Viruses and Bacteriophages

Viruses are non-cellular structures. They are obligate parasites that can multiply only inside a living cell: a virus multiplies inside a living cell.

Structure: a nucleic acid and a protein coat (capsid). The nucleic acid is either DNA or RNA.

A bacteriophage infects bacteria.

Multiplication of a virus (using a bacteriophage as an example):

1. The phage attaches to the bacterium
2. The nucleic acid enters the cell
3. Replication of the nucleic acid
4. New virus particles are assembled
5. Release from the cell, the cell dies (lysis)

In humans, viruses cause influenza, measles, hepatitis, AIDS (HIV) and other diseases.

Viruses were discovered in 1892 by D. I. Ivanovsky in tobacco mosaic disease.

Key facts

• HIV infects T-lymphocytes, and immunity weakens. It is a retrovirus (DNA is made from RNA, with the enzyme reverse transcriptase).
• Antibiotics do not act on viruses. Protection: vaccination, interferon.
• Bacteriophage cycles: lytic (the cell dies) and lysogenic (the DNA is incorporated into the bacterial genome).
• Viruses have no metabolism. Outside a living cell they can be preserved in crystalline form.
• Phages are used in medicine to treat and detect bacterial infections.