Chapter 23: Viruses
Living versus non-living?
- Scientists are still debating a full definition of ‘life’ – and until that is agreed upon, it can still be debated as to whether or not viruses are ‘alive’.
- A broad indication as to whether something is living or non-living is if it possesses all five characteristics of life: Organisation; Nutrition; Excretion; Response; Reproduction.
- Viruses do not possess all five – therefore, it can be argued that they are non-living organisms.
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.
- 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.
Viruses are obligate parasites meaning they can only replicate using a living cell.
- Attachment: the virus uses its external proteins to latch onto a target 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 are synthesised and new copies of viral DNA/RNA are also synthesised.
- Assembly: The new DNA/RNA and viral proteins are assembled 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.
- 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).