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 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.”的意思是“这种小核糖核酸分子没有参与合成蛋白质，而是附着在信使核糖核酸上（如图），来消灭它们。没有信使核糖核酸，也就产生不了蛋白质。实际上，产生这种蛋白质的基因被压制了。”，从这句话我们可进一步了解“turn off”到底是什么含义。

　　2.答案为A，属情感态度题。虽然“It is one thing to manipulate cells in a test tube, quite another to treat people.”，但是从句子“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.”和“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.”我们可看出作者对小核糖核酸分子前景是非常看好的。

　　3.答案为B，属推理判断题。文章第四段阐述了小核糖核酸分子的应用前景是非常好的，但那都是在实验室里做出来的，还没有具体应用到人体上。应用到人体上又是另一回事，要复杂的多了。

　　4.答案为A，属推理判断题。文章最后一段谈及了小核糖核酸分子是意外发现的，是“无心插柳”的意外收获和发现。“Development is programmable; discovery is not.”是对这一事实的一个很好的概括。

　　5.答案为C，属事实细节题。原文对应信息是：“No one has yet attempted a human experiment.” 。

　　参考译文

　　脱氧核糖核酸（DNA）和核糖核酸（RNA）的基本运行方式并不是不解之迷。现在大家都知道脱氧核糖核酸是由碱基(A, T, C and G)构成的。碱基通过线形排列(AATAGGCTCC...)对遗传信息进行编码。基因——长长的脱氧核糖核酸分子中的不连续的片段——把它们的序列转录成单线信使核糖核酸分子，核糖核酸又成为蛋白质的模板。简言之，脱氧核糖核酸产生了信使核糖核酸，信使核糖核酸又产生了蛋白质。产生一种独特的蛋白质是每一个基因的目标。这种认识已长达50年，它是现代生物学的基础。但是科学并没有完全解决一个相关的问题。如果生物体内的每一个细胞都含有同等满额的基因的话，那么为什么细胞之间千差万别呢？不同的基因是如何以适当的组合方式来开始（表达）或停止（沉默）自己的工作，从而制造心脏、骨骼以及脑部细胞呢？

　　于是小分子核糖核酸（microRNA）进入了我们的视野。在20世纪90年代初，研究人员在研究一种虫子的基因时发现了一种很短却又与众不同的核糖核酸。这种小核糖核酸分子没有参与合成蛋白质，而是附着在信使核糖核酸上（如图），来消灭它们。没有信使核糖核酸，也就产生不了蛋白质。实际上，产生这种蛋白质的基因被压制了。这一发现刚开始时被认为是虫子体内的奇怪物质而未被考虑，但后来科学家们在各种动植物身上发现了成百上千个小分子核糖核酸基因——单在人身上就发现了200个。有很多这样的基因以相同的形态存活在不同的物种体内，这表明它们对生命来说是必不可少的。那么它们究竟起什么作用呢？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。

　　核糖核酸的干扰作用为研究人员搞清生命生长提供了一个新工具——比如说，植物是如何展现其独特外形的，婴儿的小手在母体里是如何形成的，等等。此外，因为小分子核糖核酸的构造既小又简单，所以可以制造核糖核酸，把它当作研究工具来使用。如果科学家们怀疑某种疾病是由于某一种基因出了问题，那么他们可以设计小分子核糖核酸，用以压制实验室里受感染的动物体内的基因。如果这种疾病得以预防或治愈，那么这种基因便成为了治疗的对象。

　　核糖核酸干扰可以用来生产新药。如果科技确实能起作用，它就可以帮助我们治疗从病毒感染到癌症等各种疾病。小分子核糖核酸可以用来封闭人体细胞，使之免受致病病毒的侵扰，或者使病毒丧失侵入细胞的能力。在最近进行的试管实验研究中，研究人员发现核糖核酸干扰能使细胞抵制爱滋病病毒。早期研究发现小分子核糖核酸还可以促进干细胞的产生。通过阻止那些能促进生长发育的蛋白质的产生，小分子核糖核酸甚至还能有助于抑制癌细胞。

　　用试管做细胞实验是一回事，用于人身上又是另一回事了。使小分子核糖核酸安全进入适合的细胞体内是一项复杂的过程。至今还无人尝试人体实验。尽管这样，这个在1993年还是件新奇事物的领域现在将要改变世界——这都是因为我们花精力进行了基础研究。那些发现小分子核糖核酸的科学家们并没想要去防治爱滋病、种植干细胞或者治疗癌症。他们只想搞明白虫子体内究竟发生了什么。正如匹克米斯特·福勒评述的：“开发项目有程序可循，而发现却非如此了。”