A cell is a reservoir of proteins and performs the role of manufacture and supply of proteins. The ‘director’ behind the scene is none other than the cell nucleus//, which houses the DNA, and enables the plan of what needs to be built. If a particular protein is to be manufactured, the command is reproduced verbatim in the nucleus. The nucleus consists of the RNA, a DNA like substance, and is carried via pores in the cell nucleus to reach the cell plasma. The distinct components of the protein are assembled akin to the copy stored in the respective RNA.
This method has an Achilles heel: 'enemies' can misuse the assembly line to produce their own proteins by faking the commands. Viruses, for example, basically consist of a small protein capsule which surrounds its genetic make-up – usually an RNA molecule. This RNA mainly contains the blueprint for new capsule proteins. By injecting its RNA into the cell, the virus re-programmes it: the cell production line then produces large numbers of new virus capsules. These are filled with virus RNA and attack more cells.
'However, the cells are not completely at the mercy of a virus attack,' Professor Gunther Hartmann, head of the Bonn University Clinic's Department of Clinical Pharmacology. 'They often recognize the alien RNA and set off the alarm: for example, they then produce what is known as the beta interferon, thereby activating specific killer cells. They also initiate the cell's suicide programme – apoptosis. The viruses cannot then continue to breed.'
Up to now it was not known how cells distinguish their own RNA from that of the 'enemy'. The latest findings, which a Japanese research team was also involved in, now shed light on the matter: they show that the instructions from the cell's nucleus carry a kind of 'signature', which is missing in the virus commands. RNA is like a long string. In viruses there is a specific chemical signal, known as a triphosphate, located at one end of
this string. The RNA in the cell's nucleus basically also contains this triphosphate end. However, on top of it there is an additional short molecule, a molecular cap. 'In all animals and plants the RNA which encodes proteins has this kind of molecular signature,' Dr. Veit Hornung and Professor Stefan Endres of the Munich University Clinic's Department of Clinical Pharmacology emphasise. 'Apart from other functions it is also the signal that shows that the cell's own RNA is involved.'
However, in all cells there are also RNAs which do not have a molecular cap. 'Despite this they do not result in an immune reaction,' Dr. Hornung says. 'They sign their commands in a different way, viz. by means of a complex biochemical process which takes place in a special sub-structure of the cell's nucleus, known as the nucleolus.' These RNAs do not store information, being responsible for important tasks in 'assembling' the proteins.
Importance for therapy
The fact that RNAs without a 'signature' stimulate an immune reaction and initiate cell suicide opens up completely new perspectives for the therapy of virus infections and cancer cases: for example, RNA chains with a triphosphate end could be produced and fed into cancer cells. This could also trigger an anti-viral immune response. 'Our findings are also important for gene therapy,' Professor Hartmann stresses. 'Before we attempt to cure diseases by introducing genetic material, we ought to understand precisely how the cells react to this genetic material.'
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