ch. 7 Viruses, Viroids, Prions & Virusoids:

Viruses

• Definition

  • Strict intracellular infectious agents capable of infecting both plants and animals.

  • Consist of nucleic-acid genome (DNA or RNA) enclosed in a protein coat.

• Structural Organization

  • Genome: either DNA or RNA (never both in the same particle).

  • Capsid: protective protein shell surrounding the genome.

  • Built from repeating protein sub-units called capsomers.

  • Envelope (optional): lipid–protein layer derived from host membranes; present in some viruses (e.g., influenza).

• Host Spectrum & Examples

  • Animal viruses: HIV, Influenza virus.

  • Plant viruses: Tobacco mosaic virus (TMV), Potato virus.

• Key Biological Features

  • Obligate intracellular parasites: require host cellular machinery for replication.

  • Exhibit some properties of life only inside host cells (e.g., reproduction, evolution).

Prions

• Definition

  • Infectious proteinaceous particles that lack nucleic acid.

  • Term coined by Stanley Prusiner.

• Molecular Nature

  • Normal cellular form: PR!P^C (Prion Protein, Cellular).

  • Pathogenic form: PR!P^{SC} (Prion Protein, Scrapie).

  • Misfolded into a highly stable, β-sheet–rich conformation.

  • Catalyzes conversion of PR!P^C → PR!P^{SC} (template-directed refolding).

• Pathogenesis

  • Accumulation of PR!P^{SC} on neuronal surfaces → neuronal dysfunction → cell death → spongiform encephalopathy.

• Major Diseases

  • Bovine spongiform encephalopathy (Mad Cow Disease).

  • Creutzfeldt–Jakob disease (CJD).

  • Kuru.

• Biological & Ethical Significance

  • Demonstrate that proteins alone can transmit heredity & disease.

  • Raise concerns about food safety and iatrogenic transmission (surgical instruments, transplants).

Viroids

• Definition

  • Smallest known autonomous infectious agents; composed solely of circular, single-stranded RNA (ssRNA).

  • Lack protein coat.

• Discovery

  • First described by Theodor Otto Diener while studying potato spindle tuber disease.

• Biological Features

  • Highly base-paired, rod-like secondary structure for stability.

  • Replicate autonomously in plant cell nuclei or chloroplasts using host RNA polymerase.

• Host Range & Example

  • Primarily infect plants (rare in animals).

  • Classical example: Potato spindle tuber viroid (PSTVd).

• Significance

  • Important agricultural pathogens; cause stunting, deformation, yield loss.

Virusoids

• Definition

  • Circular ssRNA molecules similar in size to viroids but non-autonomous.

  • Require a helper virus (co-infecting the same cell) for replication & encapsidation.

• Replication Strategy

  • Virusoids “hitch-hike” inside the capsid of the helper virus.

  • Their RNA is replicated by viral polymerase supplied by the helper.

• Distinction from Viroids

  • Viroids replicate independently; virusoids cannot.

• Host Spectrum

  • Documented almost exclusively in plants.

Comparative Highlights & Exam Pointers

• Smallest infectious agent (overall): Viroids (answer to the question posed).

• Protein-only agent: Prion.

• RNA-only, self-replicating: Viroid.

• RNA-only, needs helper virus: Virusoid.

• Agents capable of infecting both plants & animals: Viruses (prions → animals, viroids/virusoids → plants).

Quick Reference Cheatsheet

Practice Question

Which of the following is the smallest infectious agent known?

A) Virus  B) Bacteria  C) Prion  D) Viroid

Correct Answer: D) Viroid

Explanation: Viroids are naked, circular ssRNA molecules (~250–400 nt) and represent the minimal known infectious entity.