BIOL Lecture 20

Viruses Overview

  • Definition: Viruses are considered infectious agents that consist of nucleic acid (either DNA or RNA) enveloped in a protein coat. Their origins are linked to the study of entities that infect bacteria, particularly bacteriophages.

Characteristics of Viruses

  • Viruses exist at the intersection of life and non-life, often described as possessing a "borrowed life."

  • They are not classified as cells; they are much smaller than cells and can only reproduce within host cells.

  • Common examples include bacteriophages that infect Escherichia coli.

Composition of Viruses

  • A virus is typically formed of:

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

    • Protein Coat: Surrounds the nucleic acid. This coat is referred to as the capsid.

    • Membranous Envelope: Some viruses have an outer envelope derived from the host cell's membrane, enhancing their ability to infect.

Structure of Capsids

  • Capsids are made of protein subunits called capsomeres.

  • Capsids can exhibit various shapes, contributing to the overall morphology of the virus.

Differences in Viral Genomes

  • Viral genomic types include:

    • Double-stranded or single-stranded DNA.

    • Double-stranded or single-stranded RNA.

  • Depending on the type of nucleic acid, they are classified as either DNA viruses or RNA viruses.

Bacteriophages

  • Bacteriophages (Phages): These are viruses that specifically infect bacteria; they feature complex capsid structures and an elongated head that contains DNA.

  • They utilize tail structures to attach and inject their genetic material into host cells.

Host Range of Viruses

  • Host range refers to the variety of host cells a virus can infect:

    • Broad Host Range: Example includes the West Nile Virus, affecting mosquitoes, birds, horses, and humans.

    • Narrow Host Range: Example includes Measles virus, which only infects humans.

  • Multicellular eukaryotic infections often target specific tissues like respiratory epithelium for cold viruses and HIV targeting specific white blood cells.

Viral Reproduction

  • Viruses must reproduce within host cells, classified as obligate intracellular parasites.

  • After infecting a host cell, they hijack the host's cellular machinery to produce viral proteins and genomes, resulting in self-assembly of new virus particles.

Phage Lifecycle

  • Lytic Cycle: Involves the destruction of the host cell and the release of newly formed phages.

  • Lysogenic Cycle: Viral DNA integrates into the host chromosome as a prophage, and is replicated without killing the host.

  • Temperate Phages: Can undergo both lytic and lysogenic cycles, with examples including those causing diphtheria and botulism.

Bacterial Defenses Against Viruses

  • Bacteria develop mutations that modify receptors to evade phage recognition.

  • They deploy restriction enzymes that can recognize and cut phage DNA.

  • The CRISPR-Cas system offers an adaptive immune response to target and dismantle viral DNA previously encountered by the bacterium.

Classification of Animal Viruses

  • Animal viruses are classified based on:

    • Type of nucleic acid (DNA or RNA).

    • Structure of the nucleic acid (single-stranded or double-stranded).

  • Many animal viruses possess membranous envelopes that help them invade host cells.

Retroviruses

  • Retroviruses can transcribe their RNA genome back into DNA using reverse transcriptase.

  • HIV, the virus responsible for AIDS, is a prime example of a retrovirus.

Effects of Viral Infections

  • Viruses can harm cells by causing the release of hydrolytic enzymes or producing toxins.

  • While humans may recover from infections like the common cold easily, others like polio cause permanent damage to non-dividing nerve cells.

Vaccination and Immune Response

  • Vaccines involve harmless versions of the virus that stimulate the immune system, preparing it for real infections.

  • Antiviral drugs are available but do not cure viral infections, unlike antibiotics for bacterial infections.

Emerging Viruses

  • New viral diseases can surface or become recognized globally, often from viruses expanding their host range or jumping species, as seen with SARS and the 1918 influenza pandemic.

Conclusion

  • Understanding viruses’ structures, reproductive cycles, and impacts on health is crucial for combating viral diseases and preparing for potential outbreaks.