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Over the years there have been many documented cases of diseases for which there appeared to be no cause. Only within the late 20 th century has the reason for these illnesses been brought into the spotlight. The symptoms of these illnesses are usually the loss of coordination followed by dementia, but in some cases an inability to sleep develops into dementia. These symptoms are caused by damaged regions of the brain. This damage occurs in the form of normal tissue changing to a sponge-like consistency. This is caused by cells clumping together and dying, leaving holes in the brain.
These diseases fall into the category of spongiform encephalopathies (Collinge, 1995). This type of disease can occur in all mammals and is named after the mammal in which it is found. For example, in cows it is called bovine spongiform encephalopathy, or more commonly known as mad cow disease. Spongiform encephalopathies may also occur in humans in the form of Creutzfeldt-Jakob disease or Kuru, the latter of which was only found in the Fore tribe of Papua, New Guinea, members of which ate the brains of the dead as a sign of respect. Cannibalism was ended in 1958 due to health concerns. After the deaths of the 2600 afflicted persons, not a single case of Kuru has been recorded.
The cause of spongiform encephalopathy is a small protein called a prion, or proteinaceous infectious particle. The protein was discovered and named in 1982 by Dr. Stanley B. Prusiner of the Department of Neurology at the University of California School of Medicine.
At first, the discovery endured a great deal of skepticism due to the long accepted theory that only microbes can cause disease, which was proposed by Louis Pasteur. The prion protein (PrP for short) was first imagined when the agent that causes scrapie (the type of spongiform encephalopathy which occurs in sheep) was thought to lack nucleic acid (Alper, 1972). This was determined by subjecting infected tissue to ultraviolet or ionizing radiation, both of which are known to degrade nucleic acid. Afterwards, the tissue was injected into the brain tissue of a healthy subject.
The infectivity of the tissue remained the same. This proved that there was no nucleic acid present in the infective agent, due to the fact that if it had been a viral or bacterial infection, the nucleic acid within the virus or bacteria would have been destroyed and infection would no longer be possible. The identification of a protein as the cause was discovered while experimenting with conditions which denature proteins, on the infectivity of the tissue sample. The question then changed from, What causes the illness?
to What creates the protein? Eventually, the amino acids, which make up PrP, were identified and genetic probes could be created. It was discovered that not only did sheep contain the gene, which codes for PrP, but that all mammals have a similar gene. The real puzzle was why doesnt everyone become ill, if we all have the protein in our bodies. Over the years various people studied PrP and discovered that it comes in two forms, one of which seems to have no function, while the other, the infectious form, has an altered shape. This altered shape somehow induces a similar shape change in nearby proteins.
These newly rearranged proteins start converting other proteins to the infectious shape. Normally this would not be a problem because cellular enzymes called proteases break down unused proteins (Prusiner, 1984). This is not the case however, the altered shape of the PrP somehow enables it to resist breakdown by the proteases. Still the question of what causes the original PrP to be misshapen remains.
The answer was found while comparing the structure of the normal PrP to the structure of the misshaped PrP. It was discovered that a mutation had caused one base pair out of over 750 to be incorrect. This caused the 102 nd condon to code for the amino acid proline instead of leucine (Prusiner, 1995). Because a mutation can occur in anyone, cases of prion caused diseases are usually one in a million with no set pattern, but due to the inherited genes that are passed onto an animals offspring, these mutations can also be inherited. For a disease like Fatal Familiar Insomnia, the mutated gene that codes for PrP may be inherited by the offspring of the affected, who in turn also become ill and die. Mutation and inheritance are not the only ways by which an animal can become infected with a prion disease.
Prion diseases have been known to be transmitted through the ingestion of infected meat. This often occurs when an animals feed contains ground meat from the same species. Although it is less common, prion diseases may even break the species barrier, the set of genetic differences that occur between all animals that prevent illnesses to be transmitted from species to species. In these cases, eating the flesh of another species can infect an animal. It is not known exactly how the PrP can change the shape of a PrP in another species, but it is known that the beginning and end portions of the amino acid sequence in PrP play an important role in bridging the gaps between species (Scott, 1985). In order to prove this, a portion of the PrP was taken from a patient who died from Creutzfeldt-Jakob disease (CJD).
On either end, the remaining portions were added from a healthy mouse donor. This hybrid PrP was then injected into healthy mice. A full human PrP was injected into control mice. The mice that had been injected with the hybrid PrP had a higher rate of infection compared to those that had been injected with the full human PrP. As of yet, there are no known cures for any diseases caused by prions, but many scientists are working on ways to reverse or prevent the damage caused by the diseases.
Since the normal PrP seems to have no actual purpose, scientists have been trying to find a way to remove or inhibit the gene that creates PrP, thus rendering the transmission or inheritance of a defect in the PrP harmless. The most success with this effort came to John Collinge, who is the head of the Prion Unit at the Medical Research Council at London. The potential success came almost by accident. As they were crossbreeding two different transgenic mice, an unusual combination was created. The new mice were normal at birth, but 3 months after birth, the mice created an enzyme that prevented the production of PrP in the neurons of the brain. Offspring of these mice were injected with the infectious prions and by the time they...
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Research essay sample on Creutzfeldt Jakob Disease Nucleic Acid
