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The basic workings of DNA and RNA are no mystery. It's now well known that DNA consists of four nucleotide “bases” (A， T， C and G)， whose linear sequences (AATAGGCTCC……) encode hereditary information. Genes——discrete segments of long DNA molecules——transcribe their sequences onto single-strand messenger RNA molecules， which then serve as templates for proteins. In short， DNA makes messenger RNA， and messenger RNA makes proteins. The production of a particular protein is the goal of each gene. This 50-year-old insight is the bedrock of modern biology， but science has not fully solved a related mystery. If every cell in an organism contains the same full complement of genes， why are the cells themselves so varied? How do different genes get turned on (“expressed”) or off (“silenced”) in just the right combinations to produce heart cells， bone cells and brain cells?

That's where microRNA enters the picture. In the early 1990s， researchers studying a species of worm discovered genes for a very short and very unusual piece of RNA. Instead of synthesizing proteins， this tiny RNA molecule latched onto messenger RNAs (chart)， causing their destruction. Without messenger RNA， no protein was produced. In effect， the gene for that protein had been silenced. The discovery was initially dismissed as an oddity in a worm， but scientists have since found genes for hundreds of microRNAs in various plants and animals——200 in humans alone. Many of these genes have survived in identical forms in different species， indicating that they are essential to life. What， exactly， is their role? We now suspect that by silencing particular genes at just the right times——a process called RNA interference——they push genetically identical cells down different paths of development， enabling some to digest food while others perceive light.

RNA interference gives researchers a new tool for understanding how living things grow——how a plant assumes a particular shape， for example， or how a baby's hand forms during gestation. Moreover， because microRNAs are so small and simple in structure， they can be manufactured for use as research tools. If scientists suspect that a particular gene is responsible for a disease， they can design microRNA to silence the gene in affected laboratory animals. If the disease is prevented or cured， the gene becomes a target for treatment.

RNA interference has yet to generate new medicines， but if the technique fulfills its promise， it could help us treat everything from viral infections to cancer. MicroRNAs could be used to seal off human cells from disease-causing viruses， or to disable viruses that gain entry. In a recent test-tube study， researchers showed that RNA interference could make cells impermeable to HIV. Early studies suggest that microRNAs can also boost the production of stem cells in culture. By blocking production of growth-promoting proteins， microRNAs may even help contain cancer cells.

It is one thing to manipulate cells in a test tube， quite another to treat people. Getting microRNAs safely into the right cells in the body will be complicated. No one has yet attempted a human experiment. Even so， a field that was just a curiosity in 1993 is now poised to change the world——all because we invested in basic research. The scientists who discovered microRNAs were not trying to prevent AIDS， grow stem cells or treat cancer. They just wanted to figure out how something happened in a worm. As Buckminster Fuller observed， “Development is programmable; discovery is not.”

注(1)：本文选自Newsweek; 12/8/2003， p96-96， 1p， 1 diagram， 1c;

注(2)：本文习题命题模仿对象2004年真题Text 3;

1. The expression “silenced”(Line 5， Paragraph 2) most probably means _________.

[A]ruined

[B]destroyed

[C]killed

[D]stopped