Flashcards in 2.0 Virology Deck (73)
What is the size range of a virus?
Most are 20-700nm
(can be smaller)
What is the smallest virus?
Foot and mouth disease virus (FMDV) - 20nm
What is the largest virus?
Mimivirus - 700nm
What is the structure of virions?
1) Nucleic acid (genome)
3) Lipid membrane (envelope)
Structure of capsids:
Composed of capsomers (repeating protein units)
Symmetrical (can be helical or icosahedral
Structure of lipid membrane:
Phosopholipid membrane (aquired from host)
Embedded viral protein
May be glycosylated
How do virions increase the coding capacity of their small genomes?
1) Densely packed genes
2) Overlapping reading frames
4) Few non-coding regions
What are the different ways to measure viruses?
1) Electron micrograph
- Useful for quantification
- No info on virulence
2) Polymerase chain reaction
- Useful for diagnosis
- No info on virulence
3) Immunological evidence of infection
- Detection of adaptive immune response
- Too slow for diagnosis (useful epidemiologically)
- Can give some info on virulence
4) Plaque assay
- Preffered method to measure infectivity
What are the stages of viral replication?
1) Adsorption and penetration
2) Eclipse phase
3) Assembly and release
What is the mean burst size?
Average yield of virus particles per cell
- Varies greatly
How does HIV bind to target cell?
gp120 (envelope glycoprotein) on HIV binds to CD4 + chemokine co-receptor
CD4 = only on T-cells
How does influenza bind to target cell?
Haemagglutin (HA) (envelope glycoprotein) on influenza binds to sialic acid
Sialic acid = on most cells
How does EBV bind to target cell?
gp340 (envelope glycoprotein) on EBV binds to CD21
How does HIV penetrate the target cell?
Binding of gp120 to CD4 → conformational change in virus gp120/gp41 → virus envelope fuses with plasma membrane
How does infleunza penetrate the target cell?
Binding of HA to sialic acid → endocytosis → endosome is acidified
↓ pH → rearrangement of HA → viral envelope is pulled closer to vesicle membrane → disruption → fusion
What happens in the eclipse phase?
No virus particle present in host cell
Virus has disassembled
Genome is being replicated
Virus proteins are being made
What are the steps for -ve ssRNA and dsRNA viral replication?
OCCURS IN CYTOPLASM
1) Viral genome transcribed to +ve sense RNA
- Enzyme = viral RNA dependent RNA polymerase
2) +ve sense RNA can be used as mRNA or to make new viral genome
Exception to this is influenza (occurs in nucleus - uses host RNA pol II)
What are the steps for +ve ssRNA viral replication?
OCCURS IN CYTOPLASM
- Translated proteins include RNA dependent RNA polymerase
2. Virus genome is replicated into complimentary (-ve sense) RNA (RNA dependent RNA polymerase)
3. Second stage of replication is to copy -ve sense to +ve RNA (RNA dependent RNA polymerase)
4. These can then be packaged into new virions
What are the steps for retrovirus replication?
1. Virus genome is copied by reverse transcriptase
- This is an RNA dependent DNA pol
- Packaged within the virus particle
- Creates dsDNA intermediate
2. dsDNA intermediate is integrated into host genome
3. mRNA is transcribed by host DNA dependent RNA pol II
- provirus = template
4. Full length transcripts can be translated
5. Or packaged into new virus capsids in the cytoplasm
What are the steps for ds DNA viral replication?
OCCURS IN NUCLEUS
1. Virus genome is transported into nucleus
- Uses host DNA-dependent RNA pol
3. mRNA translation
- Occurs in cytoplasm
4. Some proteins are transported back to nucleus
- e.g. DNA pol and capsid proteins
5. In nucleus viral DNA is replicated and progeny genomes are packed into new capsids
How are poxviridae virions an exception to ds DNA viral replication?
Replication occurs in cytoplasm
They carry their own enzymes (DNA dep. RNA pol + capping/polyadenylating enzyme)
Viral DNA alone is not infectious
In viral replication, what do early genes code for and what do late genes code for?
1) Early genes → nucleic acid replication (+ modification of host cell)
2) Late genes → structural proteins of virion
(Early proteins = low amounts, late proteins = large amounts)
What are the mechanisms virions use to make different proteins (poly-protein processing)?
1) Post-translational cleavage (using specific proteases)
2) Segemental genome (influenza)
3) Splicing (e.g. HIV → gp160 → gp120 + gp41)
What are the two mechanisms of viral release from a cell?
1) Lysis of cell
2) Budding of enveloped virus
What viruses show latency?
What modifications to host cells can viruses induce?
1) Subversion of cellular metabolism to make only viral proteins
2) Cell stimulation
3) ↑ dNTP pool
4) Membrane modifications
5) Cytopathic effect (CPE)
6) ↓ host cell signalling (↓ innate immunity)
7) Lytic/non-lytic infections
8) Cell transformation
What viruses cause lytic infections?
1) DNA viruses
2) Non-enveloped RNA
3) Viruses that cause host-cell shut off
What viruses cause non-lytic infections?
1) Enveloped RNA
What viruses cause cell transformation?
1) HPV → wart/cervical cancer (16 +18)
2) Rous sarcoma virus → sarcoma in chickens