The study of viruses is called virology.

Living versus non-living?

  • Virologists consider viruses to be simply infectious agents and not living. This is because:
    • They are not cells
    • They require living host cells in order to replicate themselves
    • They possess either DNA or RNA (never both)

Basic structure

All viruses are composed of an outer protein coat (called the capsid) surrounding a piece of DNA or RNA. They do not possess both types of nucleic acid. Some viruses possess an outer lipid membrane.

Shapes of viruses

1. Rod-shaped

  • Viruses that are rod-shaped have proteins packaged tightly into a helix; e.g. tobacco mosaic virus.
Rod-shaped viruses, e.g. tobacco mosaic virus

2. Round

  • Viruses that are round are composed of 20 identical proteins arranged into a spherical protein coat; e.g. rhinovirus, which causes colds and flu.
Round-shaped virus, e.g. rhinovirus

3. Complex

  • Complex viruses comprise a large group of viruses with shapes that vary widely. The most common shape is that of the bacteriophage – that infect bacterial cells.
Complex-shaped virus, e.g. bacteriophage

Replication of viruses

Viruses are obligate parasites meaning they can only replicate using a living host cell.

  1. Attachment: the virus uses its external proteins to latch onto a target cell by attaching to membrane proteins on the surface of the host cell.
  2. Entry: either the entire virus enters the cell or it injects its nucleic acid into the cell.
  3. Replication: the virus or the viral nucleic acid takes over the cell’s nucleus and protein synthesis system (ribosomes). New viral proteins new copies of viral DNA/RNA are made.
  4. Assembly: The new DNA/RNA and viral proteins are put together in their correct positions to make new viruses.
  5. Release: the newly formed viral particles are released from the cell either by budding out through the cell membrane or by causing the cell to burst.
Viral replication

Economic importance of viruses

Beneficial effects

  • Viruses are regularly used in genetic engineering of various types of cells. They are used as vectors (see Chapter 18).
  • It is hoped that bacteriophage viruses may in the future be used to treat serious bacterial infections in humans, eventually replacing antibiotics.

Harmful effects

  • Viruses can cause illnesses and disease; for example in humans viruses cause colds and influenza (rhinovirus), AIDS (HIV), hepatitis (hepatovirus), poliomyelitis (polio virus), measles (paramyxovirus), and chicken pox (varicella zoster virus) to name but a few; and in other organisms conditions such as foot and mouth disease in ruminants and various mosaic diseases in plants (e.g. tobacco mosaic virus in the tobacco plant).
HIV attaching to a white blood cell
Mosaic virus disease of a plant’s leaves
Bacteriophage attaching to a bacterium