Virus, Viroids and Prions — Core Principles
Core Principles
Viruses, viroids, and prions are acellular infectious agents not included in the five-kingdom classification. Viruses are nucleoprotein entities, consisting of genetic material (DNA or RNA) encased in a protein capsid, sometimes with an outer envelope.
They are obligate intracellular parasites, replicating only within living host cells, causing diseases like influenza, HIV, and COVID-19. Viroids are simpler, being naked, circular, single-stranded RNA molecules without a protein coat, primarily infecting plants and causing diseases like Potato Spindle Tuber disease.
Prions are the most unusual, composed entirely of misfolded proteins (PrPSc) that can induce normal proteins to misfold, leading to fatal neurodegenerative diseases like Mad Cow Disease and Creutzfeldt-Jakob disease.
Their study highlights the diverse mechanisms of infection and the boundaries of what we consider 'life'.
Important Differences
vs Viruses, Viroids, and Prions
| Aspect | This Topic | Viruses, Viroids, and Prions |
|---|---|---|
| Nature/Composition | Viruses: Nucleoprotein (genetic material + protein coat) | Viroids: Naked RNA (no protein coat); Prions: Misfolded protein (no nucleic acid) |
| Genetic Material | Viruses: DNA or RNA (ss or ds) | Viroids: RNA only (circular, ss); Prions: None |
| Protein Coat (Capsid) | Viruses: Present | Viroids: Absent; Prions: Absent (entirely protein, but not a capsid) |
| Size | Viruses: 20-300 nm (larger than viroids/prions) | Viroids: Smallest known infectious agents (~246-401 nucleotides); Prions: Protein aggregates, variable size |
| Host Range | Viruses: Plants, animals, bacteria, fungi | Viroids: Primarily plants; Prions: Mammals (especially nervous system) |
| Mechanism of Pathogenicity | Viruses: Replicate, hijack host cell machinery, cause cell damage/lysis | Viroids: Interfere with host gene expression (RNA interference); Prions: Induce misfolding of normal host proteins |
| Examples of Diseases | Viruses: Flu, HIV, Measles, COVID-19 | Viroids: Potato Spindle Tuber Disease; Prions: BSE, CJD, Scrapie |
| Discovery | Viruses: Ivanovsky, Beijerinck, Stanley (TMV) | Viroids: Diener (PSTVd); Prions: Prusiner (Scrapie) |