Viral Genetics and Mutations

Viral Genetics

• The study of viral genetics encompasses two areas:
  • Mutations and their effects on replication and pathogenesis
  • Interaction of two genetically distinct viruses infecting the same cell

Mutations

• Mutations in viral DNA and RNA occur through base substitution, deletion, and frameshift mutations.
• Mutations are crucial in vaccine development, as attenuated mutants (weakened but still antigenically similar) are used to develop vaccines. These vaccines do not cause disease but induce immunity.
• DNA viruses leverage host cell machinery, benefiting from proofreading and error correction mechanisms, leading to fewer mutations.
• RNA viruses undergo more mutations due to the lack of proofreading capabilities in their polymerases. This high mutation rate allows them to rapidly adapt to changing environments. This adaptability is referred to as antigenic drift.
• Two other types of mutants exist:
  • Antigenic variants: These have altered surface proteins, making them resistant to pre-existing antibodies. They can lead to disease outbreaks.
  • Drug-resistant mutants: These are insensitive to antiviral drugs as their viral targets have been modified.
  • Defective interfering (DI) particles: These defective mutants can only replicate if a "helper" virus is present. They can also interfere with the growth of normal viruses.

Viruses Interaction

• Interaction of two or more viruses in the same host cell can occur via several mechanisms:
  • Recombination: Exchange of genes between two viruses with high sequence homology. This is more common in double-stranded DNA viruses.
  • Reassortment: Occurs in RNA viruses with segmented genomes (like influenza) and leads to genetic variation.
  • Complementation: A nonfunctional viral protein from one virus can be complemented by a functional protein from another virus. This can be essential for defective viruses.
  • Phenotypic mixing: Two viruses infecting a cell can result in progeny with mixed capsid proteins, potentially altering their tropism.
  • Phenotypic masking (transcapsidation): The capsid protein of one virus surrounds the genome of another, producing progeny with altered tropism compared to the parental viruses.

Interference

• Interference occurs when one virus inhibits the growth of another, often mediated by interferons. The inhibition can be due to factors such as competition for receptors or the production of inhibitors like interferons. Sometimes simultaneous infections can enhance viral yield (rather than hinder it).

This quiz covers the essential aspects of viral genetics, focusing on mutations and their implications for replication and vaccine development. It explores how different viruses interact within the host and the significance of mutation rates in RNA and DNA viruses. Test your knowledge on the mechanisms and consequences of viral mutations.