http://www.nytimes.com/2006/10/03/science/03nobel.html?_r=1&oref=sloginhttp://www.nytimes.com/2005/06/21/science/21rna.html?ex=1160020800&en=6d52c61c0be516c7&ei=5070This stuff is pretty cool!
DNA usually grabs the headlines for its starring role as the archive of genetic information.
But many of the really difficult operations that a cell performs are carried out by RNA, DNA's close chemical cousin.
Keeping Genes in Line So deeply has RNA been overshadowed that two of its major roles in the cell have come to light only in the last few years. One, a way of fine-tuning the activity of genes, has been the subject of a flurry of recent reports documenting RNA's part in central operations like stem cells, cell differentiation, insulin production and cancer.
In its fine-tuning role, RNA is known as micro-RNA. It ratchets down the production of certain proteins in cells by binding to target sites on messenger RNA, the envoy from the genes that directs protein production. The precise dose of a protein can be very important: Down syndrome is caused by an extra chromosome and the resulting excess production of proteins whose genes exist in three copies instead of the usual two.
Though it may be inefficient for a cell to generate messenger RNA from a gene and then hobble production of the protein it specifies, micro-RNA seems to provide a sophisticated way of adjusting production levels, particularly of proteins that are needed at one stage of life but must be absent in others.
A particularly important role for micro-RNA may be choking off the production of proteins that maintain cells in an undifferentiated stemlike state, thus forcing the cells to mature into their destined adult forms. In an article in Nature this month, Todd Golub of the Broad Institute in Cambridge, Mass., reports that many kinds of cancer tissue have less micro-RNA than normal cells, suggesting that the tumor cells had backtracked on the normal maturation process and regained the growth potential of stem cells. Though it is unclear how the normal production of micro-RNA's is suppressed in cancer, Dr. Golub and colleagues wrote, too little micro-RNA in a cell could cause tumors or contribute to maintaining them.
So far, 222 human micro-RNA's have been identified, and in the current issue of Nature Genetics an Israeli biotech company, Rosetta Genomics, reports that it has detected a further 89.
Many more probably remain to be found. David Bartel, an RNA expert at the Whitehead Institute in Cambridge, estimates there are 400 to 800 human micro-RNA genes. But one micro-RNA can control several different kinds of messenger-RNA, bringing a much larger number of genes under the influence of the micro-RNA system. At least one-third of the genes in humans and other mammals are controlled by micro-RNA's, Dr. Bartel said.
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