Viruses

Viruses do not belong to the 5 kingdoms into which life is normally classified – so not animal, vegetable etc. They differ from living cells in at least three ways: 

• First they have a relatively simple non-cellular organisation

• Second, they have only one type of nucleic acid – either DNA or RNA – living cells contain both types

• Third, they are unable to reproduce independently of cells – they can only reproduce inside living cells.

Although, they are not made of cells, the nucleic acids of many viruses contain DNA sequences similar to those of their host cells. They use a variety of strategies to enter a cell. The viruses that infect bacteria – called bacteriophages – insert their DNA into the host cell and leave their protecting coat on the cell surface. Animal viruses, including those that infect us, enter the cell in their entirety.

They infect every type of living organism, from bacteria [yes even bacteria] to flowering plants and animals. There are some pretty virulent and destructive examples amongst their numbers – the HIV virus, the Ebola virus, the virus that causes smallpox and the influenza viruses. The herpes virus is best known for causing cold sores and genital herpes, but actually causes a wide range of diseases. The hepatitis B and C viruses, can cause liver cancer. If you happen to be a tobacco plant you might tremble at the name of the tobacco mosaic disease virus.

The common cold is a viral disease. It is more correctly known as acute viral rhinopharyngitis and is a viral infectious disease of the upper respiratory system, caused primarily by rhinoviruses and coronaviruses. As we all well know, it results in a cough, sore throat, runny nose, and fever – all of which are actually signs of the body’s defences. The common cold is the most frequent infectious disease in humans.

Viruses are everywhere – in the air, in the ground, in the sea – oceans are teaming with viruses, the top 1mm of water is estimated to contain 3x10 to the power of 30 virus particles. They can be carried by insects - aphids are big carriers of plant viruses. They can sit and wait dormant in cells for centuries and then burst forth in active mode. They can pass from one generation of host to the next in a dormant state only to be awakened much later.

They are tiny and come in all shapes and sizes. The smallest particles are little larger than ribosomes and can be viewed only with the aid of an electron microscope. A single virus is called a virion and consists of a DNA or RNA core enclosed in a protein coat called a capsid. This coat protects the virion and enables it to transfer itself from one host to another. Some viruses like the HIV and influenza virus have an extra outer layer or envelope surrounding the capsid, which can be a very complex mix of carbohydrates, lipids and proteins. Viruses occur in two phases: 

• The extracellular phase – is non-reproducing and contains few if any enzymes 

• The intra-cellular phase – during this phase the virus mainly exists as replicating nucleic acids which take over the metabolic machinery of the host to synthesise new viruses

Viral life cycles vary in their precise details depending on the species of virus, but they all share a general pattern: 

• Attachment to a host cell.

• Release of viral genes and possibly enzymes into the host cell.

• Replication of viral components using host-cell machinery.

• Assembly of viral components into complete viral particles.

• Release of viral particles to infect new host cells.

 Why do they exist? Well, why not?

Observations

For iPad/iPhone users: tap letter twice to get list of items.

• Bryson, Bill - On microbes
• Viruses and their crystal structure