Chapter 19: Viruses and Prions

Vocabulary terms and definitions covering the discovery, structure, replicative cycles, and pathogenicity of viruses and prions based on Chapter 19.

Virus

An infectious particle consisting of genes packaged in a protein coat, lacking metabolic machinery and structures found in cells.

Tobacco Mosaic Virus (TMV)

The first virus to be crystallized, discovered by Wendell Stanley in 1935, which affects tobacco plants with a mottled coloration.

Capsid

The protein shell enclosing the viral genome, built from protein subunits called capsomeres.

Helical Viruses

Rod-shaped viruses, such as the tobacco mosaic virus, which have a rigid capsid made from over $1,000$ molecules of a single type of protein arranged in a helix.

Icosahedral Viruses

Viruses with a polyhedral capsid containing $20$ triangular facets, such as adenoviruses.

Viral Envelope

A membranous outer layer found in some viruses, derived from the host cell's plasma membrane, containing host cell phospholipids and viral proteins/glycoproteins.

Bacteriophages (Phages)

Complex viruses that infect bacteria, often possessing an elongated icosahedral head and a protein tail piece with fibers for attachment.

Host Range

The limited number of species or tissue types a particular virus can infect, determined by a 'handshake' fit between viral surface proteins and specific receptor molecules.

Lytic Cycle

A phage replicative cycle that culminates in the death of the host cell by lysis (breaking open) to release progeny phages.

Virulent Phage

A phage that replicates only by the lytic cycle.

Lysogenic Cycle

A replicative cycle that allows the phage genome to replicate without destroying the host cell.

Temperate Phages

Phages capable of using both lytic and lysogenic modes of replicating within a bacterium, such as phage lambda ($\text{λ}$).

Prophage

Viral DNA that has been integrated into a specific site on a bacterial chromosome.

Restriction Enzymes

Bacterial cellular enzymes that identify and cut up foreign phage DNA to restrict the virus's ability to replicate.

CRISPR-Cas System

A bacterial immune system consisting of clustered regularly interspaced short palindromic repeats and associated nuclease proteins that identify and cut invading phage DNA.

Retroviruses

RNA animal viruses (Class VI) that use reverse transcriptase to transcribe an RNA template into a DNA copy.

Reverse Transcriptase

A viral enzyme that catalyzes the synthesis of DNA using an RNA template, providing an RNA-to-DNA information flow.

Provirus

Integrated viral DNA that remains a permanent resident of a host animal cell's genome.

Mimivirus

A large double-stranded DNA virus with a capsid of $400\,nm$ and a genome of $1.2\,Mb$, containing genes for translation and DNA repair.

Pandoravirus

A large virus ($1\,μ m$ in diameter) with a genome of $2–2.5\,Mb$ where over $90\%$ of its genes are unrelated to cellular genes.

Pithovirus sibericum

A virus discovered in permanently frozen soil in Siberia with a diameter of $1.5\,μ m$ that remained infectious after $30,000$ years.

Vaccine

A harmless derivative of a pathogen that stimulates the immune system to mount defenses against a harmful pathogen.

Epidemic

A widespread outbreak of an infectious disease.

Pandemic

A global epidemic.

Hemagglutinin (HA)

A viral surface protein identified in strains like $H5N1$ that helps the flu virus attach to host cells; $18$ types have been identified.

Neuraminidase (NA)

A viral enzyme found on the surface of flu viruses that helps release new virus particles from infected cells; $11$ types have been identified.

Horizontal Transmission

A route of viral infection in plants where the virus enters from an external source, often via damage by wind, injury, or herbivores.

Vertical Transmission

A route of viral infection in plants where the infection is inherited from a parent through asexual propagation or infected seeds.

Plasmodesmata

Cytoplasmic connections between adjacent plant cells through which viral genomes and proteins can spread, often facilitated by virally encoded proteins.

Prions

Infectious proteins that are misfolded versions of normal brain proteins; they aggregate and cause degenerative brain diseases such as scrapie, mad cow disease, and Creutzfeldt-Jakob disease.