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  3. Ever since Gregor Mendel’ s famous experiments with hybrid sweet peas, it has been known that there must be unitary elements wit

Ever since Gregor Mendel’ s famous experiments with hybrid sweet peas, it has been known that there must be unitary elements wit

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问题    Ever since Gregor Mendel’ s famous experiments with hybrid sweet peas, it has been known that there must be unitary elements within the cells which exert control over inherited characteristics, and for a long time there was considerable speculation about what these were. These elements came to be known as genes, and although they were long treated as hypothetical constructs, a great deal of knowledge about them slowly accumulated. It came to be known, for example, that each gene had to be passed along virtually unchanged from generation to generation; that there must be many thousands of these particles in every human cell, distributed unevenly among the twenty-three pairs of chromosomes; that each gene must occupy a very definite place (locus) on its chromosome; and that each pair of homologous chromosomes had to contain homologous assortments of genes, arranged with few exceptions in precisely the same order on each member of the chromosome pail’ s. A wonderfully complex and fruitful system thus emerged about an aspect of the world which no one has ever directly observed. Let us now briefly turn to some of the newly acquired insights which have greatly expanded the already impressive theory of genetics.
   Genes are, of course, too small to be seen even by the most powerful electron microscopes, but recent research by geneticists, microbiologists, and biochemists has rapidly advanced our information about their constitution and action. The chemical substance of which the genes and thus the chromosomes are made, is now known to be deoxyribonucleic acid (DNA), a giant molecule containing a double-spiral strand of material which embodies the genetic code. The chromosomes consist of long strands of DNA, which, although it is capable of transmitting vastly complex "code messages", is comprised of combinations of only four primary chemical subunits, or "code letters". This great insight into the structure and functioning of genetic material, which was first proposed by James D. Watson and Francis H. C. Crick in 1953, involves a new description of what genes are like. A gene is simply a specific portion of the double-spiral strand of DNA which consists of a particular combination of the code letters that spell out a particular code word.
   Various combinations of the four code letters, forming different code words, provide the biochemical information used in the construction of the different proteins in the cell. Many of these proteins act as enzymes. The enzymes, as has been pointed out above, are the biological catalysts which direct all the chemical or metabolic reactions that are going on continuously in all cells. These metabolic functions are, of course, the basis of all the physical growth and development of any living organism.
   The code is embodied in the DNA of the chromosomes and genes, but exactly how does this code deter mine the production of proteins. Obviously, the code must be transmitted to the sites at which the actual work of protein synthesis is carried out. The material which accomplishes this task is ribonucleic acid (RNA, a substance very similar to DNA and complementary to it. From the code site on the linear DNA molecule, which is the gene), RNA, the messenger, carries the code to the cellular particles out into the cytoplasm of the cell, where proteins are manufactured. This messenger RNA provides the pattern, and another type of RNA, transfer RNA, collects from within the cytoplasm the raw materials, the amino acids, from which the proteins are made. With the pattern and the materials, the proteins are formed, one step at a time. These proteins act as enzymes or biological catalysis. They exist in all living organisms and control their growth and function through the control of the chemical transformations involved in metabolism. A very large number of enzymes are present in any living creature, and the absence or malformation of any enzyme can destroy the normal sequence of metabolism of a given biochemical substance.
   We can thus see that genetic activity takes the form of biochemical regulation, the genes determining the formation of enzymes. In this sense, all genetic disorders are primarily metabolic defects (Garrod, 1908). A defective or changed gene will in turn produce a change in the protein with which it is associated. The only result of such a change may be a slight alteration in the function of the protein, and there may thus be little or no observable effect. If the change or defect takes place within the code message for an essential element of the protein, however, the enzyme activity of this protein may be rendered completely inactive. If this happens, the result can be grave trouble: perhaps death, serious disease, or severe mental retardation due to poisoning of the central nervous system by a metabolite that is toxic to this system. The error in enzyme synthesis may begin to be important, so that the structure of the central nervous system is faulty almost from the beginning of embryonic life, or it may become important much later in the life cycle.
   It is quite likely that, in the foreseeable future, many essential biochemical processes will be understood in terms of the precise genetic codes responsible for them. All of the amino acids have already yielded to such analysis; their codes have been identified. With understanding may come control and prevention, such as may be possible by administration of the lacking enzymes, dietary control of substances which the individual is unable to metabolize, or transplantation of normal tissue to the diseased individual to correct the metabolic error.  
From generation to generation genes ______ .
见文章第一段第三句前半部分。
选项 A、change location in the chromosome
B、change order but not position in the chromosome
C、are constantly changing their chemical makeup
D、remain virtually unchanged
答案
解析 D
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