The upcoming discussion will update you about the difference between viroids, prions, virno and virusoids.
Difference # Viroids:
Viroids are covalently closed, circular, single-standard RNA molecules of low molecular weight that multiply in plants and cause diseases. It has so far not been reported in animals. These are the smallest known agents of infectious diseases, and represent the minimal biological and genetic systems.
Viroids were discovered in 1971 by T. O. Diener as a strange and new kind of plant pathogenic agent associated with potato spindle disease which he termed as viroid. It paled viruses into insignificance in its ingenuity of parasitism and small size. Its physico-chemical structure has been worked out to a large extent though its replication and pathogenic behaviour are baffling.
Difference # Prions:
Prions were discovered by Prusiner in 1982 during the search for the cause of scrapie, a neurological disease of sheep. Similar neurological disorders occur in human beings also viz., Kuru, CJ and GS disease. Before the discovery of prions, these diseases were thought to be caused by a ‘slow virus’.
All these diseases show presence of vacuoles or fluid-filled cavities containing clumps of proteins in the brain cells. These proteins are called PrP (Protease-resistant Protein). In normal conditions, these proteins are present in the membranes of the nerve cells. The PrP are resistant to proteases and this leads to their accumulation.
The prions are considered to be ‘infectious proteins’ which have no nucleic acids and, therefore, have no means of replicating (multiplying) themselves. Prusiner and others have suggested that prions are normal proteins but are folded abnormally. This is due to a defect in the gene that produces PrP. The PrP formed by the defective gene has proline in place of leucine and these results in wrong folding of the protein. The defective PrP gene is inherited, but it is different from other inherited diseases in that the infected material of the brain is infectious. The hereditary diseases are not infectious.
Difference # Virno:
Some scientists believe that the causal agent of neurological diseases, scrapie of sheep and Kuru, CJ, and JS of human beings, is a virno, a tiny piece of nucleic acid surrounded by protein of the host cell.
Difference # Virusoids:
Viroid-like RNA (small, circular single-stranded) enclosed in a virion is called a virusoid. The virion, thus, contains viroid-like (i.e., virusoid) RNAs, besides its own RNA e.g., Velvet tobacco mottle virus and related viruses. Both RNAs (viral and the virusoids) are essential for infection. But in Lucerne transient streak virus (LTSV), the virusoids act as a satellite virus and are not necessary for replication of viral RNA.
The virusoids resemble viroids, at least superficially, in – (1) structure (2) lack of coding capacity, and (3) rolling-circle mode of replication. But these differ from viroids in – (1) being encapsulated, and (2) in beingincapable of causing infection independently.