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阅读RC-9661 (第9/12题) Time Cost00:00
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All the cells in a particular plant start out with the same complement of genes. How then can these cells differentiate and form structures as different as roots, stems, leaves, and fruits? The answer is that only a small subset of the genes in a particular kind of cell are expressed, or turned on, at a given time. This is accomplished by a complex system of chemical messengers that in plants include hormones and other regulatory molecules. Five major hormones have been identified: auxin, abscisic acid, cytokinin, ethylene, and gibberellin. Studies of plants have now identified a new class of regulatory molecules called oligosaccharins.

Unlike the oligosaccharins, the five well-known plant hormones are pleiotropic rather than specific ; that is, each has more than one effect on the growth and development of plants. The five have so many simultaneous effects that they are not very useful in artificially controlling the growth of crops. Auxin, for instance, stimulates the rate of cell elongation, causes shoots to grow up and roots to grow down, and inhibits the growth of lateral shoots. Auxin also causes the plant to develop a vascular system, to form lateral roots, and to produce ethylene.

The pleiotropy of the five well-studied plant hormones is somewhat analogous to that of certain hormones in animals. For example, hormones from the hypothalamus in the brain stimulate the anterior lobe of the pituitary gland to synthesize and release many different hormones, one of which stimulates the release of hormones from the adrenal cortex. These hormones have specific effects on target organs all over the body. One hormone stimulates the thyroid gland, for example, another the ovarian follicle cells, and so forth. In other words, there is a hierarchy of hormones.

Such a hierarchy may also exist in plants. Oligosaccharins are fragments of the cell wall released by enzymes: different enzymes release different oligosaccharins. There are indications that pleiotropic plant hormones may actually function by activating the enzymes that release these other, more specific chemical messengers from the cell wall.

The passage suggests that which of the following is a function likely to be performed by an oligosaccharin?

  • 分析A选项
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  • 分析B选项
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  • 分析C选项
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  • 分析D选项
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  • 分析E选项
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当前版本由 kisenana 更新于2015-11-05 16:13:36 感谢由 kisenana 对此题目的解答所做出的贡献。
问的是O的作用 第一段提到o可以控制基因表达,促进不同细胞长成一棵植物的不同部位。
选项A 正确 ,可以促进长成根
选项B O 产生其他O 文章未提及
选项C O activate enzymes 定位到最后一段 是H荷尔蒙→Enzyme→O;O不可以反过来产生E
选项D O复制基因组成 文章未提及
选项E produce multiple effects 错误定位到第二段,O是专一的

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    捉急0707

    问的是likely to,因此原文中直接出现的描述应该是错误的。从第二段第一句可以推测出,O的作用是specific的。A的描述的作用是最specific的。
    1 0 回复 2017-10-26 19:16:20
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