Viruses: Structure, Replication, and Pathogenicity

Introduction to Viruses

Viruses are infectious particles that consist of genetic material encased in a protein coat. They are much simpler than prokaryotic cells and cannot reproduce or carry out metabolism outside of a host cell. Viruses occupy a unique position between living organisms and chemicals, leading a "borrowed life" dependent on host cells.

• Definition: A virus is an infectious particle made of genes (DNA or RNA) packaged in a protein coat.

• Diseases: Viruses cause a wide variety of diseases in humans, animals, and plants.

• Reproduction: Viruses require host cells to replicate.

The Discovery of Viruses

The study of viruses began with the investigation of tobacco mosaic disease, which stunted plant growth and caused mosaic coloration. Early researchers hypothesized bacteria as the cause, but later work showed the infectious agent did not share bacterial features. In 1935, Wendell Stanley crystallized the tobacco mosaic virus (TMV), confirming its viral nature.

• Key Example: Tobacco mosaic virus (TMV) was the first virus to be crystallized.

Structure of Viruses

Viruses are not cells; they are small infectious particles composed of nucleic acid, a protein coat, and sometimes a membranous envelope. Their simple structure makes them useful for biological research.

• Nucleic Acid: DNA or RNA, single- or double-stranded.

• Protein Coat: Called a capsid, built from subunits called capsomeres.

• Envelope: Some viruses have a membranous envelope derived from host cell membranes.

Viral Genomes

Viral genomes vary in type and structure.

• Types: Double- or single-stranded DNA or RNA.

• Classification: DNA viruses or RNA viruses.

• Structure: Linear or circular nucleic acid molecules.

• Gene Count: 3 to 2,000 genes per genome.

Capsids and Envelopes

The capsid is the protein shell enclosing the viral genome. Capsids can be helical or icosahedral in structure. Some viruses have accessory structures, such as viral envelopes, which help them infect hosts.

• Capsid: Built from capsomeres; protects the genome.

• Envelope: Contains viral and host molecules; surrounds capsids in many animal viruses.

• Bacteriophages: Viruses that infect bacteria, with elongated capsid heads and protein tails for DNA injection.

Viral Replication

Viruses are obligate intracellular parasites, meaning they replicate only within host cells. Each virus has a host range, which is the set of species it can infect.

• Host Range: Some viruses infect only one species; others have broad host ranges.

• Replication: Viral genome enters host cell, host machinery produces viral proteins, and new viruses self-assemble.

Replicative Cycles of Phages

Phages (bacteriophages) are well-studied viruses with two main reproductive mechanisms: the lytic cycle and the lysogenic cycle.

• Lytic Cycle: Produces new phages, lyses host cell, releases progeny viruses. Virulent phages use only this cycle.

• Lysogenic Cycle: Viral DNA integrates into host chromosome as a prophage, replicates with host without killing it. Temperate phages use both cycles.

Comparison of Lytic and Lysogenic Cycles

Bacterial Defenses Against Phages

Bacteria have evolved several mechanisms to defend against phage infection.

• Surface Proteins: Mutations prevent phage recognition.

• Restriction Enzymes: Cut foreign DNA; bacterial DNA is protected by methylation.

• CRISPR-Cas System: Stores DNA fragments from previous phage infections, allowing recognition and destruction of matching phage DNA upon reinfection.

Replicative Cycles of Animal Viruses

Animal viruses are classified by their genome type (DNA or RNA, single- or double-stranded) and the presence or absence of an envelope. Many animal viruses have both an envelope and an RNA genome.

• Viral Envelope: Glycoproteins bind to host cell receptors; envelope derived from host plasma membrane or other cellular membranes.

• Herpesvirus: Envelope formed from nuclear envelope, replaced by Golgi-derived envelope.

Retroviruses and Proviruses

Retroviruses, such as HIV, use reverse transcriptase to copy their RNA genome into DNA, which integrates into the host genome as a provirus.

• Retrovirus: Uses reverse transcriptase; HIV causes AIDS.

• Provirus: Permanently integrated viral DNA in host genome; transcribed by host RNA polymerase.

• Difference from Prophage: Provirus remains permanent; prophage can excise and re-enter lytic cycle.

Evolution of Viruses

Viruses likely evolved from bits of cellular nucleic acid, such as plasmids and transposons, which are mobile genetic elements. The largest viruses have genomes encoding proteins for translation, DNA repair, and more, raising questions about their evolutionary origins.

• Mobile Genetic Elements: Plasmids, transposons, and viruses.

• Large Viruses: Genome size and complexity challenge traditional views of viral evolution.

Viruses and Prions as Pathogens

Viruses and prions are formidable pathogens affecting animals and plants.

• Viral Diseases: Cause cell damage, toxin production, and disease symptoms.

• Vaccines: Stimulate immune response; prevent viral illnesses.

• Antiviral Drugs: Inhibit viral DNA synthesis and assembly; antibiotics are ineffective against viruses.

Emerging Viral Diseases

Emerging viruses are those that suddenly become apparent, often due to mutation, spread from isolated populations, or transmission from other animals. Examples include HIV, Ebola, Zika, and influenza strains.

• Mutation: Creates new viral strains.

• Animal Transmission: Three-quarters of new human diseases originate from animals.

• Pandemics: Global epidemics, such as H1N1 in 2009.

• Environmental Change: Alters virus spread, e.g., mosquito range expansion.

Influenza Virus Classification

Influenza A viruses are named based on surface proteins hemagglutinin (HA) and neuraminidase (NA). There are multiple types of each, leading to diverse strains.

• H5N1: Highly deadly, not transmitted person-to-person.

• H1N1: Pandemic strain, complex genetic origin.

Viral Diseases in Plants

Plant viruses cause numerous diseases, leading to spots, stunted growth, and damaged organs. Most plant viruses have RNA genomes and helical or icosahedral capsids.

• Transmission: Horizontal (through damaged cell walls) and vertical (inherited from parent).

Prions: Proteins as Infectious Agents

Prions are infectious proteins that cause degenerative brain diseases in animals. They are misfolded proteins that can convert normal proteins into the misfolded, disease-causing form.

• Diseases: Scrapie (sheep), mad cow disease, Creutzfeldt-Jakob disease (humans).

• Properties: Transmitted in food, act slowly, virtually indestructible.

• Propagation: Prions aggregate and convert more proteins, possibly involved in Alzheimer's and Parkinson's.

Summary Table: Key Concepts in Viral Replication

Relevant Images

Figure 19.1a: A human immune cell infected by HIV, releasing new viruses. This image visually demonstrates the process of viral infection and replication, directly supporting the explanation of how viruses infect host cells and produce progeny.

Figure 19.1b: How does a virus make more viruses? This image is directly relevant to the explanation of viral replication cycles, illustrating the process by which viruses produce new viral particles within host cells.