infect potatoes, causing potato, spindle tuber disease. They infect chrysanthe-
mums, stunting their growth. Viroids cause cucumber pale fruit disease. Millions
of dollars are lost each year in crop failures caused by viroids.
A viroid is similar to a virus in that it can reproduce only inside a host cell as
particles of RNA. However, it differs from a virus in that each RNA particle con-
tains a single specific RNA. In addition, a viroid does not have a capsid or an
envelope. Some viruses do not have an envelope.
Prions
Aprionis a small infectious particle that contains a protein. Some researchers
believe that a prion consists of proteins without nucleic acids because a prion is
too small to contain a nucleic acid and because a prion is not destroyed by agents
that digest nucleic acids.
Prion diseases referred to as transmissible spongiform encephalopathies
(TSEs) are progressive neurological diseases that are fatal to humans and ani-
mals. Researchers believe that prions cause Creutzfeldt-Jakob disease.
Creutzfeldt-Jakob disease is a neurological disease that causes progressive
dementia first observed by Hans Gerhard Creutzfeldt and Alfon Maria Jakob in
the 1920s. In 1976, Carlton Gajdusek won the Nobel prize for his work with the
TSE Kuru. Kuru is characterized by progressive ataxia incapacitation and death.
In 1982, neurobiologist Stanley Prusiner proposed that proteins cause the
neurological disease Scrapie,which is a degenerative neural conditon in sheep.
Prusiner named this infectious protein prion. Prions also cause other neurologi-
cal diseases such as Kuruand Gerstmann-Strausler-Sheinker syndrome.
However, scientists are still studying prions to learn their origins and how pri-
ons replicate and cause disease.
Quiz
- A virion is:
(a) another name for a virus
(b) a virus particle
(c) a synthesized virus
(d) a mature virus
CHAPTER 12 Viruses, Viroids, and Prions^193