2. • Viroids are small, covalently closed, circular, single standard RNA (ssRNA)
molecule without protein coat (nucleic acid devoid of protein coat).
• First reported by Diener and Raymer in 1971
• World’s first reported viroid is Potato Spindle Tuber Viroid (Diener and Rayer,
1971) and it is the most studies viroid.
4. General properties
• They are usually smaller than plant viruses and are known to infect only plants.
• Genome size ranges form few hundred (250-401) nucleotides long, with high degree of
• They do not code for any proteins and thus depend on the host plant or with associated
plant virus for their replication and other functions
• Viroids are only transmitted through mechanical or sap transmission, no other (vectors)
transmission type has been reported so far.
• So far 40 viroid diseases have been reported to be caused plant diseases in the world.
• The viroid moves from cell to cell through ‘Plasmodesmata’ and ‘cell division’
• The replication mechanism of viroid is ‘rolling circle mechanism’
7. Coconut cadang cadang viroid
• Reduced the nut size and husk
• Reduced tree length and
• Withering of leaves and look
like telephone pole appearance
• Virusoids are circular single-stranded RNAs dependent on
plant viruses for replication and encapsidation
• Discovered by Randles
• Contains several hundred nucleotides and does not code for
• They are thus similar to viroids in their means of replication
(rolling circle replication),
• Virusoids act as a satellite virus and are not necessary for
• The molecular weight of virusoids ranges from 1.5 × 106
• A prion is a small infectious particle composed of
abnormally folded protein and has a ability to
transmit their misfolded shaped onto normal
variants of the same protein.
• Discovered by Stanley B. Prusiner
10. Animals Diseases
Sheep, Goat Scrapie
Cattle Mad cow disease (Bovine
Deer and Mouse Chronic wasting disease
Creutzfeldt -Jacob disease (CJD)
Fatal familial insomnia