og-29-176

EASYSUMMER

Passage 29 (44/63)

It was once assumed that all living things could be divided into two fundamental and exhaustive categories. Multicellular plants and animals, as well as many unicellular organisms, are eukaryotic—their large, complex cells have a well-formed nucleus and many organelles. On the other hand, the true bacteria are prokaryotic cell (prokaryotic cell: 原核细胞), which are simple and lack a nucleus. The distinction between eukaryotes and bacteria, initially defined in terms of subcellular structures visible with a microscope, was ultimately carried to the molecular level. Here prokaryotic and eukaryotic cells have many features in common. For instance, they translate genetic information into proteins according to the same type of genetic coding. But even where the molecular processes are the same, the details in the two forms are different and characteristic of the respective forms. For example, the amino acid sequences of various enzymes tend to be typically prokaryotic or eukaryotic. The differences between the groups and the similarities within each group made it seem certain to most biologists that the tree of life had only two stems. Moreover, arguments pointing out the extent of both structural and functional differences between eukaryotes and true bacteria convinced many biologists that the precursors of the eukaryotes must have diverged from the common ancestor before the bacteria arose.

Although much of this picture has been sustained by more recent research, it seems fundamentally wrong in one respect. Among the bacteria, there are organisms that are significantly different both from the cells of eukaryotes and from the true bacteria, and it now appears that there are three stems in the tree of life. New techniques for determining the molecular sequence of the RNA of organisms have produced evolutionary information about the degree to which organisms are related, the time since they diverged from a common ancestor, and the reconstruction of ancestral versions of genes. These techniques have strongly suggested that although the true bacteria indeed form a large coherent group, certain other bacteria, the archaebacteria (archaebacteria: [复] n.[微]原始细菌( 一种不同于细菌和动植物细胞且要求完全厌氧条件并能产生甲烷的微生物)), which are also prokaryotes and which resemble true bacteria, represent a distinct evolutionary branch that far antedates the common ancestor of all true bacteria.

According to the passage, investigations of eukaryotic and prokaryotic cells at the molecular level supported the conclusion that

(A) most eukaryotic organisms are unicellular

(B) complex cells have well-formed nuclei

(C) prokaryotes and eukaryotes form two fundamental categories

(D) subcellular structures are visible with a microscope（C）

(E) prokaryotic and eukaryotic cells have similar enzymes

答案是C我认同，可是OG对B的解释，是在理解不了。

The

passage does not address what proportion of eukaryotic organisms are unicellular (choice A) or

whether all complex cells have well-formed nuclei (choice B).

原文Multicellular plants and animals, as well as many unicellular organisms, are eukaryotic—their large, complex cells have a well-formed nucleus and many organelles.

破折号不就是说明 EUKARYOTIC是COMPLEX CELLS ,且是 A WELL-FORMED NUCLEUS吗？？

多谢赐教

EASYSUMMER

自己先解释一下，下面题目中的红字是题眼（According to the passage, investigations of eukaryotic and prokaryotic cells at the molecular level supported the conclusion that：），也就是要找在molecular level下的结论，而在这样的前提下的结论实际上只有4点明确了：

1。prokaryotic and eukaryotic cells have many features in common. For instance, they translate genetic information into proteins according to the same type of genetic coding.

2。the details in the two forms are different and characteristic of the respective forms. For example, the amino acid sequences of various enzymes tend to be typically prokaryotic or eukaryotic.

3。The differences between the groups and the similarities within each group made it seem certain to most biologists that the tree of life had only two stems.

4。Moreover, arguments pointing out the extent of both structural and functional differences between eukaryotes and true bacteria convinced many biologists that the precursors of the eukaryotes must have diverged from the common ancestor before the bacteria arose.

所以，依这样的标准来看，下面的5个答案只有C了。

(A) most eukaryotic organisms are unicellular(B) complex cells have well-formed nuclei

(C) prokaryotes and eukaryotes form two fundamental categories(D) subcellular structures are visible with a microscope(E) prokaryotic and eukaryotic cells have similar enzymes

请指教

EASYSUMMER

不好意思，自己再罗索一次：下面是原文的第一段，红字"HERE" 这里我理解是一个引导词，说明此下面的言论是依据上面的molecular level 而来。 求证！！！

It was once assumed that all living things could be divided into two fundamental and exhaustive categories. Multicellular plants and animals, as well as many unicellular organisms, are eukaryotic—their large, complex cells have a well-formed nucleus and many organelles. On the other hand, the true bacteria are prokaryotic cell (prokaryotic cell: 原核细胞), which are simple and lack a nucleus. The distinction between eukaryotes and bacteria, initially defined in terms of subcellular structures visible with a microscope, was ultimately carried to the molecular level. Here prokaryotic and eukaryotic cells have many features in common. For instance, they translate genetic information into proteins according to the same type of genetic coding. But even where the molecular processes are the same, the details in the two forms are different and characteristic of the respective forms. For example, the amino acid sequences of various enzymes tend to be typically prokaryotic or eukaryotic. The differences between the groups and the similarities within each group made it seem certain to most biologists that the tree of life had only two stems. Moreover, arguments pointing out the extent of both structural and functional differences between eukaryotes and true bacteria convinced many biologists that the precursors of the eukaryotes must have diverged from the common ancestor before the bacteria arose.

wangyu73cn

The passage does not address what proportion of eukaryotic organisms are unicellular (choice A) or whether all complex cells have well-formed nuclei (choice B).
原文Multicellular plants and animals, as well as many unicellular organisms, are eukaryotic—their large, complex cells have a well-formed nucleus and many organelles.
破折号不就是说明 EUKARYOTIC是COMPLEX CELLS ,且是 A WELL-FORMED NUCLEUS吗？？

楼主看得确实很认真啊。这个问题促使我又把整个文章看了看，发现解释说得没有错。文章中确实没有提到B选项的内容。
在文章中，仅第2句话中出现了complex cells，同时，第3句中出现了simple。两个形容词都是分别描述eu或pro类的性状的，并没有界定complex或simple cells的情况，没有表明凡是complex cells都有well formed nucleus.

danlon77

没有表明凡是complex cells都有well formed nucleus.

同意!eukaryotic—their large, complex cells have a well-formed nucleus and many organelles.这句只表明eukaryotic cells是complex cells,但并不代表两者等价，即可能后者的范畴大于前者，所以不能说明所有complex cells都有 well-formed nucleus.

jzchina

The distinction between eukaryotes and bacteria, initially defined in terms of subcellular structures visible with a microscope, was ultimately carried to the molecular level. Here prokaryotic and eukaryotic cells have many features in common. For instance, they translate genetic information in proteins according to the same type of genetic coding. But even where the molecular processes are the same, the details in the two forms are different and characteristic of the respective forms.

可以理解here指的是molecular level，那么where指的是什么呢？也是molecular level吗？

wangyu73cn

我觉得这里的where有点相当于“when”或者“当the molecular processes are the same的情况下”的意思。

jzchina

感谢版主！

swlfx

向楼主学习。

多谢王牛牛。

clarkzdyhit

原文：

Multicellular plants and animals, as well as many unicellular organisms, are eukaryotic—their large, complex cells have a well-formed nucleus and many organelles.

选项

(B) complex cells have well-formed nuclei

单复数也不对呀