Diseases are caused by things called infectious agents. Until recently scientists thought that there are four types of infectious agents: viruses, bacteria, fungi and parasites. All of these organisms have some kind of nucleic acid that carries genetic information, either the deoxyribonucleic acid (DNA) in their cells or ribonucleic acid (RNA). When scientists first began studying TSEs, they went searching for the usual infectious agents by looking for nucleic acids. Scientists tested with radiation, chemicals, and enzymes to find infectious agents with DNA or RNA, but did not found any. Some scientists believe that these methods for detecting the genetic information have failed in the case of TSEs because the infectious agent must just have a smaller amount of nucleic acid than most agents of disease.

     Many scientists however, are starting to believe the protein-only theory that J.S. Griffith proposed in 1967. His theory says that it is a protein, a prion protein, that is responsible for TSEs. This was considered, and still is by some scientists, a very radical idea since rather than containing genetic information, proteins are sequences of amino acids and the product of the genetic encoding of DNA. No protein has ever been thought to be infectious before. Griffith thought that prions could reproduce without nucleic acids.

>>> What are Proteins?

     Proteins are made up of amino acids. The amino acids are in different sequences in each protein, making them unique. Rather than being a straight chain, proteins fold into distinctive shapes. Their shape determines their function in a cell or organism. It is like those children’s toys where different shapes fit into their corresponding hole. Only circular pieces fit into circular holes. However if one of those circular pieces could be reshaped into a triangle, it wouldn’t fit into the circular hole and the toy wouldn’t function. It is the same with proteins. If one changes its shape then it can no longer function the way it should.

     Proteins, which are chains of amino acids, are known to fold to form a number of shapes. Some take the form of a helix while others are shaped into pleated sheets. In some proteins sections will form helices while other parts of the same protein can form pleated sheets (see Fig. 2).

>>> What is a Prion?

     “Prion” is a term coined by Stanley Prusiner, a scientist who won the Nobel Prize in 1997 for his work identifying and classifying the prion and linking it with various neurological diseases. It stands for proteinaceous infectious particle. The regular prion protein (PrP) is found in the cells of all mammals and does not cause disease, but the abnormal prion protein (PrPSc) is not and if it does enter a mammal it can cause disease. PrP and PrPSc have the identical amino acid sequences. PrP causes disease when it transforms, or folds differently, and becomes PrPSc. PrP sections that are shaped like a helix become pleated sheets in PrPSc (see Fig. 3).

     When a regular PrP enters a cell infected with PrPSc, contact with the PrPSc causes the normal PrP to fold into the abnormal PrPSc shape. Rather than reproducing itself, the abnormal PrPSc is responsible for transforming normal PrP. Because of its abnormal shape, PrPSc is not broken down by the enzyme protease, causing a buildup of the abnormal protein. Either the host cell bursts, releasing all of the PrPSc or the abnormal proteins collect on the surface of cells, forming plaques and killing the cells.