Chapter 11: Viruses

Overview

• Viruses have influenced human history and continue to affect human health.
• Some forms of cancer are caused by viruses, raising questions about the transmissibility of cancer.
• The chapter explores the structure and behavior of viruses and virus-like agents.

Definition and Characteristics of Viruses

• Viruses: Infectious agents too small to be seen with a light microscope.
  • Lack a cell nucleus, organelles, or cytoplasm.
  • Considered non-living as they can't replicate on their own.
  • Obligate intracellular parasites, replicating only in living host cells.

Components of Viruses

• Nucleic Acid Core: Can be DNA or RNA, single or double-stranded.
• Capsid: Protein coat surrounding the nucleic acid.
  • Plays a key role in attachment to host cells.
• Envelope: Lipid bilayer membrane surrounding some viruses; acquired from the host cell.
• Spikes: Glycoproteins extending from the viral envelope, aiding in attachment to host cells.

Sizes and Shapes

• Viruses vary in size from 30 nm (e.g., enteroviruses) to 1500 nm.
• Can have complex shapes with specialized structures like tails and fibers.

Origins and Classification of Viruses

• Viruses must infect host cells to reproduce.
• Debate exists about whether viruses are living or non-living.
• Initially classified by host; now by nucleic acid type, structure, and replication method.
• Over 40,000 strains exist, classified by ICTV.

Viral Nucleic Acids

• RNA Viruses: Can be positive-sense or negative-sense.
  • Positive-sense RNA acts like mRNA.
  • Negative-sense RNA requires transcription to make positive-sense mRNA.
• DNA Viruses: Can be double-stranded or single-stranded.

Viral Replication Cycles

Lytic Cycle

1. Adsorption: Virus attaches to host cell wall.
2. Penetration: Viral DNA injected into the host cell.
3. Synthesis: Viral DNA takes over host cell activities to replicate.
4. Maturation: New viral particles are assembled.
5. Release: Host cell lyses, releasing new virions.

Lysogenic Cycle

• Virus integrates into the host DNA as a prophage, replicating with the host cell.
• Can remain dormant; conditions may trigger switch to the lytic cycle.

Bacteriophages

• Infect bacterial cells specifically, often used in phage therapy.
• Can be highly effective against antibiotic-resistant bacterial infections.

Replication of Animal Viruses

• Adsorption: Attachment to host cell proteins.
• Penetration: Entire virus enters the host cell, often by endocytosis.
• Synthesis: Varies by virus type; retroviruses use reverse transcriptase.
• Maturation and Release: Virions assemble and leave the host cell, possibly budding without lysis.

Human Cancer Viruses

• Certain viruses like HPV and HBV are linked to cancers such as cervical and liver cancer.
• Oncogenic viruses can lead to unregulated cell division and tumor formation.

Key Takeaways

• Viral infections and behavior are complex, influencing human health significantly.
• Understanding viral structure and replication is crucial for disease prevention and treatment.