state
- extra is virion
- intra is virus
Structure of a virion
- protein coat surrounding the nucleic acid is called a capsid
- coat and nucleic acid is referred to as nucleocapsid
- some have an additional phospholipid membrane called envelope outside nucleocapsid
2 features that distinguish viruses
- nature of nucleic acid
- virion shape
Viral genomes encode what kind of protein?
- viral structural proteins
- proteins that interact with the host for example proteases, DNA/RNA polymerase, reverse transcriptase, immune system inhibitors
Viral genomes can have RNA or DNA, both of which can be … or … stranded and the RNA can be … or …
- single or double
- positive or negative sense
3 ways viral mRNA can be synthesised
- transcribed from viral DNA (DNA virus)
- synthesis of RNA complementary to viral DNA (negative sense RNA virus)
- used directly as mRNA (positive sense RNA)
How to synthesise mRNA from negative sense RNA virus?
synthesis of RNA complementary to viral RNA
Capsids are made of what?
- repeating protein subunits or capsomers
- a capsomer is composed of a single repeating protein to save on gene
How are polyhedral virions formed?
- capsid is roughly spherical (shape is like a geodesic dome)
- capsomers are pentamers or hexamers
- most commonly forms an icosahedron with 20 sides
Advantage of polyhedral virion?
- efficient at enclosing space and making a robust structure just from one single repeating protein
What shape is T4 bacteriophage?
complex
What shape is poliovirus?
polyhedral
How does HIV replicate?
- fusion of HIV to host cell surface
- HIV RNA, reverse transcriptase, integrase and other proteins enters host cell
- viral DNA formed by reverse transcriptase
- viral DNA transported across nucleus and integrates into host DNA
- new viral RNA is used as genomic RNA and makes viral proteins
- new viral RNA and proteins move to cell surface and new immature HIV forms
- virus is released and viral protease cleaves new polyprotein to create mature infectious virus
