chapter 6 pt 1

Chapter Overview

• Viruses: Focus on structure and classification.

• Classifying based on host type and genetic makeup.

• Discussion on viral replication in animal and bacterial hosts.

• Addresses latency, viruses and cancer, emerging viruses, prions, and viroids.

Key Concepts

• Viruses: Infectious agents that are acellular, requiring a host for replication.

• Obligate Intracellular Parasites: Viruses must infect host cells to reproduce.

Structure of Viruses

• Virion: Complete infectious viral particle, contains:

  • Nucleic acid (DNA/RNA)

  • Protein coat (capsid) made of capsomeres

• Some viruses have an envelope (lipids/proteins from host cell membranes) which can have spikes for attachment.

Size and Classification of Viruses

• Viruses are significantly smaller than bacteria, visible only under electron microscopy.

• Morphological classification:

  • Helical: Spiraled structure.

  • Polyhedral: Many-sided shapes, e.g., icosahedral (20 faces).

  • Complex: Combination of shapes, e.g., bacteriophages.

Historical Context

• First identified virus: Tobacco mosaic virus (late 1800s).

• Innovations like the electron microscope were pivotal for studying viruses.

Classification by Host Type

• Traditional classifications of animal viruses based on targeted body systems:

  • Pneumatotropic: Affects respiratory system (e.g., influenza).

  • Dermatotropic: Affects skin (e.g., herpes).

  • Viscerotropic: Targets organs (e.g., hepatitis).

  • Neurotropic: Affects nervous system (e.g., rabies).

• Newer classifications focus on genetic structure and replication methods.

Genetic Makeup of Viruses

• DNA Viruses: Can be single-stranded or double-stranded. Replication occurs in the nucleus.

  • Example: Human parvovirus 19 (single-stranded), associated with Fifth disease.

• RNA Viruses: Can be single-stranded or double-stranded. Generally replicate in cytoplasm.

  • Single-stranded RNA viruses can be:

    • Positive sense: Functions as mRNA.

    • Negative sense: Needs to synthesize mRNA from a template.

  • Example: Rotavirus, causes diarrhea.

  • Retroviruses: Carry reverse transcriptase, converting RNA to DNA which integrates into the host genome (e.g., HIV).

Viral Replication in Animal Hosts

1. Attachment: Virus binds to host's specific receptors.

2. Entry: Capsid and nucleic acid enter through endocytosis (naked viruses) or membrane fusion (enveloped viruses).

3. Uncoating: Removal of capsid, releasing viral nucleic acid.

4. Synthesis: Viral proteins made using host's machinery.

5. Maturation: Assembly of new virions.

6. Release: Viruses exit via budding (enveloped viruses) or cell lysis (naked viruses).

Bacteriophage Replication

• Lytic Cycle: Bacteriophage infects, reproduces, and causes cell lysis.

  • Steps: Attachment, penetration, biosynthesis, maturation, lysis.

• Lysogenic Cycle: Viral DNA incorporates into host chromosome; can later switch to lytic cycle under stress.

Multiplication Curve for Bacteriophages

• Inoculation: Viruses bind to host cells.

• Eclipse Phase: Initial decrease in detectable viral particles as they enter host cells.

• Burst Phase: Sudden increase as new virions are released.

• Burst Size: Maximum number of virions produced per bacterial cell.

Influenza Viruses

• Influenza is an enveloped virus with two critical spikes (H for hemagglutinin, N for neuraminidase).

• Important for vaccine development; strains can mutate, necessitating annual updates to vaccines.

Prions and Viroids

• Distinctions from viruses; both are acellular pathogens not classified as viruses.

Important Historical Figures

• Development by the International Committee on Taxonomy of Viruses in 1966 to standardize viral classification.