Two-stranded RNA have a function, too
dna and rna are the two so-called 'molecules of life'. dna is the famous double helix and the rna a single strand. It is known that the dna carries with it, in the form of a code, the information for making proteins. For a protein to be made, the information in any one of the two dna strands is first converted to a single strand of rna and only then is it used to synthesise a protein.
In humans, because dna is passed on from parents to children, the code that it contains is also transmitted. But, among pathogens, though a small number of viruses have rna , they do not have dna as their genetic material. Thus, rna can form double-stranded structures but only rarely and, when it does happen, the structures degrade rapidly. In short, for all practical purposes, dna is a double-stranded molecule and rna is a single-stranded one.
Curiously, it turns out that when double-stranded rna (ds rna ) is artificially-generated in a cell, it has fairly strong effects. It can interfere with the expression of the information encoded within it. How this happens, and whether interference of gene expression caused by ds rna has any role to play in the normal life of a cell or organism, remains unknown. H. Tabara and colleagues at the University of Massachusetts, usa offer an insight to this problem ( Cell , Vol 99).
The group worked with the nematode worm Caenorhabditis elegans . They engineered ds rna within the worm and carried on breeding the "sick" animals until they obtained lines that consisted of individuals that were resistant to the interference caused by the ds rna . These resistant worms appeared to be entirely normal in respect to growth and development even though they still contained the ds rna within their cells, but they had other problems.
The controls that they normally exercised over the normally resident and benign parasitic dna sequences (also known as transposable elements) that they carried appeared to have been loosened. As a result, the transposable elements started moving about from place to place within the dna of the worm, obviously not a good thing for the worm. Based on this finding, the Tabara group came to the conclusion that ds rna may have a function after all -- it may be to prevent transposable elements from running amok.
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