SAT阅读官方指南全解析——第3篇
本篇是SAT阅读官方指南1-20篇全解析系列之第三篇，正文见下：
This passage is adapted from J.D. Watson and F.H. C.Crick, “Genetical Implications of the Structure of Deoxyribonucleic Acid.” ○ C 1953 by Nature Publishing Group. Watson and Crick deduced the structure of DNA using evidence from Rosalind Franklin and R.G. Gosling’s X-ray crystallography diagrams of DNA and from Erwin Chargaff’s data on the base composition of DNA.
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 far 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 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 2 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 would not be room for it.
We believe that the bases will 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 it were, the complement of the other, and it is this feature which suggests how the deoxyribonucleic acid molecule might duplicate itself.
The table shows, for various organisms, the percentages of each of the four types of nitrogenous bases in that organism’s DNA.
具体解析请见下：
22. The authors use the word “backbone” in lines 2 and 21 to indicate that
(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
正确答案：B
分析:文章 2-4 行将 DNA 描述成一个长链条，主干由糖和磷酸盐交替组成。换句话说，主干(backbone)是由糖和磷酸盐通过反复重复交替构成的 DNA 的主要结构。
23. A student claims that nitrogenous bases pair randomly with one another. Which of the following statements in the passage contradicts the student’s claim?
(A) Line 3 (“To each…types”)
(B) Line 5 (“So far….irregular”)
(C) Lines 12-13 (“The bases…other”)
(D) Lines 14-15 (“One member…chains”)
正确答案：D
分析：文章中作者解释说氢键能够将含氮碱基进行配对。含氮碱基能够配对是因为有一种特殊的结构:配对双方其中一个是嘌呤，另一个是嘧啶,因此能够进行连接(第 14-15 行)。因为这种特性，所以称他们的配对是随机的是不准确的。
SAT阅读官方指南全解析——第3篇
24. In the second paragraph (lines 7-10), what do the authors claim to be a feature of biological interest ?
(A) The chemical formula of DNA
(B) The common fiber axis
(C) The X-ray evidence
(D) DNA consisting of two chains
正确答案：D
分析：文章 7-8 行提到，这种结构在生物学上很有趣，因为它包含 2 条链，而不是 1 个。
25. The authors’ main purpose of including the information about X-ray evidence and density is to
(A) establish that DNA is the molecule that carries the genetic information
(B) present an alternate hypothesis about the composition of a nucleotide
(C) provide support for the authors’ claim about the number of chains in a molecule of DNA
(D) confirm the relationship between the density of DNA and the known chemical formula of DNA
正确答案：C
分析：文章 7-8 行提到 DNA 有两条链，而 15-17 行说在此之前大家都以为只有 1 条链。作
者为了支持 2 条链这一观点，使用了 X 射线作为证据。
26. Based on the passage, the authors’ statement “If a pair consisted of two purines, for example, there would not be room for it” (lines 15-16) implies that a pair
(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 pyrimidines.
正确答案：B
分析：作者在 13-14 行提到，仅仅有一种配对形式能够适合 DNA 的这种结构，即一个嘌呤和一个嘧啶。其他配对形式都是不适合 DNA 特殊结构的。而且 15-16 行提到：2个嘌呤的配对形式就会出现空间不足， 所以可以推测： 2 个嘌呤所占空间比一个嘌呤和一个嘧啶大。
27. The authors’ use of the words “exact,” “specific,” and “complement” in lines 25-26 in the final paragraph functions mainly to
(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 a template to be copied during DNA replication.
正确答案：D
分析：文章解释了 DNA 的复制方式：DNA 的双链结构，可以保证一条链能够准确复制到另一条链上。作者采用“exact ”“specific” “complementary”这样的词汇来描述这种复制的过程。
28. Based on the table and passage, which choice gives the correct percentages of the purines in yeast DNA?
(A) 17.1% and 18.7%
(B) 17.1% and 32.9%
(C) 18.7% and 31.3%
(D) 31.3% and 32.9%
正确答案：C
分析：文章 3-5 行提到 DNA 嘌呤包括腺嘌呤(adenine)和鸟嘌呤(purine)两种,在表格中，这种嘌呤对应的比例分别是 31.3%和 18.7%。
29. Do the data in the table support the authors’ proposed pairing of bases in DNA?
(A) Yes, because for each given organism, the percentage of adenine is closest to the percentage of thymine, and the percentage of guanine is closest to the percentage of cytosine.
(B) Yes, because for each given organism, the percentage of adenine is closest to the percentage of guanine, and the percentage of cytosine is closest to the percentage of thymine.
(C) No, because for each given organism, the percentage of adenine is closest to the percentage of thymine, and the percentage of guanine is closest to the percentage of cytosine.
(D) No, because for each given organism, the percentage of adenine is closest to the percentage of guanine, and the percentage of cytosine is closest to the percentage of thymine.
正确答案：A
分析： 作者在文中 17-19 行提到： 嘌呤存在的可能形式是： adenine with thymine和 guanine with cytosine.表格中显示 adenine/thymine 和 guanine/cytosine 有相同的比例。因此，表格信息支持了文章中的内容。
30. According to the table, which of the following pairs of base percentages in sea urchin DNA provides evidence in support of the answer to the previous question ?
(A) 17.3% and 17.7%
(B) 17.3% and 32.1%
(C) 17.3% and 32.8%
(D) 17.7% and 32.8%
正确答案：A
分析：当胞嘧啶(cytosine)和鸟嘌呤(guanine)的比例最接近时，可以支持文中作者关于 DNA结构的猜想。因此选 A。
31. Based on the table, is the percentage of adenine in each organism’s DNA the same or does it vary, and which statement made by the authors is most consist with that data?
(A) The same;”Two of…pyrimidines”(lines 3-5)
(B) The same; “The important ….structure”(lines 13-14)
(C) It varies; “Adenine….thymine”(lines 19-20)
(D) It varies; “It follows…information” (lines 22-24)
正确答案：D
分析：通过表中数据可以看出，不同生物 DNA 中 adenine 的比例是不同的:从大肠杆菌的(E.coli)的 24.7%到章鱼(octopus)的 33.2%。在 22-23 行中，这种多样性被提到：在一个长分子结构中，各种多样性是可能的。