The chemical formula of deoxyribonucleic acid (DNA) is now well established. The molecule is a very long chain, the backbone of which consists of a regular alternation of sugar and phosphate groups. To each sugar is attached a nitrogenous base, which can be of four different types. Two of the possible bases—adenine and guanine — are purines, and the other two — thymine and cytosine — are pyrimidines. So are as is known, the sequence of bases along the chain is irregular. The monomer unit, consisting of phosphate, sugar and base, is known as a nucleotide.
The first feature of our structure which is of biological interest is that it consists not of one chain, but of two. These two chains are both coiled around a common fiber axis. It has often been assumed that since there was only one chain in the chemical formula there would only be one in the structural unit. However, the density, taken with the X-ray evidence, suggests very strongly that there are two.
The other biologically important feature is the manner in which the two chains are held together. This is done by hydrogen bonds between the bases. The bases are joined together in pairs, a single base from one chain being hydrogen-bonded to a single base from the other. The important point is that only certain pairs of bases will fit into the structure. One member of a pair must be a purine and the other a pyrimidine in order to bridge between the two chains. If a pair consisted of two purines, for example, there could not be room for it.
We believe that the bases will be present almost entirely in their most probable forms. If this is true, the conditions for forming hydrogen bonds are more restrictive, and the only pairs of bases possible are: adenine with thymine, and guanine with cytosine. Adenine, for example, can occur on either chain; but when it does, its partner on the other chain must always be thymine.
The phosphate-sugar backbone of our model is completely regular, but any sequence of the pairs of bases can fit into the structure. It follows that in a long molecule many different permutations are possible, and it therefore seems likely that the precise sequence of bases is the code which carries the genetical information. If the actual order of the bases on one of the pair of chains were given, one could write down the exact order of the bases on the other one, because of the specific pairing. Thus, one chain is, as if were, the complement of the other, and it is this feature which suggests how the deoxyribonucleic acid molecule might duplicate itself.
1.The authors use the word “backbone” (underlined in 1th and 5th paragraphs) to indicate that （ ）.
2.In the second paragraph, what do the authors claim to be a feature of biological interest?
3.Based on the passage, the authors’ statement “If a pair consisted of two purines, for example, there would not be room for it” (italicized) implies that pair （ ）.
4.The authors9 use of the words “exact,” “specific,” and “complement” blackened in the final paragraph functions mainly to（ ）.
问题1选项
A.only very long chains of DNA can be taken from an organism with a spinal column
B.the main structure of a chain in a DNA molecule is composed of repeating units
C.a chain in a DNA molecule consists entirely of phosphate groups or of sugars.
D.nitrogenous bases form the main structural unit of DNA
问题2选项
A.DNA consisting of two chains.
B.The common fiber axis.
C.The X-ray evidence.
D.The chemical formula of DNA
问题3选项
A.of purines would be larger than the space between a sugar and a phosphate group
B.of purines would be larger than a pair consisting of a purine and a pyrimidine
C.of pyrimidines would be larger than a pair of purines
D.consisting of a purine and a pyrimidine would be larger than a pair of pyrimidine
问题4选项
A.confirm that the nucleotide sequences are known for most molecules of DNA
B.counter the claim that the sequences of bases along a chain can occur in any order
