Viruses Discovered in 1898 as the cause of hoof-and-mouth disease in cattle. The disease-causing organism was much smaller than bacteria—it could pass through filters that stopped all bacteria. Viruses that attack bacteria are called “bacteriophage”, or just “phage”. Phage means “eater”.

Virus Structure Viruses consist of nucleic acids (DNA or RNA) wrapped in a protein coat. Sometimes there is also a surrounding membrane. The membrane is covered with protein spikes. The spikes are used to attach the virus to the cells it is invading. Viruses have no internal metabolism: no ATP, no enzymes transforming chemical compounds to new forms. Viruses reproduce by invading living cells and taking over their metabolic processes. Are viruses living or non-living? The answer to this question is closely related to the observation that there are 2 kinds of people in this world: those who divide the world into 2 categories and those who don’t. Viruses are both and neither: they have some properties of life but not others. For example, viruses can be killed, even crystallized like table salt, but they can’t maintain a constant internal state (homeostasis). Most scientists consider viruses to be parasites that evolved from cells: essentially parasitic cells that degenerated. However, there is some thought that perhaps very simple viruses or just RNA molecules evolved before the appearance of metabolism.

Virus Replication Viruses are very specific as to which species they attack: humans rarely share viral diseases with other naimls, for instance. The general virus life cycle involves 5 steps: 1. attachment to the cell 2. penetration by either the whole virus or just its DNA or RNA 3. replication of new viral proteins and nucleic acids 4. assembly of the new viruses 5. release of the new viruses into the environment Bacteriophage inject their nucleic acid, and they lyse (break open) the bacterial cell when replication is finished.

Virus Replication In Eukaryotes Eukaryotic viruses usually have membranes. The membranes are derived from the cell membrane. They coat the virus as it buds out of the cell. The cell remains alive and permanently infected.

Viral Latency Some viruses have the ability to become dormant inside the cell. They are inactive for long periods of time, and then activate to produce new viruses in response to some external signal. In bacteriophage, the phage DNA incorporates itself into the bacterial chromosome. When an activation signal occurs, the phage DNA comes out of the chromosome and starts replicating.

Latency in Eukaryotes Some eukaryotic viruses move into nervous system tissues and remain dormant for many years. Chickenpox (caused by the virus Varicella zoster) is a childhood infection that can reappear later in life as shingles, a painful itching rash limited to small areas of the body. Herpes viruses also become latent in the nervous system. A herpes infection lasts for a person’s lifetime.

Retroviruses Retroviruses contain RNA as their genetic material, but convert it into DNA inside the cell. This process is carried out by the enzyme “reverse transcriptase”. The retroviral DNA then inserts itself into one of the cell’s chromosomes, where it makes more copies of the retroviral RNA. HIV, the AIDS virus, is a retrovirus. So is Feline Leukemia Virus

Viroids Some plants are infected by small circular RNA molecules that have no protein coat. These are called viroids. Viroids resemble the introns that are spliced out of most eukaryotic genes.

Prions Prions are “infectious proteins”. They are normal body proteins that get converted into an alternate configuration by contact with other prion proteins, They have no DNA or RNA. The main protein involved in human and mammalian prion diseases is called “PrP”. Similar proteins have been found in other organisms, including some with useful, positive effects. (For the literary types: prions work approximately like “ice-9” did in the novel Cat’s Cradle, by Kurt Vonnegut.)

Prion Diseases When normal proteins are converted to the prion form, they form insoluble deposits in the brain. This gives rise to rapid neural degeneration, similar to a fast acting form of Alzheimer’s disease. Mad cow disease (bovine spongiform encephalitis: BSE). Apparently started as scrapie, a disease of sheep. In England, cattle were fed ground up sheep carcasses and developed BSE. Later, humans eating the infected cows developed a form of Creutzfeld-Jacob Syndrome, a human prion disease. After this, virtually all cattle in England were slaughtered to prevent spread of the disease. Deer and elk in the US (including northern Illinois and Wisconsin) get “chronic wasting disease”, a prion disease. So far, no cases of transfer to humans. People in New Guinea used to suffer from kuru, which they got from eating the brains of their enemies. Once this practice ended, kuru disappeared.