Chapter 23: Viruses

The study of viruses is called virology.

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.

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.

Shapes of viruses

1. Rod-shaped

Viruses that are rod-shaped have proteins packaged tightly into a helix; 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.

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.

Replication of viruses
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.

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).