【2017年整理】GMAT阅读解析 by 申友Alice.docVIP

每周一篇 By 申友Alice老师 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? // (提出现象一组基因中产生的细胞形成不同的根茎叶结构。) 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.（专有名词第一次出现需引起重视，生词可用首字母代入法表示。） // (解释现象，只有一部分基因通过五大荷尔蒙和OL物质被表达出来。) // Unlike（出现比较词，注意对比的双方）the oligosaccharins, the five well-known plant hormones are pleiotropic rather than specific ; that is,（解释说明，可以略读） each has more than one effect on the growth and development of plants. // (五大荷尔蒙与OL物质对比，具有多效性、不专一性。) The five have so many simultaneous effects that they are not very useful（负评价词） in artificially controlling the growth of crops. Auxin, for instance,（举例可以略读，注意看前一句总结句。） 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.（五大荷尔蒙因为具有多效性所以在控制植物生长方面不是很有作用。）// The pleiotropy of the five well-studied plant hormones is somewhat analogous to that of certain hormones in animals. For example, （举例略读）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. // (植物中五大荷尔蒙的多效性和动物相似。) 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 fo