Introduction to Viruses Flashcards
What are nucleoproteins?
Viral proteins that stabilise and protect the genome.
They help in packaging and replication
What are viruses?
Obligate intracellular parasites; depend entirely on host cells for replication
What are two types of viral structures?
Enveloped and non-enveloped
What does it mean for a virus to be “enveloped”?
Envelope is a lipid bilayer derived from the host membrane, embedded with viral glycoproteins
What are the functions of the envelope in viruses?
- Receptor recognition
- Host cell entry via membrane fusions or endocytosis
- Immune evasion by mimicking host components
What is the structure of non-enveloped viruses?
They have a protein coat composed of capsomeres called a capsid
What is the function of the capsid in non-enveloped viruses?
- Protects genome form nucleases
- Provides structural integrity
- Aids in host attachment
Out of enveloped and non-enveloped viruses, which are generally more stable in a host environment?
Non-enveloped
What is the main differences between enveloped and non-enveloped viruses?
Non-enveloped are generally more stable, but enveloped are better at immune evasion
What is RdRP?
RNA-dependent RNA polymerase
Why must RNA viruses encode their own polymerase?
Host cells cannot replicate RNA from an RNA template, so they don’t have the necessary enzyme.
Therefore, RNA viruses will encode their own polymerase (RdRP)
What is the exception to RNA viruses requiring RdRP?
Retroviruses, which instead use reverse transcriptase
How does RdRP work?
Synthesises complementary RNA from the viral RNA template
What is a feature of RNA-dependent RNA polymerase, and why is this significant?
RdRP lacks proofreading activity. The consequences of this are:
- High mutation rates (rapid viral evolution)
- Production of quasi-species
- Risk of lethal mutagenesis (too many errors = nonviable genome)
Example of drug that targets RdRP?
Remdesivir targets the RdRP of SARS-CoV-2 to inhibit viral replication
What is the error rate of RdRP?
1 error per 10,000 (high)
What are the main drivers of viral evolution?
- Massive progeny output
- High mutation rate
- Quasispecies concept
- Selection
How does a massive progeny output drive evolution?
- Rapid replication cycles
- Single infected cell can produce 10^4 - 10^9 virions
- Huge population for selection to act on
What is the mutation rate of DNA viruses?
1 error per 10^6 - 10^8
Lower than RNA viruses (as have proofreading) but still high, especially considering rapid replication rate
Who introduced the quasispecies concept?
Manfred Eigen
What is the quasispecies concept, and how does it drive viral evolution?
Proposes that viral populations exist as a “mutant cloud” of genetically diverse, closely related variants.
Provides a strong robustness to environmental changes (like immune/drug attack), as there is a huge variety of genetic differences, so what may work to attack/kill one variant, will not work with another
How does selection drive viral evolution?
Evolution favours variants with increased fitness (survival of the fittest).
Selection pressures like:
- Host immune responses
- Antiviral drugs
- Temperature
allow the selection of an ever evolving, stronger virus.
What is an acute viral infection (with example)?
Rapid replication and spread, but is usually self-limiting and cleared by adaptive immunity.
Example is Influenza A (in healthy individuals)
What is a chronic viral infection (with example)?
Continuous replication at low levels, with immune evasion mechanisms to prolong infection
Example is HIV
What is a latent viral infection (with example)?
Viral genome persists in host cells without active replication, although can reactivate under stress or immunosuppression
Example is Herpes Simplex Virus in trigeminal ganglion
