Viruses

Introduction

• Virus: small infectious particle that consists of nucleic acid enclosed in a protein coat called a capsid
• Host range: number of species and cell types that can be infected
• Structure: viral capsids vary in shape and complexity
  • Some have viral envelope derived from the host cell’s plasma membrane
• Genome
  • DNA vs RNA, single stranded (ss) vs. double stranded (ds), linear vs circular

Reproduction

• Viruses are not alive
  • Not cells or composed of cells
  • Cannot carry out metabolism on its own
• Viral reproductive cycle can be quite different among types of viruses
  • A virus may have alternative cycles

Viral Reproductive Cycle

1. Attachment

2. Entry

3. Integration (depending on the virus)

4. Synthesis of viral components

   1. Host cell enzymes such as DNA polymerase make many copies of the phage DNA and transcribe the genes within these copies into mRNA
   2. In the case of HIV, the DNA provirus is not excised from the host chromosome. Instead, it is transcribed in the nucleus to produce many copies of viral RNA 
      • Translated to make viral proteins
      • Serve as genome for new viral particles

5. Viral assembly

   1. Some viruses self-assemble
   2. Others are too complicated to self-assemble
   3. Proteins modify capsid proteins or serve as scaffolding

6. Release

   1. Phages must lyse their host cell to escape
   2. Enveloped viruses bud from the host cell

Latency In Human Viruses

• Two different mechanisms

  1. Virus integrates into host genome and may remain dormant for long periods of time
  • ex: HIV

  1. Other viruses can exist as episomes

     1. Episomes: genetic elements that replicate independently but occasionally integrate into host DNA
     • ex: Herpes simplex type I and II, varicella zoster \n (chicken pox)

Origin of Viruses

• Many biologists argue that cells evolved first, before viruses
  • Viruses evolved from macromolecules inside living cells (maybe plasmids)
• Others argue for regressive evolution
• Another theory is that viruses did not evolve from cells but evolved in parallel with cellular organisms

Viroids and Prions

• Viroids: composed solely of a single-stranded circular RNA molecule a few hundred nucleotides in length
  • Infect plant cells
  • Some replicate in host cell nucleus, others in chloroplasts
  • RNA genome does not code for proteins
  • Disease mechanism not well understood
• Prions: composed entirely of protein; converts normal proteins to abnormal form
  • Disease causing conformation PrPSc
  • Normal conformation PrPC
  • Normal protein expressed at low levels on surface of neurons
  • Several types of neurodegenerative diseases of human and livestock
  • Group of diseases called transmissible spongiform encephalopathies (TSE)

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