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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.
According to the passage, which of the following best describes a function performed by oligosaccharides?
选项 A、Regulating the daily functioning of a plant’s cells.
B、Interacting with one another to produce different chemicals.
C、Releasing specific chemical messengers from a plant’s cell walls.
D、Producing the hormones that cause plant cells to differentiate to perform different functions.
E、Influencing the development of a plant’ s cells by controlling the expression of the cells’ genes.
答案E
解析 哪一个指出低聚糖的一个功能?本题和19题重复，见上文解释。E．正确。影响植物细胞的发育，通过控制细胞基因的表达。A、B原文未提。C．低聚糖本身就是本选项所说的从细胞壁释放的“chemical messengers”，怎么可能再释放它。D．造荷尔蒙。无。
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