Viruses and Prions

Retroviruses

  • Some viruses use RNA instead of DNA as their genetic material. These are called retroviruses.
  • Retroviruses have a complex replication cycle.
  • The most well-known retrovirus is HIV (human immunodeficiency virus). Some cancer-causing viruses also belong to this group.
  • Like all viruses, retroviruses have a protein capsid.
  • Surrounding the capsid is a lipid envelope, which is obtained from the plasma membrane of a host cell.
  • Inside the core of the virus, there is RNA and an enzyme called reverse transcriptase.
    • Reverse transcriptase transcribes DNA from the viral RNA.
  • When HIV attaches to a cell, the virus enters the cytoplasm of the host cell, and the viral RNA is released.
  • Reverse transcriptase synthesizes DNA using the viral RNA as a template.
  • The synthesized DNA moves into the nucleus of the host cell and integrates into a chromosome.
  • The viral DNA might remain inactive for years before it is activated.
  • Once activated, RNA is transcribed from the viral DNA, and the host cell manufactures and assembles new HIV particles.

Prions

  • A proteinaceous infectious particle, or prion, is a protein that can cause infection or disease.
  • Diseases caused by prions have been studied for decades but were not well understood until 1982 when Stanley B. Prusiner identified that the infectious particle was a protein.
  • Prions normally exist in cells, but their function is not well understood.
  • Normal prions are shaped like a coil.
  • Mutations in the genes that code for these proteins cause the protein to be misfolded.
  • Mutated prions are shaped like a piece of paper folded many times.
  • Mutated prions are associated with diseases known as transmissible spongiform encephalopathies (TSEs).
  • Examples of diseases caused by prions:
    • Mad cow disease in cattle
    • Creutzfeldt-Jakob disease (CJD) in humans
    • Scrapie in sheep
    • Chronic wasting disease in deer and elk
  • Misfolded prions can cause normal proteins to mutate.
  • Prions infect nerve cells in the brain causing them to burst.
    • This results in spaces in the brain, hence the description of spongy form encephalopathy (brain disease).
  • In the mid-1980s, a new variant of CJD (vCJD) was discovered in England.
  • The origin of vCJD is not fully agreed upon, but a leading hypothesis is that the prions are transmitted from cattle.
    • Abnormal prions can be found in the brains and spinal cords of cattle.
    • The hypothesis is that if the spinal cord is cut during butchering, the prions might contaminate the beef and be transmitted to humans who eat the beef.
  • Due to this potential transmission route, the United States government has strict regulations concerning the importation of cattle and beef from other countries.