The virusoid genomes are small, only to nucleotides long. A virusoid genome does not code for any proteins, but instead serves only to replicate virusoid RNA. Virusoids belong to a larger group of infectious agents called satellite RNAs, which are similar pathogenic RNAs found in animals.
Unlike the plant virusoids, satellite RNAs may encode for proteins; however, like plant virusoids, satellite RNAs must coinfect with a helper virus to replicate. One satellite RNA that infects humans and that has been described by some scientists as a virusoid is the hepatitis delta virus HDV , which, by some reports, is also called hepatitis delta virusoid. Then, in , Stanley Prusiner, a medical doctor studying scrapie a fatal, degenerative disease in sheep discovered that the disease was caused by proteinaceous infectious particles, or prions.
A prion is a misfolded rogue form of a normal protein PrPc found in the cell. This rogue prion protein PrPsc , which may be caused by a genetic mutation or occur spontaneously, can be infectious, stimulating other endogenous normal proteins to become misfolded, forming plaques see Figure 2.
Today, prions are known to cause various forms of transmissible spongiform encephalopathy TSE in human and animals. Figure A shows the process of how normal prion protein is converted to disease causing forms. Figure B is a micrograph that shows holes in brain tissue. Figure 2. Endogenous normal prion protein PrPc is converted into the disease-causing form PrPsc when it encounters this variant form of the protein.
PrPsc may arise spontaneously in brain tissue, especially if a mutant form of the protein is present, or it may originate from misfolded prions consumed in food that eventually find their way into brain tissue.
The accumulation of rogue proteins causes the brain tissue to become sponge-like, killing brain cells and forming holes in the tissue, leading to brain damage, loss of motor coordination, and dementia see Figure 3.
Infected individuals are mentally impaired and become unable to move or speak. There is no cure, and the disease progresses rapidly, eventually leading to death within a few months or years. The CJD brain has larger spaces as seen by more black regions in the image of the whole brain. The micrograph shows holes in the brain tissue.
Figure 3. Creutzfeldt-Jakob disease CJD is a fatal disease that causes degeneration of neural tissue. Laughlin Dawes; credit b top : modification of work by Suzanne Wakim; credit b bottom : modification of work by Centers for Disease Control and Prevention TSEs in humans include kuru, fatal familial insomnia, Gerstmann-Straussler-Scheinker disease, and Creutzfeldt-Jakob disease see Figure 3.
TSEs in animals include mad cow disease, scrapie in sheep and goats , and chronic wasting disease in elk and deer. Go back to the previous Clinical Focus box. As an Amazon Associate we earn from qualifying purchases. Want to cite, share, or modify this book? This book is Creative Commons Attribution License 4.
Skip to Content Go to accessibility page. Microbiology 6. My highlights. Table of contents. Review Questions. Answer Key. Learning Objectives By the end of this section, you will be able to: Describe viroids and their unique characteristics Describe virusoids and their unique characteristics Describe prions and their unique characteristics Research attempts to discover the causative agents of previously uninvestigated diseases have led to the discovery of nonliving disease agents quite different from viruses.
Figure 6. What is the genome of a viroid made of? What is the main difference between a viroid and a virusoid? PrPsc may arise spontaneously in brain tissue, especially if a mutant form of the protein is present, or it may originate from misfolded prions consumed in food that eventually find their way into brain tissue.
Laughlin Dawes; credit b top : modification of work by Suzanne Wakim; credit b bottom : modification of work by Centers for Disease Control and Prevention. Does a prion have a genome? Previous Next. Order a print copy As an Amazon Associate we earn from qualifying purchases. We recommend using a citation tool such as this one. Not known; possibly by alteration of normal prior protein PrP to rogue form due to somatic mutation.
Eating contaminated cattle products and by secondary bloodborne transmission. Contaminated neurosurgical instruments, corneal graft, gonadotrophic hormone, and, secondarily, by blood transfusion. Viroids are an independent class of plant pathogens.
Two rounds of rolling Viroids were discovered in the year by T. O Diener. It was found after the huge loss in the potato industry after examining the potato spindle tuber Viroid. The structure and form of Viroids are different from that of viruses. Viroids contain short strands of circular and single-stranded RNA without the protein coats.
The plants that are usually affected by Viroids are potatoes, tomatoes, coconut palms, avocados, etc. The crops that are infected by Viroids are the reason for crop failure causing huge losses to the agriculture industry.
The agriculture industry also loses a lot of revenue every year due to this infection in crops. In comparison with other infectious plant pathogens, Viroids are very small in size and hence contain only Ten Thousand atoms. Viroids cause mechanical damage to the cell wall of the crops by infecting the epidermis.
The 30 known viroids belong to two families- Pospiviroidae and Avsunviroidae. They have a rod-like structure with a small single-stranded region. They also have a central conserved region and replicate in the nucleus. The viroid is copied by a rolling circle mechanism which produces complementary linear and concatemeric RNAs.
Their ends are joined by a host enzyme to form circles. This viroid also has rod-like branched regions. However, this viroid lacks a central conserved region and replicates in the chloroplast. RNA molecules are working ribozymes, and this activity is crucial for replication. In plants infected with members of the Avsunviroidae family, viroid RNA is imported into the chloroplast, and complementary concatemeric RNAs are produced by chloroplast-dependent RNA polymerase.
Cutting of cleavage of these molecules is done by a ribozyme, an enzyme encoded in the viroid RNA. Viroids are known to affect only plants. This affliction of the disease can cause economic loss and destroy important crops, fruits, and plants. Two clear examples of economically significant viroids are coconut cadang-cadang viroid. It creates a deadly infection in coconut palm trees. The second example is apple scar skin viroid that causes a disease in apple trees that result in visually unappealing apples.
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