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)
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
- Viruses that are rod-shaped have proteins packaged tightly into a helix; e.g. tobacco mosaic virus.
- Viruses that are round are composed of 20 identical proteins arranged into a spherical protein coat; e.g. rhinovirus, which causes colds and flu.
- 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.
Replication of viruses
Viruses are obligate parasites meaning they can only replicate using a living host cell.
- 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.
- Entry: either the entire virus enters the cell or it injects its nucleic acid into the cell.
- 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.
- Assembly: The new DNA/RNA and viral proteins are put together in their correct positions to make new viruses.
- 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.
Economic importance of viruses
- 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.
- 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).