单选题
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.
单选题
Genes determine through complex biochemical processes the structure and
rate in which ______ .
A. proteins are formed
B. chromosomes are paired
C. enzymes are produced
D. cells multiply
【正确答案】
B
【答案解析】[解析] 见文章第一段中间。
单选题
The genetic material in the nucleus of the cell is called the ______ .
A. cytoplasm
B. chromosome
C. chromatin
D. gene
【正确答案】
D
【答案解析】[解析] 见文章第一段开头。
单选题
In man, the body cells contain how many pairs of chromosomes?
A. Twenty-two.
B. Twenty-three.
C. Twenty-six.
D. Forty-six.
【正确答案】
B
【答案解析】[解析] 见文章第一段第三句后半部分。
单选题
From generation to generation genes ______ .
A. change location in the chromosome
B. change order but not position in the chromosome