4.9 Virus Structure and Replication Flashcards
What are virions?
What is the symmetry of the virus capsid?
To protect the genome from breakdown by nucleases, the capsid proteins are assembled symmetrically around the nucleic acid
What are the characteristics of viruses with icosahedral capsid symmetry?
- Icosahedral capsid may or may not be surrounded by a membrane
- Envelope/membrane is a host-derived lipid from intracellular membrane: nuclear, endoplasmic reticulum, golgi or plasma membrane
- Envelope may be flexible (pleiomorphic) giving rise to unusual shapes
Which viruses have helical capsid symmetry?
- Rod shaped coat consisting of repeating units
- A single protomer associates with nucleic acid in a spiral or helical arrangement
Which viruses have complex symmetry?
- The poxviruses are the largest and most complex of all viruses. There are >100 different proteins in the virion and the genome is 130 kbp dsDNA
Which viruses have multiple shelled capsids?
- Rotaviruses infect the lining of the intestine and cause diarrhea, especially in children. The virus is very hardy, surviving passage through the gut and may even survive for long periods in sewage despite stringent treatment
How have viruses been grouped in taxonomy?
The International Committee on Taxonomy of Viruses (ICTV) has grouped viruses into orders, families, genera and species based on their…
- Genome (nucleic acid type, fragmentation, organisation, sequence)
- Strategy for replication (reverse transcription, integration, site of replication)
- Morphology (envelope…)
How does a plaque assay show virus replication?
- Show us if virus is infection cells because induce cell death
- Add virus to cells and over several days virus will replicate
- But will not spread far because of agar overlay, forming plaque
- Wash cells take agar off and stain alive cells
Explain each phase
- Eclipse period when virus just entered cell starting to undergo early stages of replication
- Latent period replication cycle reached its maximum accelerating very quickly
- Delay in the virus inside the cell compared to when released in the extracellular space
- 16 hours after infection to see virus in extracellular space
Name each stage
How does a virus enter the cell?
- Either fusion with plasma membrane OR endocytosis then endosome escape
- Both pathways require interaction with a receptor on the host cell surface
- PH dependent process, low pH of endosome needed to drive that process of fusion and release into the cytosol
Where does the virus go once it has entered the cell?
- Once the virus has entered the cell and the genome has been uncoated it has to go to the right place in the cell for replication
- Cytoplasm for RNA viruses (exception is influenza)
- Nucleus of DNA viruses (exception is poxviruses)
How does poliovirus replication occur?
- Polio encodes for a couple of special - enzymes that must be made before the genome can be replicated
- RNA-dependent RNA polymerase (RdRp) to copy the genome into a (-) strand RNA that is used for a template to prodyce mRNA and genome copies
- Viral protease that cleaves at specific sites in the translated polyprotein to produce the mature proteins
What is the ultimate step of all viruses when invading host cell?
- ultimate step is to produce mRNA to produce proteins
- Some need to bring in the viral polymerase as complimentary strand does not look like mRNA and will not be recognised by host cell ribosome
- Copy negative strand into positive which acts as mRNA
How do viruses mature and exit the cell?
- Viral proteins that end up in the envelopes of viruses traffic through the ER and golgi and are deposited on the surface of the cell. As a consequence they are often glycosylated
- Once the viral genome is copied and proteins produced, the components are assembled into virions
- Non-enveloped viruses build up within the cell until the cell lyses and virions are released
- For enveloped viruses, release involves budding
How does virus exit by budding occur?
