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."

　　1.The expression “silenced”(Line 5, Paragraph 2) most probably means _________. 　　
　　[A]ruined
　　[B]destroyed
　　[C]killed
　　[D]stopped

　　2.How does the author feel about the study of MicroRNA
　　[A]Optimistic.
　　[B]Confused.
　　[C]Frightening.
　　[D]Shocking.

　　3.When mentioning “It is one thing to manipulate cells in a test tube, quite another to treat people.”(Line 1, Paragraph 5), the author implies that __________.
　　[A]the test of cells and the treatment of people are unrelated to each other
　　[B]the treatment of people is more complex than the cells in the laboratory
　　[C]more emphasis should be laid on the human tests
　　[D]human treatment may not be successful

　　4.What does Buckminster Fuller mean by “Development is programmable; discovery is not.”(Last Line, Last Paragraph)
　　[A]Discovery is always made beyond one’s expectation.
　　[B]Development is easier than discovery.
　　[C]Development lacks curiosity while discovery does not.
　　[D]Development is less important than discovery.

　　5.Which of the following is not true about RNA interference
　　[A]It prevents disease-causing viruses from entering human body.
　　[B]It improves the production of stem cells.
　　[C]It can cure of all kinds of human diseases.
　　[D]It silences certain protein to prevent tumor.

　　答案：DABAC

　　篇章剖析

　　本文采用提出问题——分析问题的模式，指出小分子核糖核酸的发现、作用机理及其应用。第一段指出脱氧核糖核酸、信使核糖核酸、蛋白质之间的关系；第二段指出小分子核糖核酸的发现及其作用机理；第三段和第四段指出核糖核酸干扰的具体作用及其应用；第五段指出这一发现的偶然性。

　　词汇注释

　　unravel[Qn5rAv(E)l]v.分散并澄清（神秘或让人迷惑的事物）的构成元素；解开

　　come to light v.暴露;被发现,众所周知

　　nucleotide[`njU:klIEtaId, -tId]n.核苷

　　encode[In5kEJd]vt.把(电文.情报等)译成电码(或密码)；编码

　　hereditary[hI5redItErI; (@) -terI]adj.世袭的, 遗传的

　　synthesize[5sInWIsaIz]v.综合, 合成

　　gestation[dVe5steIF(E)n]n.怀孕, 酝酿, 妊娠

　　seal off v.把...封锁起来

　　HIV abbr. 人体免疫缺损病毒，艾滋病病毒

　　impermeable[Im5p\:mIEb(E)l]adj.不能渗透的, 不渗透性的

　　manipulate[mE5nIpjJleIt]vt.(熟练地)操作,巧妙地处理

　　be poised to 准备好的

　　难句突破

　　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.

　　主体句式：We suspect that …

　　结构分析：本句是一个宾语从句。介词by做伴随状语，其中“a process called RNA interference”是“silencing particular genes at just the right times”的同位语；现在分词“enabling”做伴随状语，其中连词“while”表示对比。

　　句子译文：我们现在猜测它们在某一适当的时候通过压制一些特殊的基因——这是一个被称为核糖核酸干扰的过程——推动基因相同的细胞发展成不同的性状，从而使一些细胞能够消化食物，另一些细胞能够感知光线。

　　题目分析

　　1.答案为D，属猜词题。从文章第一段“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”我们可看出“silence”的意思是“turn off”。句子“Instead of synthesizing proteins, this tiny