Virus, Viroids and Prions — Revision Notes

⚡ 30-Second Revision

Viruses: — Nucleoprotein (DNA or RNA + protein capsid). Obligate intracellular parasites. E.g., HIV, Influenza.
Viroids: — Naked, circular, single-stranded RNA. No protein coat. Primarily plant pathogens. E.g., Potato Spindle Tuber Viroid.
Prions: — Misfolded proteins (PrPSc). No nucleic acid. Induce normal proteins to misfold. Cause neurodegenerative diseases. E.g., BSE, CJD.
Commonality: — All are acellular and obligate parasites.
Not in 5-Kingdom: — Due to acellular nature.

2-Minute Revision

Viruses, viroids, and prions are acellular infectious agents, hence excluded from the five-kingdom classification. Viruses are nucleoprotein particles, meaning they consist of genetic material (either DNA or RNA, never both) enclosed within a protein coat called a capsid.

Some viruses also have an outer lipid envelope. They are obligate intracellular parasites, replicating only by hijacking host cell machinery, causing diseases like flu, AIDS, and COVID-19. Viroids are simpler, composed solely of a naked, circular, single-stranded RNA molecule, lacking a protein coat.

They are the smallest known infectious agents and primarily infect plants, causing diseases like Potato Spindle Tuber disease. Prions are the most unusual, being infectious misfolded proteins without any genetic material.

They cause neurodegenerative diseases (e.g., Mad Cow Disease, Creutzfeldt-Jakob disease) by inducing normal host proteins to adopt their abnormal, disease-causing conformation. Key differences lie in their composition (nucleic acid type, protein coat presence) and host specificity.

5-Minute Revision

For NEET, a robust understanding of viruses, viroids, and prions requires a comparative approach. Viruses are obligate intracellular parasites, meaning they cannot replicate outside a living host cell.

Structurally, a virus (virion) comprises genetic material (either DNA or RNA, which can be single or double-stranded) encased in a protein coat called a capsid, made of capsomeres. Some viruses, particularly animal viruses, acquire an outer lipid envelope from the host cell membrane.

Their replication involves attachment, penetration, uncoating, biosynthesis, assembly, and release. Examples include bacteriophages (infect bacteria), Tobacco Mosaic Virus (TMV, infects plants), and human viruses like HIV and influenza.

Viroids, discovered by Diener, are even simpler. They are naked, circular, single-stranded RNA molecules, significantly smaller than viruses, and crucially, lack a protein coat. Their RNA does not code for proteins but interferes with host gene expression. They are predominantly plant pathogens, causing diseases such as Potato Spindle Tuber Disease.

Prions, discovered by Prusiner, are the most unique, being infectious misfolded proteins. They contain no genetic material (DNA or RNA). The abnormal prion protein (PrPSc) induces normal cellular prion protein (PrPC) to misfold into the infectious form, leading to accumulation and neurodegeneration. These cause Transmissible Spongiform Encephalopathies (TSEs) like Bovine Spongiform Encephalopathy (BSE or Mad Cow Disease) in cattle and Creutzfeldt-Jakob Disease (CJD) in humans.

All three are acellular and obligate parasites, hence not classified in the five kingdoms. Focus on their distinct compositions, genetic material types, presence/absence of protein coats, and specific disease examples for NEET.

Prelims Revision Notes

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Viruses:

* Nature: Acellular, obligate intracellular parasites. * Composition: Nucleoprotein (genetic material + protein capsid). * Genetic Material: DNA or RNA (never both in one virion); can be ss or ds, linear or circular.

* Protein Coat: Capsid (made of capsomeres) present. Some have an outer envelope. * Size: 20-300 nm, filterable. * Replication: Lytic (virulent) or Lysogenic (temperate) cycles in bacteriophages.

* Diseases: Common cold, influenza, measles, mumps, polio, AIDS, hepatitis, COVID-19. * Discovery: Ivanovsky (filterable agent), Beijerinck (Contagium vivum fluidum), Stanley (crystallized TMV).

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Viroids:

* Nature: Acellular, obligate intracellular parasites. * Composition: Naked, circular, single-stranded RNA molecule. * Genetic Material: RNA only (ss, circular, non-coding). * Protein Coat: Absent. * Size: Smallest known infectious agents (246-401 nucleotides). * Host: Primarily plants. * Diseases: Potato Spindle Tuber Disease (PSTVd), Coconut Cadang-Cadang Viroid. * Discovery: Theodor Diener (1971).

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Prions:

* Nature: Acellular, infectious proteins. * Composition: Misfolded proteins (PrPSc). * Genetic Material: Absent (no DNA or RNA). * Protein Coat: Absent (entirely protein, but not a capsid structure).

* Mechanism: Induce normal cellular proteins (PrPC) to misfold into the abnormal PrPSc conformation. * Diseases: Transmissible Spongiform Encephalopathies (TSEs) - Bovine Spongiform Encephalopathy (BSE/Mad Cow Disease), Scrapie, Creutzfeldt-Jakob Disease (CJD), Kuru.

* Discovery: Stanley Prusiner (1982).

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Commonalities: — All are acellular, obligate parasites, and not included in the five-kingdom classification.

Vyyuha Quick Recall

To remember the key features of Viroids and Prions:

Viroids are Very Vulnerable (no protein coat), Very small RNA, Very plant-specific.

Prions are Pure Protein, Pathogenic, Promote misfolding, Primarily brain (neurodegenerative).