01 / 21

Subviral Particles
Prions & Viroids

Explore infectious agents smaller than viruses that challenge our understanding of biology: protein-only prions and RNA-only viroids.

MCAT Biology

What Are Subviral Particles?

Subviral particles are infectious agents simpler than viruses. While viruses contain both nucleic acid and a protein coat, subviral particles lack one or more of these components yet can still cause disease.

Prions
Misfolded proteins only: no nucleic acid at all. Cause neurodegenerative diseases.
Viroids
Short, circular RNA only: no protein coat. Infect plants exclusively.

Challenging the Central Dogma

The central dogma states that genetic information flows from DNA → RNA → Protein. Subviral particles break these expectations in fascinating ways:

DNA RNA Protein Viroids: RNA only Prions: Protein only No DNA, no protein No nucleic acid

Prions: Infectious Proteins

A prion (proteinaceous infectious particle) is a misfolded version of a normal cellular protein called PrP (prion protein). The normal form is PrPC and the infectious form is PrPSc.

PrPC (Normal) Mostly alpha-helices Soluble, non-infectious MISFOLD PrPSc (Prion) Mostly beta-sheets Insoluble, infectious Same amino acid sequence, different conformation PrPSc converts PrPC on contact (chain reaction)

How Prions Propagate

Prions replicate without nucleic acid. The misfolded PrPSc physically contacts normal PrPC and templates its misfolded conformation, creating an exponential chain reaction.

1
Contact: PrPSc binds to PrPC on the cell surface.
2
Conformational change: PrPC refolds from alpha-helices into beta-sheets, becoming PrPSc.
3
Aggregation: Misfolded prions aggregate into insoluble amyloid fibrils.
4
Neurodegeneration: Amyloid plaques cause spongiform (sponge-like) holes in brain tissue.

Prion Diseases (TSEs)

Transmissible spongiform encephalopathies (TSEs) are fatal neurodegenerative diseases caused by prions. They can arise through three routes:

Sporadic
Spontaneous misfolding: ~85% of CJD cases arise with no known cause.
Inherited
Mutations in the PRNP gene increase susceptibility to misfolding.
Acquired
Exposure to infected tissue: surgical instruments, contaminated food (vCJD from BSE).

Viroids: Naked RNA Pathogens

Viroids are the smallest known pathogens: short (246-401 nucleotides), circular, single-stranded RNA molecules with no protein coat and no encoded proteins. They infect plants only.

Circular ssRNA ~250-400 nucleotides Extensive base-pairing No capsid No envelope No mRNA activity Uses host enzymes

Viroid Replication Strategy

Viroids do not encode any proteins, so they hijack host RNA polymerase to replicate via a rolling circle mechanism.

1
Entry: Viroid RNA enters the plant cell (often via mechanical damage or insect vectors).
2
Nuclear import: Viroid migrates to the nucleus (or chloroplast, depending on family).
3
Rolling circle replication: Host RNA polymerase II (normally reads DNA) is tricked into copying the RNA template, producing long concatemers.
4
Self-cleavage: Ribozyme activity in some viroids cleaves concatemers into monomers, which circularize.

Prions vs. Viroids

Feature Prion Viroid
Composition Protein only (PrPSc) RNA only (ssRNA)
Nucleic acid? None Yes (RNA)
Protein coat? None needed None
Hosts Mammals (humans, cattle) Plants only
Replication Conformational conversion Rolling circle (host enzymes)
Encodes proteins? N/A No
Key diseases CJD, BSE, kuru Potato spindle tuber

The Infectious Agent Spectrum

Prion Protein only PrP No DNA No RNA No capsid ~20 kDa Viroid RNA only ssRNA No DNA No protein No capsid ~120 kDa Virus DNA or RNA + Protein NA DNA or RNA Capsid protein +/- envelope ~104–108 kDa Increasing complexity and size

Why Prions Evade the Immune System

Prion diseases are uniquely difficult to treat because the immune system does not recognize PrPSc as foreign. Here is why:

1
Self-protein: PrPSc has the same amino acid sequence as normal PrPC. The immune system is tolerant to self-antigens.
2
No nucleic acid: Without DNA or RNA, there are no PAMPs (pathogen-associated molecular patterns) for innate immune receptors to detect.
3
No inflammation: Prion accumulation causes neuronal death without triggering a typical inflammatory response.

MCAT Problem-Solving Strategy

When you see a question about subviral particles, follow this systematic approach:

1
Identify the agent: Does the question describe protein-only, RNA-only, or nucleic acid + protein? This immediately narrows to prion, viroid, or virus.
2
Check the host: Plant pathogen = likely viroid. Animal/human neurodegeneration = likely prion.
3
Assess replication mechanism: Conformational conversion (no new synthesis) = prion. Rolling circle with host enzymes = viroid.
4
Evaluate treatment/resistance: Resistant to nucleases and protease = prion. Susceptible to RNase = viroid.
5
Connect to broader concepts: Link to protein folding, central dogma, RNA world hypothesis, or immune evasion as needed.

Example 1: Identifying the Agent

1
RNase destroys it → the agent contains RNA essential for infectivity.
2
Protease has no effect → the agent has no essential protein component.
3
Plant host → consistent with a viroid (viroids infect plants only).

Example 2: Prion Resistance

1
CJD is a prion disease. The infectious agent is PrPSc, a misfolded protein.
2
Prions resist standard autoclaving. The beta-sheet-rich structure makes PrPSc extremely thermostable.
3
Special protocols required: 134 degrees Celsius for 18+ minutes, or treatment with 1M NaOH or concentrated sodium hypochlorite.

Example 3: RNA World Connection

The RNA world hypothesis proposes that early life used RNA as both genetic material and catalyst, before DNA and proteins evolved.

RNA as genome
Viroids store all their genetic information in RNA alone: no DNA needed.
RNA as catalyst
Some viroids have ribozyme activity: self-cleaving RNA, demonstrating catalytic function without proteins.
Self-replicating
Viroids replicate using RNA-dependent mechanisms, hinting at a pre-DNA world where RNA replicated RNA.

Common Mistakes

✕ Confusing Prions with Viroids
Prions are protein-only, viroids are RNA-only. They share nothing in composition. Mnemonic: Prion = Protein.
✕ Saying Viroids Infect Animals
Viroids infect plants only. If a question describes animal disease, it is NOT a viroid. Hepatitis D is a satellite virus, not a viroid.
✕ Thinking Prions Have Mutations
PrPSc has the same amino acid sequence as PrPC. The difference is conformation (tertiary structure), not primary structure.

Question 1 of 5

A researcher treats an infectious agent with protease K. The agent loses its infectivity. Treatment with DNase and RNase has no effect. What is this agent most likely?

Question 2 of 5

Which structural feature distinguishes PrPSc from PrPC?

Question 3 of 5

Viroids replicate using the host cell's RNA polymerase II. What is unusual about this enzyme's role in viroid replication?

Question 4 of 5

Why does the immune system fail to mount an effective response against prion infections?

Question 5 of 5

A scientist discovers a new pathogen that is a small circular RNA, has ribozyme activity, encodes no proteins, and infects crop plants. This agent supports which hypothesis about early life?