Sunday, 20 June 2021

Major Differences between Viroids and Prions

  Both viroids and prions are Strict intracellular, Subcellular infectious agents, that always requires a living host for replication. Besides there are many differences between Viroids and Prions.

Differences between Viroids and Prions

• Definition :

Viroids

• Viroids are infectious RNA particles without protein coat.
• Viroids are just virus-like.
• It was discovered by T. Diener.
• The first viroid discovered was PSTV (Potato spindle tuber viroid) and he called it as virus like or viroid.
• Viroids are the smallest known Infectious agents.

Prions

• Prions are infectious protein particles that is discovered by Stanley Prisoner in 1982.
• Prions cause Neurodegenerative diseases.
• Prions is actually a misfolded protein.

• Chemical Nature and Size

Viroids

• Viroids are made up of RNA.
• It is always small circular single stranded RNA molecule.
• The size of the genome is 5 to 10 times smaller than the smallest DNA and RNA viral genomes.
• Generally the size of viroids genome ranges from 246 to 463 nucleotides.

Prions

• Prions are actually misfolded proteins.
• It is called as PRPc (Prionprotein cellular)
• Prion protein is made up of 209 amino acids and the function is it is having a neuroprotective function.
• This prion (PRPc) is converted to PRPsc which is called as Prion Protein Scrapie; the disease that occurs in sheep.
• The structure of PRPsc is very stable with beta sheets and it accumulates in nerve cells as amyloids and ultimately kill neurons.

• The Replication

Viroids

• In viroid replication is done by an RNA-based rolling circle mechanism; using RNA polymerase 2 of the host.
• Viroid the RNA does not code for any protein but it can act as ribozyme. so it can effectively use the machinery of the host.

Prions

• In the case of prion, The PRPc is converted to PRPsc.
• The normal protein PRPc is converted to non-abnormal protein or disease causing protein PRPsc.
• There are two models proposed regarding the replication of Prion,
One is heterodimer model and the second is fibrillar model; both suggest that conversion of PRPc to PRPsc is happening during the replication process.

• The Hosts and Examples

Viroids

• Viroids infects only plants.
• Potato spindle tuber viroid (PSTV) is causing potato spindle tuber disease.
• Other viroids diseases include :
- Tomato plant maco viroid (TPMV).
- Avocado sunblotch viroid (ASBV)
- Peach latent mosaic viroid (PLMV) etc.

Prions

• Prions causes Neurodegenerative diseases in animals.
• in cattle it causes bovine spongi form encephalopathy (BSE) which is called a "mad cow disease".
• In humans it causes neurodegenerative diseases like CJD disease and Kuru disease etc.

These are the basic differences between Prions and Viroids. 

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