Virus And Bacteriophage

Viruses are unlike any other form of life. They are sub-microscopic infectious organisms and vary in size from 10 to 300 nanometers .Viruses are on a mysterious border between living and non-living. They are too small to be seen under the ordinary light microscope, they are revealed only under the electron microscope, which show them to be packets of genetic material –DNA or the closely related RNA surrounded by a protective coating of proteins.

Viruses are the most familiar to us as the cause of human, animal and plant diseases. Smallpox, rabies and polio are ancient examples.Viruses such as the human immunodeficiency virus (HIV), the cause of AIDS, seen to have arisen in more recent times.

Viruses often gain entry to cells by latching onto proteins on the cell surface – proteins are normally used by the cells for quite different purposes. The AIDS virus, for example attacks only a particular set of white blood cells, T-lymphocytes. These carry a special protein on their surface to which the virus sticks. Once attached to its “receptor” protein, the virus is then taken into the cell.

Upon entering a cell, a virus discards the proteins and replicate itself by copying and translating its own genetic material, then begins copying and translating the virus nucleic acid instead. The genes encoded in viral nucleic acid direct the manufacture of more virus coating protein and the nucleic acid itself is also copied may times. So the cell is tricked into making new virus particles that are eventually shed from the cell, sometimes destroying the cell in the process. Most viruses have relatively few genes, all of which are directed to replicating the virus.

Example: The human adenovirus is of the type responsible for colds and sore throats and many other viruses exist like rhino virus, retro virus, influenza virus etc.

Bacteriophages: are viruses that infect bacteria. They achieve this by injecting DNA through the cell wall.

Bacteriophage lamda, which infects E.coli bacteria in the human intestine, has a simple structure. The DNA is stored in a polyhedral protein head attached to a hollow tail with a single tail fibre. The phage attaches itself to a bacterium at the tail and injects its DNA. This can result in an infection that causes the cells to lyse (break down) releasing replica phages. During such a lytic infection, the phage DNA remains separate from the bacterial DNA and manipulates the cell’s enzymes to synthesize the proteins that form the components of new phage.

The phage DNA replicates and as a result large numbers of phage get assembled. In the process, the bacterial DNA is used and by the time the cell lyses may be completely destroyed. In some cases, the injection of the phage DNA results in a lysogenic infection. When the phage DNA becomes part of the bacterial chromosome, cell division then produces numerous replicas of the phage DNA. During the course of lysogenic growth, damage to the cell may result in a lytic infection by causing the phage DNA to be ejected from the bacterial chromosome.