GRE阅读题目解析:自然界螺旋结构

　　1

　　What causes a helix in nature to appear with either a dextral ("right-handed, "or clockwise) twist or a sinistral ("left-handed," or counterclockwise) twist is one of the most intriguing puzzles in the science of form.

　　什么因素决定了自然界中的螺旋结构呈右旋(“右手的” 或顺时针的)或是左旋(“左手的” 或逆时针的)，这是研究形态的科学中最吸引人的一个谜。

　　2

　　Most spiral-shaped snail species are predominantly dextral.

　　多数螺旋状蜗牛种都是右旋个体占多数的。

　　3

　　But at one time, handedness (twist direction of the shell) was equally distributed within some snail species that have become predominantly dextral or, in a few species, predominantly sinistral.

　　然而现在已经变为右旋占多数的蜗牛种类，其旋向性(壳的螺旋方向)一度也曾均衡分布，在某些种类中，甚至左旋曾占优势。

　　(at one time 一度，从前，在过去的一段时间内)

　　4

　　What mechanisms control handedness and keep left-handedness rare?

　　什么机制控制了旋向性并压制了左旋个体的数量?

　　5

　　It would seem unlikely that evolution should discriminate against sinistral snails if sinistral and dextral snails are exact mirror images, for any disadvantage that a sinistral twist in itself could confer on its possessor is almost inconceivable.

　　如果左旋与右旋是完全镜像的，似乎不大可能归结为进化选择淘汰左旋，因为任何由左旋产生的个体劣势都几乎是难以取信的。

　　(any disadvantage is almost inconceivable，翻译成汉语的逻辑，应该是：目前还没有发现任何一种劣势是由左旋带来的。)

　　6

　　But left- and right-handed snails are not actually true mirror images of one another.

　　但左旋与右旋蜗牛实际上并非彼此的镜像。

　　7

　　Their shapes are noticeably different.

　　它们在外形上有明显区别。

　　8

　　Sinistral rarity might, then, be a consequence of possible disadvantages conferred by these other concomitant structural features.

　　那么，左旋变少可能是其他伴随性的结构特点，带来的可能的劣势造成的。

　　(concomitant 伴随的)

　　9

　　In addition, perhaps left- and right-handed snails cannot mate with each other, having incompatible twist directions.

　　另外，也许左旋与右旋的蜗牛无法交配，会产生混乱的壳旋向。

　　10

　　Presumably an individual of the rarer form would have relative difficulty in finding a mate of the same hand, thus keeping the rare form rare or creating geographically separated right- and left-handed populations.

　　可推测的是，外形上更稀缺的个体会相应地更难找到相同旋向的配偶，因此维持了这种外形的稀缺，或造成了左旋与右旋在地理上相互区隔的种群分布。

　　11

　　But this evolutionary mechanism combining dissymmetry, anatomy, and chance does not provide an adequate explanation of why right-handedness should have become predominant.

　　然而这种结合了非对称、解剖学以及概率观点的进化机制，不能有效解释为何右旋理应变成多数。

　　12

　　It does not explain, for example, why the infrequent unions between snails of opposing hands produce fewer offspring of the rarer than the commoner form in species where each parent contributes equally to handedness.

　　比如，它无法解释某些罕见的情况下，不同旋向的蜗牛交配后，更常见旋向的后代，仍然多于另一种旋向的后代，本来双亲对后代旋向的影响应该是相当的。

　　(union 在本句中作交配)

　　13

　　Nor does it explain why, in a species where one parent determines handedness, a brood is not exclusively right- or left-handed when the offspring would have the same genetic predisposition.

　　它也无法解释，为什么在单亲决定旋向的物种中，一窝后代并非全左旋或全右旋，而本来这些后代都有相同的基因倾向。

　　14

　　In the European pond snail Lymnaea peregra, a predominantly dextral species whose handedness is maternally determined, a brood might be expected to be exclusively right- or left-handed — and this often occurs.

　　在欧洲池塘蜗牛锥实螺中，右旋多数的种群的旋性是母亲决定的，一窝后代可能是完全右旋或左旋的 —— 现实情况也经常如此。

　　15

　　However, some broods possess a few snails of the opposing hand, and in predominantly sinistral broods, the incidence of dextrality is surprisingly high.

　　然而，有些家庭的一窝后代中包括两种旋性，而且在左旋占多数的情况下，出现右旋的概率出人意料地高。

　　16

　　Here, the evolutionary theory must defer to a theory based on an explicit developmental mechanism that can favor either right- or left-handedness.

