Thursday, 15 March 2012

Viral Reproduction and Genetics

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Viruses that attack bacteria are known as bacteriophages or phages. They consist of DNA surrounded by a protein coat. A long thread of DNA of about 150 genes is packed into a head from which a short tail projects.

The phage attaches itself to the bacterial surface by its tail. The DNA thread is injected into the bacterium where it proceeds to replicate freely. New virus heads and tails are manufactured and then assembled within the cell under the influence of viral DNA. After some time the bacterium may burst open releasing the viruses, which then repeat the process in other bacterium.

In 15, the American scientists Alfred Hershey and Martha Chase demonstrated the only the viral DNA is responsible for directing operations in the bacterial cell. They used two samples of bacteriophage. In one they labelled the viral DNA with radioactive P. In the other they labelled the protein of the head and tail with 5S. DNA contains phosphorus and not sulphur, whereas protein contains sulphur and not phosphorus. They then took cultures of E. coli, and in one they added phages that had their DNA labelled, and in the other they added phages that had their proteins labelled. They left them with enough time to infect the bacteria. The bacteria were then agitated in a blender to separate them from any viral coats attached to them. The viral coats were then isolated from the bacteria through centrifugation; the bacteria were thrown down to the bottom of the tube. They found that in the P experiment the infected bacteria were highly radioactive due to the high levels of P. In the 5S experiment most of the labelled sulphur and radioactivity were restricted to the protein coat and there was little present in the infected bacteria.

This shows that on infection only the viral DNA enters the bacterium. The viral DNA takes over the entire metabolic machinery of the bacterium, suppressing its normal metabolism, and causing it to produce new protein coats and nucleic acid threads identical to those of the invading phage.

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However, new research shows that in some cases the whole virus enters the cell, not just the DNA.

Viruses which actively attack and reproduce in cells are described as virulent. Some viruses may infect a cell and then remain in a dormant state for a long period. Instead of replicating, the viral DNA inserts itself into the host’s DNA, replicating only when the host’s DNA replicates. The virus in this temperate state is known as a provirus. Eventually it may lose its benign state and give rise to new viruses which will burst out of the cell.

Cancers can be caused by viruses in two ways. Firstly, some of the viral genes may make proteins which affect the regulation of the host’s genes causing them to produce more of a certain type of mRNA. This may result in proteins being produced that cause the cell to grow and divide more. Secondly, some viruses carry genes that are not required for the manufacture of their own proteins. Instead these genes cause the host cell to become cancerous. They do this by producing proteins which resemble those produced by the host to increase the arte of cell growth and division. These genes are known as oncogenes.

Many viruses contain RNA instead of DNA as their genetic material. In some, the RNA functions as mRNA once it gets into the host cell, directing the production of viral protein. Other RNA viruses, known as retroviruses, possess an enzyme called reverse transcriptase, which transcribes their RNA into DNA. The new viral DNA enters the host’s DNA as a provirus remaining in a temperate state for months or years.

Human Immunodeficiency Virus (HIV) which can cause AIDS is an example of a retrovirus. They do not conform to the central dogma of molecular genetics, instead they have their own.

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