Viroids and Prions — Revision Notes

⚡ 30-Second Revision

Viroids: — Naked, circular, single-stranded RNA. No protein coat. No protein coding. Plant pathogens. Discovered by Theodor Diener. Replicate via rolling circle using host RNA polymerase. Ex: Potato Spindle Tuber Viroid (PSTVd).
Prions: — Infectious proteinaceous particles. No nucleic acid. Misfolded normal protein (PrP^Sc). Cause neurodegenerative diseases (TSEs). Discovered by Stanley Prusiner. Propagate by conformational conversion of PrP^C to PrP^Sc. Ex: Creutzfeldt-Jakob Disease (CJD), Bovine Spongiform Encephalopathy (BSE), Scrapie.
Key Distinction: — Viroids = RNA only. Prions = Protein only.

2-Minute Revision

Viroids and prions are sub-viral infectious agents, each representing a distinct departure from conventional pathogens. Viroids are the simplest, consisting of a small, circular, single-stranded RNA molecule without any protein coat or protein-coding capacity.

They exclusively infect plants, causing diseases like Potato Spindle Tuber Disease, and replicate using the host's RNA polymerase via a 'rolling circle' mechanism. Their pathogenicity is thought to involve interference with host gene expression, possibly through RNA silencing.

Prions, on the other hand, are infectious proteins, devoid of any genetic material. They are misfolded versions of a normal cellular protein (PrP^C) that induce other normal PrP^C molecules to adopt the abnormal, disease-causing PrP^Sc conformation.

This leads to the accumulation of insoluble protein aggregates in the brain, causing fatal neurodegenerative diseases known as Transmissible Spongiform Encephalopathies (TSEs), such as Creutzfeldt-Jakob Disease in humans and Mad Cow Disease in cattle.

Key scientists are Theodor Diener for viroids and Stanley Prusiner for prions. Remember their unique compositions and the diseases they cause for NEET.

5-Minute Revision

For NEET, a solid grasp of viroids and prions requires understanding their unique structures, replication/propagation, and the diseases they cause. Viroids, discovered by Theodor Diener, are the smallest known infectious agents.

They are essentially 'naked' genetic material: a small (200-400 nucleotides), circular, single-stranded RNA molecule. Crucially, they lack a protein coat and do not encode any proteins. This means they are entirely dependent on the host cell's machinery for replication, often employing a 'rolling circle' mechanism using host RNA polymerase.

They are obligate plant pathogens, causing diseases like Potato Spindle Tuber Viroid (PSTVd), by interfering with host gene expression, possibly via RNA silencing.

Prions, discovered by Stanley Prusiner, are even more unusual, being infectious proteinaceous particles with no nucleic acid whatsoever. They are misfolded isoforms (PrP^Sc) of a normal cellular protein (PrP^C) found in the brain.

The pathogenic PrP^Sc acts as a template, inducing normal PrP^C molecules to misfold into the PrP^Sc form. This self-propagating conformational conversion leads to the accumulation of insoluble PrP^Sc aggregates, causing neurodegeneration.

These diseases are called Transmissible Spongiform Encephalopathies (TSEs), including Scrapie (sheep), Bovine Spongiform Encephalopathy (BSE/Mad Cow Disease), and Creutzfeldt-Jakob Disease (CJD) in humans.

Prions are highly resistant to heat, radiation, and proteases. The core distinction for NEET is: Viroids = RNA only, plant pathogens; Prions = Protein only, animal/human neurodegenerative diseases.

Prelims Revision Notes

Viroids: The Naked RNA Pathogens

Definition: — Smallest known infectious agents, composed solely of a short, circular, single-stranded RNA molecule.
Structure:

* RNA: 200-400 nucleotides, circular, single-stranded, highly structured (often rod-like due to base pairing). * Protein Coat: ABSENT. * Protein Encoding: ABSENT (does not encode any proteins).

Discovery: — Theodor O. Diener (1971), while studying Potato Spindle Tuber Disease.
Host: — Primarily higher plants.
Replication: — Uses host DNA-dependent RNA polymerase. Follows a 'rolling circle' mechanism.
Pathogenicity: — Interferes with host gene expression, possibly via RNA silencing, leading to developmental abnormalities.
Examples: — Potato Spindle Tuber Viroid (PSTVd), Coconut Cadang-Cadang Viroid.

Prions: The Infectious Proteins

Definition: — Proteinaceous infectious particles, devoid of nucleic acid.
Structure:

* Composition: ONLY protein (misfolded isoform of a normal cellular protein). * Nucleic Acid: ABSENT.

Discovery: — Stanley B. Prusiner (1980s), coined 'prion' (proteinaceous infectious particle).
Normal Protein (PrP^C): — Cellular prion protein, found in healthy mammals, rich in alpha-helices.
Infectious Protein (PrP^Sc): — Scrapie prion protein, abnormally folded isoform of PrP^C, rich in beta-sheets, resistant to proteases.
Propagation: — PrP^Sc acts as a template, inducing PrP^C to misfold into PrP^Sc (conformational conversion).
Diseases: — Cause Transmissible Spongiform Encephalopathies (TSEs) – fatal neurodegenerative diseases.
Examples:

* Animals: Scrapie (sheep), Bovine Spongiform Encephalopathy (BSE / Mad Cow Disease). * Humans: Creutzfeldt-Jakob Disease (CJD), variant CJD (vCJD), Kuru, Fatal Familial Insomnia (FFI).

Key Feature: — Highly resistant to heat, radiation, and common disinfectants.

Key Distinctions for NEET:

Viroids: — RNA only, no protein, plant diseases.
Prions: — Protein only, no nucleic acid, neurodegenerative diseases in animals/humans.
Viruses: — Nucleic acid (DNA/RNA) + protein coat, infect all life forms.

Vyyuha Quick Recall

Very Rare Plants Die (Viroids = RNA, Plants, Diener) Proteins Cause Neuro Sickness (Prions = Protein, CJD/Neurodegenerative, Scrapie/Prusiner)