Bio chapter 26

Introduction to Viruses

  • Nature of Life: Viruses exist on the line between being considered alive and not alive.

  • Evolutionary Impact: Despite the debate on their living status, viruses are crucial for shaping the evolution of organisms throughout Earth's history.

  • Diversity: There is tremendous diversity among viruses present on Earth.

Basic Viral Structure

  • Genome: All viruses contain genetic material, either RNA or DNA. Most viruses ( \approx 70\% ) have RNA genomes, while some have DNA genomes.

  • Capsid: The viral genome (DNA or RNA) is surrounded by a protein sheath called a capsid.

    • Nucleocapsid: If the capsid is very tightly associated with the genome, it may be called a nucleocapsid, reminiscent of the nucleus in eukaryotic cells (though not a nucleus).

    • Composition: The capsid is typically composed of many copies of the same protein molecule or a few different types of protein molecules.

    • Assembly: These proteins assemble to form highly predictable shapes.

Viral Morphologies (Shapes of Capsids)

  • Helical:

    • Resembles a winding staircase or helix.

    • Common in plant viruses, e.g., Tobacco Mosaic Virus (TMV) (often found as Filamentous).

  • Icosahedral:

    • Composed of 20 triangular facets.

    • Common in most animal viruses, e.g., Poliovirus.

  • Complex/Binal Symmetry:

    • Combination of helical and icosahedral shapes.

    • Example: Bacteriophages (bacterial viruses) have an icosahedral head (enclosing DNA) and a helical tail portion.

    • These often appear as