Chapter 13: Characterizing Viroids, Prions, and Viruses

Vocabulary practice flashcards covering structural features, replicative cycles, entry mechanisms, persistent infection categories, and characteristics of viroids, prions, and viruses.

Viroid

An infectious, pathogenic plant agent composed solely of circular single-stranded RNA ($\text{ssRNA}$) with complementary base pairings and loops, lacking a protein capsid coat.

Prion

A proteinaceous infectious agent that lacks nucleic acids and induces normal mammalian proteins to refold into abnormal, protease-resistant, disease-causing configurations.

Cellular c-PrP

The normal, functional form of prion protein found in all mammalian cells and anchored to cell membranes, structurally composed primarily of several $\alpha$-helices.

Prion p-PrP

The abnormal, disease-causing isoform of the prion protein that is enriched with $\beta$-pleated sheets, resistant to proteases, and capable of converting c-PrP into p-PrP.

Spongiform Encephalopathies

Prion-induced fatal neurodegenerative diseases characterized by the formation of large vacuoles in brain tissue that produce a spongy appearance, such as Kuru and bovine spongiform encephalopathy (BSE).

Virion

A complete, fully developed, infectious viral particle consisting of nucleic acid enclosed within a protein coat (capsid) and sometimes surrounded by an outer envelope.

Capsid

The outer proteinaceous coat of a virion composed of protein subunits called capsomeres, which protects viral nucleic acids and facilitates host attachment.

Capsomeres

The individual proteinaceous subunits, constructed from single or multiple protein types, that fit together to form the capsid shell.

Viral Envelope

A lipid, protein, and carbohydrate outer bilayer acquired from the host cell's membrane during viral replication or release.

Viral Spikes

Virally coded glycoprotein projections that protrude from the outer surface or envelope of a virion and function in host recognition and receptor binding.

Bacteriophage

A virus that specifically targets, infects, and replicates inside bacterial cells.

Lytic Replication

A bacteriophage replication process consisting of attachment, entry, synthesis, assembly, and release that causes the lysis and death of the host bacterial cell.

Lysogenic Replication

A bacteriophage cycle where viral DNA integrates into the bacterial chromosome as a prophage, allowing the host to survive and reproduce normally for generations.

Prophage

An inactive bacteriophage genome that has integrated directly into the host bacterial chromosome during the lysogenic cycle.

Direct Penetration

A viral entry mechanism used by non-enveloped animal viruses where the viral capsid binds to host receptors and injects the nucleic acid directly into the cell, leaving the capsid outside.

Membrane Fusion

An entry mechanism used by enveloped animal viruses where the viral envelope fuses directly with the host cytoplasmic membrane, releasing the intact capsid into the host cytoplasm.

Endocytosis (Viral Entry)

An entry mechanism in animal viruses where host cytoplasmic membrane invaginates to engulf the entire virus inside an endocytic vesicle.

Viral Uncoating

The removal or disassembly of the viral protein capsid following entry into an animal host cell, exposing the viral genome for synthesis.

RNA-dependent RNA Polymerase (RdRP)

A viral enzyme utilized by dsRNA and ssRNA viruses that directs the replication of viral RNA genomes using RNA as a template.

Reverse Transcriptase (RT)

A retroviral enzyme that converts viral single-stranded RNA ($\text{ssRNA}$) into complementary viral DNA capable of integrating into the host genome.

Budding

A viral exit mechanism where the virion extrudes through the host plasma membrane, acquiring an envelope without causing immediate cell lysis.

Exocytosis (Viral Release)

A viral release mechanism in which newly assembled virions are enclosed in intracellular vesicles that fuse with the cytoplasmic membrane to release naked viruses outside the cell.

Latent Viral Infection

A persistent infection wherein the virus remains dormant or hidden inside specific host cells without causing symptoms until it is reactivated (e.g., Varicella-zoster virus causing chickenpox and later shingles).

Chronic Viral Infection

A persistent infection characterized by continuous viral presence, wherein the virus remains dormant in certain cells while actively replicating and producing virions in others (e.g., HIV).