Subviral Particles: Prions and Viroids (2B) - Bio & Biochem

Identify the correct statement about viroid protein-coding capacity.

Viroids do not encode proteins. Viroids lack open reading frames and rely entirely on host machinery, without translating any proteins.
Which option correctly matches agent to genetic material: prion versus viroid?

Prion: protein only; viroid: RNA only. Prions transmit via misfolded protein without nucleic acids, while viroids use RNA as their infectious genetic material.
What is the normal cellular prion protein isoform called?

PrP $^C$ . PrP $^C$ is the endogenous, properly folded prion protein expressed in healthy cells, particularly in neurons.
What type of organisms are classically infected by viroids?

Plants. Viroids primarily infect plant hosts, where they replicate and cause diseases like potato spindle tuber disease.
Which option best describes the resistance of prions to standard sterilization methods?

Highly resistant to heat, radiation, and proteases. The stable $\beta$ -sheet structure of PrP $^{Sc}$ confers resistance to denaturation by physical and chemical agents.
What is a prion, in terms of its molecular composition and infectious unit?

Infectious misfolded protein; no nucleic acid genome. Prions consist solely of misfolded proteins that propagate by inducing conformational changes in normal proteins, without any nucleic acid component.
Identify the correct statement about the presence of an envelope in prions and viroids.

Neither prions nor viroids have an envelope. Both agents lack lipid envelopes, consisting only of protein (prions) or RNA (viroids) without additional structures.
Which option best describes why prions are considered "subviral" agents?

They lack nucleic acids yet can transmit disease. Prions are subviral because they transmit infectivity through protein alone, defying the need for nucleic acids in replication.
Which host enzyme is most directly used by viroids to replicate their RNA?

Host RNA polymerase (typically RNA polymerase II). Viroids hijack the host's RNA polymerase II to transcribe their RNA, as they encode no replicative enzymes.
What is the typical clinical course of prion diseases once symptomatic?

Progressive neurodegeneration leading to death. Prion diseases are relentlessly progressive, with accumulating PrP $^{Sc}$ causing irreversible neuronal loss and fatal outcomes.
What is a viroid, in terms of its molecular composition and basic structure?

Small circular single-stranded RNA; no protein coat. Viroids are composed of short, circular ssRNA molecules that infect plants without any protective protein structure.
Which option correctly distinguishes viroids from viruses by the presence of a capsid?

Viroids lack a capsid; viruses have a protein capsid. Viroids are naked RNA molecules, whereas viruses package their genomes inside a protein capsid for protection and infectivity.
Which option best describes why viroids are considered "subviral" agents?

They are naked RNA without a capsid or envelope. Viroids are subviral as they consist only of RNA, lacking the protein capsid that encapsulates viral genomes.
Identify the correct statement about prion replication compared with viral replication.

No genome replication; propagation occurs by protein conformational conversion. Unlike viruses that replicate genomes, prions propagate solely by inducing host protein misfolding without nucleic acid involvement.
What is the typical genome form of a viroid?

Circular single-stranded RNA. Viroid genomes are compact, circular ssRNA structures that enable efficient replication in host cells.
What is the pathogenic prion protein isoform called?

PrP $^{Sc}$ . PrP $^{Sc}$ denotes the scrapie-associated, misfolded isoform that accumulates and causes prion diseases.
What type of host immune response is typically elicited by prion infections?

Minimal or absent adaptive immune response. Prions, being self-proteins, evade strong adaptive immunity, leading to little inflammation or antibody production.
Which option correctly distinguishes prions from viroids by the presence of nucleic acid?

Prions: none; viroids: RNA genome only. Prions lack any nucleic acid, relying on protein for infectivity, while viroids use RNA as their sole genetic material.
What characteristic neuropathology is classically seen in prion diseases?

Spongiform encephalopathy (vacuolization of brain tissue). Prion-induced misfolding leads to vacuole formation in neurons, creating a sponge-like appearance in brain tissue.
Choose the agent most likely to remain infectious after UV irradiation: prion or viroid?

Prion. UV irradiation damages nucleic acids, inactivating viroids but not prions, which lack any genome.
Identify the key mechanism by which prions propagate in a host.

Template-induced misfolding of PrP $^C$ into PrP $^{Sc}$ . Prions replicate by acting as templates that convert normal PrP $^C$ proteins into the pathogenic PrP $^{Sc}$ form through direct interaction.
Which option best describes how viroids cause disease without encoding proteins?

RNA-mediated disruption of host gene expression (e.g., RNA silencing). Viroid RNA interferes with host RNA processing or triggers silencing pathways, disrupting normal gene expression without protein products.
What conformational change is most associated with conversion of PrP $^C$ to PrP $^{Sc}$ ?

Increase in $\beta$ -pleated sheet content. The pathogenic PrP $^{Sc}$ form has higher $\beta$ -sheet structure, making it insoluble and aggregation-prone compared to the $alpha$ -helical PrP $^C$ .
Which tissue is primarily affected in prion diseases?

Central nervous system. Prion diseases primarily target the brain, where PrP $^{Sc}$ accumulation causes neuronal damage and spongiform changes.
What replication strategy is classically associated with viroids?

Rolling-circle replication. Viroids use a rolling-circle mechanism to generate multimeric RNA intermediates, which are processed into monomeric units.

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