Classification_of_organisms                   (2/26/06)

Viruses are not living, and are not classified within the kingdoms of life. They are simply complex organic chemicals (either DNA or RNA, usually enclosed within a protein coat) that are capable of parasitizing various organisms. The viruses below are T4 bacteriophages attacking an Escherichia coli bacterium. Several viruses are attached to the cell wall, while others have entered the bacterium and are reproducing within it.

Diagrams of viral diversity.

Details of viral structure.

Viruses can either reproduce and cause a cell to rupture (lytic cycle), or combine their DNA into the genome of the host (lysogenic cycle).

The AIDS virus injects its RNA into one type of T cell (one of our white blood cells, activated by the thymus gland, and part of our immune system).

The HIV virus produces reverse transcriptase, which catalyzes the synthesis of a DNA copy of the viral RNA. This then causes the host cell to make a complementary strand of the DNA.

The DNA then directs the synthesis of both the HIV RNA and HIV proteins, which produce more viral particles.

Once more viral particles are produced, they leave T cells by lysis, and they leave macrophages (other white blood cells) by incorporating part of the cell membrane through exocytosis.

13 types of important human viral diseases, the virus types that produce the diseases, the reservoir for each virus, the vector of each disease, and information on epidemiology .


Prions (Pronounced Pre-ons) and Viroids are also non-living cellular parasites. Prions were named for "proteinaceous infectious particle" in a 1982 paper by Dr. Stanley Prusiner. When misfolded prion proteins contact normal prion proteins, they cause them to misfold, also. This causes the large holes in the brain of victims of Creutzfeldt-Jacob disease, known as scrapie in sheep and mad cow disease in cattle. This group of diseases is known as TSEs (transmissible spongiform encephalopathies). Viroids are naked molecules of RNA that cause infectious diseases in plants.