　　至此，进化理论必须遵从一种能够清楚解释发育机制的理论，无论偏向哪种旋向。

　　17

　　In the case of Lymnaea peregra, studies indicate that a dextral gene is expressed during egg formation; i.e., before egg fertilization, the gene produces a protein, found in the cytoplasm of the egg, that controls the pattern of cell division and thus handedness.

　　在锥实螺的例子中，研究表明右旋基因是在卵的形成期表达的;也就是说，卵受精前基因在细胞质中制造一种蛋白质，控制细胞分裂，因此也控制旋性。

　　18

　　In experiments, an injection of cytoplasm from dextral eggs changes the pattern of sinistral eggs, but an injection from sinistral eggs does not influence dextral eggs.

　　实验中，给左旋卵注射右旋卵的细胞质可以改变其旋向，反之则不成立。

　　19

　　One explanation for the differing effects is that all Lymnaea peregra eggs begin left-handed but most switch to being right-handed.

　　这种效果上的差异，有一种解释是所有的锥实螺开始都是左旋的，但多数后来转为右旋。

　　20

　　Thus, the path to a solution to the puzzle of handedness in all snails appears to be as twisted as the helix itself. (472 words)

　　所以，蜗牛旋性这个谜题，似乎就像螺旋结构本身一样复杂难解。

　　4. The second paragraph of the passage is primarily concerned with offering possible reasons why

　　(A) it is unlikely that evolutionary mechanisms could discriminate against sinistral snails

　　(B) sinistrality is relatively uncommon among snail species

　　(C) dextral and sinistral populations of a snail species tend to intermingle

　　(D) a theory based on a developmental mechanism inadequately accounts for the predominance of dextrality across snail species

　　(E) dextral snails breed more readily than sinistral snails, even within predominantly sinistral populations

　　选 B

　　第二段对应句 5 - 10，为左旋蜗牛为什么相对少见提出了可能的原因。

　　5. In describing the "evolutionary mechanism" (the first sentence of the third paragraph), the author mentions which of the following?

　　(A) The favorable conditions for nurturing new offspring

　　(B) The variable environmental conditions that affect survival of adult snails

　　(C) The availability of potential mates for breeding

　　(D) The structural identity of offspring to parents of the same hand

　　(E) The frequency of unions between snails of different species

　　选 C

　　根据题干定位 evolutionary mechanism 在句 11，但你要注意，它前面还有个至关重要的 this，这意味着句 11 以前 evolutionary mechanism 的本体就已经出现了，于是我们回到第二段末尾，句 10 中找作者提到了什么，选 C，因为难找配偶，所以左旋蜗牛变少。

　　6. According to the passage, which of the following is true of Lymnaea peregra?

　　(A) Handedness within the species was at one time equally distributed between left and right.

　　(B) Under laboratory conditions, dextral eggs from Lymnaea peregra can be artificially induced to develop into sinistral snails.

　　(C) Broods of Lymnaea peregra are, without variation, exclusively sinistral or dextral.

　　(D) Handedness in Lymnaea peregra offspring is determined by two parents together.

　　(E) fewer sinistral offspring in dextral broods than dextral offspring in sinistral broods

　　选 E

　　右旋窝里的左旋，比左旋窝里的右旋少。

　　举个虚构的例子，比如一般右旋窝里如果出左旋，十个里平均只出一个;而如果左旋窝里出右旋，十个里平均出4个。可定位到句 15 。

　　A 是事实，但这是蜗牛总体发展的一个趋势，不单归于 Lymneae peregra，不符合题干要求。

　　B 刚好说反了。

　　C 错了，有时会混入一些旋性相反的后代。

　　D 错了，由妈决定的，定位句 14，maternally 。

　　7. Which of the following accurately describes the relationship between the evolutionary and developmental theories discussed in the passage?

　　(A) Although the two theories reach the same conclusion, each is based on different assumptions.

　　(B) They present contradictory explanations of the same phenomenon.

　　(C) The second theory accounts for certain phenomena that the first cannot explain.

　　(D) The second theory demonstrates why the first is valid only for very unusual, special cases.

　　(E) They are identical and interchangeable in that the second theory merely restates the first in less technical terms.

　　选 C

　　进化理论和发育理论的关系，句 16：

　　Here, the evolutionary theory must defer to a theory based on an explicit developmental mechanism that can favor either right- or left-handedness.

　　再结合句 12-13 提到的，进化理论不能解释的现象，综合判断，选 C 。

　　略枯燥的一个长文段，最后给大家看看蜗牛的左右旋，从中心向外顺时针发散状的叫右旋(黑色那只)，反之叫左旋，左旋的蜗牛比较少见，养蜗牛的贴吧也这样聊。

　　以上就是新东方在线GER频道为你带来的GRE阅读题目解析:自然界中的螺旋结构，更多精彩敬请关注新东方在线GRE频道。