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.