Viral Structure and Classification

Viral Structure and Classification

• Viruses are significantly smaller than bacteria, typically 20-300 nm in diameter, while bacteria range from 0.5-5 micrometers.
• Viruses consist of a nucleic acid core (DNA or RNA) enclosed by a protein coat (capsid). Some have a lipid envelope derived from the host cell.
• Bacteria are unicellular organisms with a cell wall, plasma membrane, cytoplasm, and a circular DNA chromosome. They can independently metabolize, reproduce, and grow.
• Viruses require a host cell to replicate, and cannot carry out metabolism independently. They hijack the host's cellular machinery for reproduction.
• Bacteria reproduce asexually through binary fission.

Viral Structure: Enveloped vs. Nonenveloped

• Enveloped Viruses: Contain a lipid envelope derived from the host cell membrane; this envelope contains viral glycoproteins used for cell entry. The capsid surrounds the viral genome. These are sensitive to environmental factors like heat and detergents. Examples: Influenza, HIV.
• Nonenveloped Viruses: Lack a lipid envelope; the genome is entirely enclosed within a protein shell (capsid). Generally more stable than enveloped viruses and can withstand harsher conditions (e.g., heat, detergents, drying). Examples: Norovirus, Poliovirus.

Viral Species Definition

• A viral species is a group of viruses sharing the same genetic information and ecological niche. These viruses have similar physical characteristics and often infect similar host organisms. Definition is based on genome sequence, host organism, and biological properties.

Viral Naming Example

• Family: Herpesviridae
• Genus: Herpesvirus
• Common Name: Herpes simplex virus (HSV)

Bacteriophage Culture

• Bacteriophages, viruses that infect bacteria, are cultured by infecting bacterial cultures in nutrient-rich media.
• Infected bacteria undergo lysis, producing clear zones (plaques) on the bacterial lawn. These plaques are counted to determine viral quantity.

Animal Virus Culture

• Animal viruses are cultured using living animals (though less common), embryonated eggs (exploiting developing embryos to support viral replication), or cell cultures. Cell cultures involve growing animal cells in the lab; the virus's effect on cells (cytopathic effect) or viral particles are used for observation and quantification.

Virus Identification Techniques

• Electron Microscopy: Visualizes viral morphology, shape, and size.
• Polymerase Chain Reaction (PCR): Amplifies viral DNA or RNA for detection.
• Serological Tests: Detect viral antigens or antibodies using methods like ELISA or western blot.

Bacteriophage Lytic Cycle

• The lytic cycle is a destructive viral reproductive cycle.
  • Attachment: Phage attaches to host.
  • Penetration: Viral nucleic acid enters the cell.
  • Biosynthesis: Host cell makes viral components.
  • Maturation: New viral particles assemble.
  • Release: Host cell lyses, releasing new phages.

Bacteriophage Lysogenic Cycle

• The lysogenic cycle incorporates the viral DNA into the host's genome (becoming a prophage) allowing for dormant viral replication along with the host cell.
  • Attachment/Entry: Phage injects DNA.
  • Integration: Viral DNA integrates.
  • Latency: Viral genome replicates with cell division.
  • Induction: Prophage exits; enters the lytic cycle.

Animal Virus Multiplication (DNA & RNA)

• DNA Viruses:
  • Attachment, penetration, uncoating, replication and transcription, protein synthesis, assembly, budding (or lysis).
• RNA Viruses:
  • Attachment, penetration, uncoating, replication and translation (direct or via reverse transcription), protein synthesis, assembly, release.

Oncogenes and Transformed Cells

• Oncogene: A gene that can cause a cell to become cancerous when mutated or activated.
• Transformed Cell: Cells that have undergone changes (e.g., increased growth, loss of contact inhibition) often due to oncogenes or viral infections.

Viruses and Cancer

• Some DNA viruses (like HPV) and RNA viruses (like Hepatitis B and C) can cause cancer by integrating their genomes into the host genome or producing proteins that disrupt cell cycle regulation, leading to uncontrolled growth.

Latent Viral Infections

• A latent viral infection involves a dormant period of viral replication, with reactivation occurring later due to various factors (e.g., stress, immune suppression). Herpes simplex virus (HSV) is an example.

Persistent vs. Latent Viral Infections

• Persistent Infection: Virus remains, continuously replicates. Hepatitis B is an example
• Latent Infection: Virus is dormant, stops replication, and can reactivate later. HSV is an example.

Virus, Viroid, and Prion Differentiation

• Virus: Nucleic acid (DNA or RNA) and protein coat needing a host cell for replication.
• Viroid: Small circular RNA, infectivity in plants, lacks a protein coat.
• Prion: Infectious protein, causes neurodegenerative diseases, no nucleic acid.

Plant Virus Lytic Cycle

• Attachment of the virus to the plant cell.
• Entry of the viral genome into the plant cell (via vectors or damage).
• Replication of viral genome and protein synthesis.
• Assembly of new viral particles.
• Release of new complete virions (either via cell rupture or plasmodesmata).

Infectious Protein (Prion) Description

• Prions are infectious proteins. They induce misfolding of normal proteins within the brain, damaging neural tissues and causing neurodegenerative diseases (e.g., mad cow disease, Creutzfeldt-Jakob disease). They lack nucleic acids.