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  3. All of the cells in a particular plant start out with the same complement of genes. How then can these cells differentiate and f

All of the cells in a particular plant start out with the same complement of genes. How then can these cells differentiate and f

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问题     All of 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 sys- tern of chemical messengers that in plants include hormones and other regulatory molecules. Five major hormones have been identified: auxin, abscisic acid, cytokinin, ethylene, and gibberel- lin. Studies of plants have now identified a new class of regulatory molecules called oligosaccharide.
    Unlike the oligosaccharide, 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 has 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 animal. 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 thy- roid 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. Oligosaccharides are fragments of the cell wall released by enzymes: different enzymes release different oligosaccharides. There are indications that pleiotropic plant hormones may ac- tually 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 oligosaccharide?
选项 A、To stimulate a particular plant cell to become part of a plant’ s root system.
B、To stimulate the walls of a particular cell to produce other oligosaccharides.
C、To activate enzymes that release specific chemical messengers from plant cell walls.
D、To duplicate the gene complement in a particular plant cell.
E、To produce multiple effects on a particular subsystem of plant cells.
答案A
解析 以下哪一种是低聚糖的功能?低聚糖在文中是化学传送物质的一种(L15—17)，而这些传送物质的一个作用就是促使特定细胞中一小部分基因被表达，从而解决L3—5提出的疑问。∴A正确，刺激特定的细胞使其成为植物根系的一部分。正是L3—5设问的内容。B．“produce other oligosaccharides”文中未提。C．“activate enzymes” 的是多向性荷尔蒙(如five well—known hormones)，见L53—56。D．“duplicate”文中无。E．“multiple effects on a particular subsystem”文中无。
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