Campbell Biology, Twelfth Edition - Chapter 19: Viruses

Definition of a Virus:
- A virus is described as an infectious particle consisting of genes packaged in a protein coat.
- Structurally, viruses are much simpler than even prokaryotic cells; they consist of nucleic acid and proteins.
- Viruses can cause a variety of diseases.
- Notably, they cannot reproduce or carry out metabolism outside of host cells.
- Viruses occupy a unique position between living organisms and chemicals, leading to the concept of “borrowed life.”

Tobacco Mosaic Disease:
- Observations indicated the disease stunts growth and creates mosaic coloration in tobacco plant leaves.
- In the late 1800s, researchers hypothesized that unusually small bacteria were responsible.
- Subsequent tests failed to establish growth on nutrient media, leading to the idea of an infectious agent distinct from bacteria.
- In 1935, Wendell Stanley confirmed this by crystallizing the tobacco mosaic virus (TMV).

Characteristics:
- Viruses are not cellular.
- Composed of nucleic acid encased in a protein coat (capsid) and may also possess a membranous envelope.
- Their simple structure makes them useful biological systems for research.
Viral Genomes:
- Viral genomes may consist of either:
- Double-stranded DNA
- Single-stranded DNA
- Double-stranded RNA
- Single-stranded RNA
- Classifications of viruses are made based on whether they are DNA viruses or RNA viruses.
- Genome structure can be a single linear molecule or circular, containing between 3 and 2,000 genes.

Capsids:
- A capsid is defined as the protein shell enclosing the viral genome.
- Composed of protein subunits called capsomeres.
- Capsids vary in structure; related viruses can be classified as helical or icosahedral.
Viral Envelopes:
- Some viruses feature accessory structures that facilitate infection.
- Envelopes are derived from host cell membranes and consist of a mix of viral and host molecules.
- An example includes enveloped viruses like influenza.
Bacteriophages (Phages):
- Viruses specifically targeting bacteria, characterized by an elongated capsid head and a tail for attaching to host cells and injecting DNA.

Obligate Intracellular Parasites:
- Viruses can only replicate within host cells; each virus has a specific host range.
- Some viruses can infect multiple host species, while others, like the measles virus, infect only humans.
General Features of Viral Replication:
- Viral genomes enter host cells through various mechanisms.
- Host cells utilize viral nucleic acids and proteins to manufacture new viral components.
- Nucleic acid molecules and capsomeres self-assemble into new virus particles.

Lytic Cycle:
- This cycle results in the death of host cells via lysis (breaking open).
- Virulent phages reproduce solely through the lytic cycle, producing numerous phages.
Lysogenic Cycle:
- In this cycle, the viral DNA integrates into the host genome without destroying the cell.
- Integrated viral DNA (prophage) replicates with the host's chromosome.
- Environmental signals may trigger a shift to lytic mode, leading to potential toxin production in host cells.

Defensive Mechanisms:
- Bacteria can develop mutations that render them resistant to phage binding.
- Restriction enzymes identify foreign DNA, while bacterial DNA is protected through methylation.
- The CRISPR-Cas system provides adaptive immunity against phages.
- CRISPRs consist of sequences derived from previous phage infections, allowing for targeted immune response upon re-infection.

Two critical factors classify animal viruses:
- Type of genomic material (RNA or DNA, single-stranded or double-stranded).
- Presence or absence of a membranous envelope.
Many animal viruses possess both an envelope and RNA genome, facilitating infection of host cells through glycoprotein receptors.

RNA and DNA Variations:
- Viruses exhibit a broad spectrum of RNA genome types, with retroviruses reversing transcribing their RNA into DNA (e.g., HIV)
- Proviruses remain permanently integrated into host cells, transcribed by RNA polymerase into functional mRNA.

Living Organisms:
- Viruses do not conform to the definition of living organisms.
- They likely evolved from portions of cellular nucleic acids, with mobile genetic elements like plasmids and transposons serving as sources.
Controversy in Evolution:
- The largest identified virus challenges the distinction between viruses and cellular organisms, with unresolved questions on its evolutionary origins.

Impact on Health:
- Viral infections can severely damage or kill cells, release toxins, or lead to immune response excitement.
- Vaccines serve as preventative measures by allowing the immune system to build defenses against pathogens.
- Antiviral drugs can mitigate infection but not cure it.
Emerging Viruses:
- Examples include HIV, Ebola, chikungunya, and Zika virus.
- New strains often arise from mutation or animal-to-human spread, emphasizing the adaptability of viral pathogens.

Over 2,000 plant viral diseases cause significant agricultural damage.
Transmission routes include:
- Horizontal transmission – through damaged cellular structures
- Vertical transmission – inheriting from parent plants.

Prions are unique pathogenic proteins contributing to degenerative diseases in animals (e.g., mad cow disease, Creutzfeldt-Jakob disease).
Misfolded proteins may induce normal proteins to adopt the prion conformation, creating aggregates.
Prions may be implicated in other diseases like Alzheimer’s and Parkinson’s.

Figures included illustrate viral structures, replicative cycles, and examples of emerging viruses.
Each figure supports the relevant discussion by visually representing key concepts from the chapter for enhanced understanding.