C.support the claim that the phosphate-sugar backbone of the author’s model is completely regular
D.emphasize how one chain of DNA may serve as template to be copied during DNA replication
第1题:
【选项释义】
The authors use the word “backbone” (underlined in 1th and 5th paragraphs) to indicate that（ ）. 作者用第一段和第五段中的划线词“backbone”是为了指出（ ）。
A. only very long chains of DNA can be taken from an organism with a spinal column A. 只有很长的DNA链才能从有脊柱的生物体中提取出来
B. the main structure of a chain in a DNA molecule is composed of repeating units B. DNA分子链的主要结构是由重复单元组成的
C. a chain in a DNA molecule consists entirely of phosphate groups or of sugars C. DNA分子中的一条链完全由磷酸基团或糖组成
D. nitrogenous bases form the main structural unit of DNA D. 含氮碱基构成DNA的主要结构单元
【考查点】推理判断题。
【解题思路】根据题干关键词backbone定位文章第一段第二句The molecule is a very long chain, the backbone of which consists of a regular alternation of sugar and phosphate groups.（这种分子是一条很长的链，其主干由糖和磷酸基团的规律交替组成），以及文章第五段首句The phosphate-sugar backbone of our model is completely regular, but any sequence of the pairs of bases can fit into the structure.（我们人类的DNA分子模式的磷酸糖主干完全是有规律的，但任何碱基对序列都可以适应这种结构）可推断B选项“DNA分子链的主要结构是由重复单元组成的”正确。
【干扰项排除】
A、D选项均未在文中体现，属于无中生有；
C选项“DNA分子中的一条链完全由磷酸基团或糖组成”，文章中提到DNA的主干由糖和磷酸基团共同组成，不是二者中的其中一个，属于曲解原文。
第2题:
【选项释义】
In the second paragraph, what do the authors claim to be a feature of biological interest? 在第二段中，作者声称什么是令人感兴趣的生物学特征？
A. DNA consisting of two chains. A. 由两条链组成的DNA。
B. The common fiber axis. B. 公共纤维轴。
C. The X-ray evidence. C. X光证据。
D. The chemical formula of DNA. D. DNA的化学式。
【考查点】事实细节题。
【解题思路】根据题干关键词biological interest定位文章第二段首句The first feature of our structure which is of biological interest is that it consists not of one chain, but of two.（在生物学研究方面我们人类的DNA结构令人感兴趣的第一个特征是它由两个链条而不是一个链条组成），可知A选项“由两条链组成的DNA”正确。
【干扰项排除】
B、C、D选项均在第二段有所体现，但与题干无关，属于答非所问。
第3题:
【选项释义】
Based on the passage, the authors’ statement “If a pair consisted of two purines, for example, there would not be room for it”（italicized）implies that pair（ ）.
根据这篇文章，作者的陈述“例如，如果一对嘌呤由两个嘌呤组成，那么就没有空间容纳它”（斜体）意味着一对（ ）。
A. of purines would be larger than the space between a sugar and a phosphate group A. 嘌呤的间距要比糖和磷酸基团之间的间距大
B. of purines would be larger than a pair consisting of a purine and a pyrimidine B. 嘌呤比一个嘌呤加上一个嘧啶对更大
C. of pyrimidines would be larger than a pair of purines C. 嘧啶的原子比一对嘌呤大
D. consisting of a purine and a pyrimidine would be larger than a pair of pyrimidine D. 由一个嘌呤和一个嘧啶组成的会比一对嘧啶大
【考查点】事实细节题。
【解题思路】根据选项关键词a purine and a pyrimidine定位文章第三段倒数第二句. One member of a pair must be a purine and the other a pyrimidine in order to bridge between the two chains.（为了在两条链之间架起桥梁，一对中的一个必须是嘌呤，另一个必须是嘧啶），结合斜体部分（例如，如果一对嘌呤由两个嘌呤组成，那么就没有空间容纳它）可知因为嘌呤的体积比嘧啶大，所以才没有空间容纳嘧啶，因此B选项“嘌呤比一个嘌呤加上一个嘧啶对更大”正确。
【干扰项排除】
A、C、D选项均未在文中体现，属于无中生有。
第4题:
【选项释义】
The authors use of the words “exact” “specific” and “complement” blackened in the final paragraph functions mainly to（ ）. 作者在最后一段使用了“确切”“具体”和“补充”等词，主要是为了（ ）。
A. confirm that the nucleotide sequences are known for most molecules of DNA A. 证实大多数DNA分子的核苷酸序列是已知的
B. counter the claim that the sequences of bases along a chain can occur in any order B. 对一个链上含氮碱基能够以任何顺序排列的声称进行反驳
C. support the claim that the phosphate-sugar backbone of the author’s model is completely regular C. 支持作者所谓的DNA模型的磷酸糖支柱完全是有规律的声称
D. emphasize how one chain of DNA may serve as template to be copied during DNA replication D. 强调一条DNA链如何在自我复制过程中作为模板被复制
【考查点】推理判断题。
【解题思路】根据选项关键词replication定位文章最后一段尾句Thus, one chain is, as if were, the complement of the other, and it is this feature which suggests how the deoxyribonucleic acid molecule might duplicate itself.（因此，一条链似乎是另一条链的补充，正是这一特征表明了脱氧核糖核酸分子如何自我复制），duplicate与replication意思相吻合，可知DNA可以自我复制，因此D选项“强调一条DNA链如何在自我复制过程中作为模板被复制”正确。
【干扰项排除】
A选项“证实大多数DNA分子的核苷酸序列是已知的”未在文中体现，属于无中生有；
B选项“对一个链上含氮碱基能够以任何顺序排列的声称进行反驳”，根据最后一段的“由此可见，在一个长分子中，许多不同的排列是可能的”可知作者并没有反驳这种观点，属于曲解原文；
C选项“支持作者所谓的DNA模型的磷酸糖支柱完全是有规律的声称”并不是作者使用这些词的目的，属于答非所问。
