四六级

　　阅读作为英语六级考试中的重点拿分项，考前一定要多加练习，保持题感才行。下面是小编为大家准备的英语六级阅读冲刺仿真模拟训练题与答案解析，供广大同学参考、练习！

　　If you go down to the woods today, you may meethigh-tech trees genetically modified to speed theirgrowthor improve the quality of their wood. Genetically-engineered food crops have becomeincreasingly common, albeit controversial. overthe past ten years. But genetic engineering of treeshas lagged behind.

　　Part of the reason is technical. Understanding. and then altering, the genes of a big pine treeare more complex than creating a better tomato. While tomatoes sprout happily, and rapidly, in the laboratory, growing a whole tree from a single, genetically altered cell in a test tube isa tricky process that takes years, not months. Moreover. little is known about tree genes. Some trees, such as pine trees. have a lot of DNA-roughly ten times as much as human. And, whereas the Human Genome Project is more than half-way throughits task of isolating andsequencing the estimated 100,00 genes in human cells. similar efforts to analyzetree genesare still just saplings (幼苗).

　　Given the large number of tree genes and the little that is known about them, tree engineersare starting with a search for genetic "markers". The first step is to isolate DNA from treeswith desirable propertiessuch as insect resistance. The next step is to find stretches of DNAthat show the presence of a particular gene. Then, when you mate two trees with differentdesirable properties, it is simple to check which offspring contain them all by looking for thegenetic markers. Henry Amerson, at North Carolina State University, is using genetic markersto breed fungal resistance into southern pines. Billions of these are grown across America forpulp and paper, and outbreaks of disease are expensive. But not all individual trees aresusceptible. Dr. Amerson’s group has found markers that distinguish fungus-resistantstock from disease-prone trees.Using traditional breeding techniques, they are introducingthe resistance genes into pines on test sites in America.

　　Using generic markers speeds up old-fashioned breeding methods becauseyou no longer haveto wait for the tree to grow up to see if it has the desiredtraits. But it is more a sophisticatedform of selective breeding. Now. however.interest in genetic tinkering (基因修补) is alsogaining ground. To this end, Dr.Amerson and his colleagues are taking part in the Pine GeneDiscovery Project. an initiative to identify and sequence the 50,000-odd genes in the pinetree's genome. Knowing which gene does what should make it easier to know what to alter.

　　1. Compared with genetic engineering of foodcrops, genetic engineering oftrees____________________.

　　A) began much later

　　B) has developed more slowly

　　C) is less useful

　　D) was less controversial

　　2. What does the author think about the genetic engineering of pine trees?

　　A) Time-consuming.

　　B) Worthwhile.

　　C) Significant.

　　D) Technically impossible.

　　3. What can we learn about the research on tree genes?

　　A) The research methods are the same as the analysis of human genes.

　　B) The findings are expected to be as fruitful as the analysis of human genes.

　　C) It will take as much time and effort as the analyst, of human genes.

　　D) The research has been mainly concentrated on the genes of young trees.

　　4. It is discovered by Henry Amerson’s team that_______________.

　　A) southern pines cannot resist fungus

　　B) all southern pines are not susceptible

　　C) the genetic marker in southern pines was the easiest to identify

　　D) fungus-resistant genes came originally from outside the U.S.A.

　　5. What is the primary objective of carrying out the Pine Gene Discovery Project?

　　A) To speed up old-Fashioned breeding methods.

　　B) To identify all the genes in the pine tree's genome.

　　C) To find out what desired traits the pine trees have.

　　D) To make it easier to know which gene needs altering.

　　参考答案：

　　1.[B]本文并没有提到树木基因改良技术从什么时候开始，所以有可能树木的基因改良技术和别的基因改良技术在开始的时间上相差不大，但在所取得的成果方面却有很大的差别，因此，本题关键在于理解首段末句中的lagbehind指的是程度上的落后，而非时间上的落后，故B正确。

　　2.[B]第2段第3句中的that takes years表明研究树木的基因改造技术将花费很长时间，因此选项A为本题答案。原文没有就选项B和C两方面做出讨论，因此不能推断出这两个选项;虽然第2段首句提到technical一词，但文章表明树木基因改造技术是可行的。只是会花费较多的时问，因此选项D也不正确。

　　3.[A]第2段末句中的similar cfforts指的就是task of isolating and sequencing，即分析基因的常用方法。该句表明人类基因和树种基因的分析方法相同，但进展有差别，因此选项A为正确的推断。

　　4.[B]选项B是一个半否定的句式，因此本题关键在于理解选项B的Al1...are not...结构在意义上等同于第3段倒数第3句的not all... are...。第3段倒数第2句中的fungus-resistant stock指的是南方松树中能抗真菌的树种，并非别的与南方松树毫不相关的树种，因此A的说法不正确。

　　5.[B]末段倒数第2句中的an initiative...是the Pine Gene Discovery Project的同位语，表明开展该计划的目的，因此选项B为本题答案。其他选项都是在完成该计划后能够达成的工作，它们都要以松树基因的排列组合为基础，因此，这些选项都不是primary objective，而是在实现了primary objective之后继续进行的工作。

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　　另外，2017年12月四六级真题答案与解析专题已上线，考后小编会为大家及时上传答案与解析，届时欢迎大家前来围观~

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