Viruses

Viruses are only visible with an electron microscope and do not have a proper cell structure; no cytoplasm, no organelles and no chromosomes.

Viruses can only reproduce inside a host cell - only non-living thing that can reproduce. Outside the cell they exist as inert virus particles called virons. They are highly specific (bacteriophagesm specific to bacteria)

When they invade the host cell they are able to take over the cells metabolic machinery and make new virus particles. Either the virus brings the enzymes it needs or its nucleic acid codes for them.

VIruses consist of:

• A nucleic acid
• A protein coat or caspid

• The core of the nucleic acid can be RNA or DNA. Th einfluenza virus and HIV contain RNA. Herpes simples (cold sores) and Parvovirus bothe conatin DNA
• Regarded as parasites as they are totally dependent on the host cell 

Life Cycle

• The viral nucleic acid enters the host cell. This can occur by the viral membrame joining to the cell surface membrane of the host cell or, as in the case pf bacteriophages, the /dna is injected into a bacterial cell
• The viral DNA/RNA is 'read'
• The host cell uses the information to make new viral DNA/RNA and new viral protein
• The viral DNa is surrounded by protein coats and the jost cell bursts, releasing the new viruses

The t2 bacteriophage lives on the gut bacterium E.coli.

Acquired immune deficiency syndrome (AIDS) is caused by the human immunodeficiency virus (HIV). The virus attacks helper T-cells. 

• HIV is a retovirus - uses RNA to produce a single strand of DNA called copy DNA
• The enzyme reverse transcriptase catalyses this reverse transcription of DNA from an RNA template

The virus can remain latent for many years before activating, when it will start to replicate and destroy the host cell. As the number of viruses increases, so the number of helper T-cells is reduced. Therefore HIV weakens the body's ability to fight disease.