OG-29-Q-180

Passage 29

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 unicellu-

lar organisms, are eukaryotic—their large, complex cells

(5) have a well-formed nucles and many organelles. On the

other hand, the true bacteria are prokaryotic cell, 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 ulti-

(10) mately 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

(15) the two forms are different and characteristic of the respec-

tive forms. For example, the amino acid sequences of vari-

ous 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

(20) that the tree of life had only two stems. Moreover, argu-

ments pointing out the extent of both structural and func-

tional differences between eukaryotes and true bacteria

convinced many biologists that the precursors of the

eukaryotes must have diverged from the common

(25)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

(30)from the true bacteria, and it
                        now appears that there are

three stems in the tree of life.
                            New techniques for deter-

mining 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

(35) from a common ancestor, and the reconstruction of ances-

tral versions of genes.
                        These techniques have strongly

suggested that
                        although the true bacteria indeed form a
                    

large coherent group, certain other bacteria, the archaebac-

teria, which are also prokaryotes and which resemble true

(40) bacteria, represent a distinct evolutionary branch that

far
                        antedates the common ancestor of all true bacteria.

180. According to the passage, researchers working under the two-category hypothesis were correct in thinking that

(A) prokaryotes form a coherent group

(B) the common ancestor of all living things had complex properties

(C) eukaryotes are fundamentally different from true bacteria

(D) true bacteria are just as complex as eukaryotes

(E) ancestral versions of eukaryotic genes functioned differently from their modern counterpart

答案C。这题做对了，但是想不通为什么A错。 因为在two-category hypothesis 下，A项应是正确选项，见红体字。

并且，Q181：

181. All of the following statements are supported by the passage EXCEPT:

(A) True bacteria form a distinct evolutionary group.

(B) Archaebacteria are prokaryotes that resemble true bacteria.

(C) True bacteria and eukaryotes employ similar types of genetic coding.

(D) True bacteria and eukaryotes are distinguishable at the subcellular level.

(E) Amino acid sequences of enzymes are uniform for eukaryotic and prokaryotic organisms.

OG对181的解释：

Lines 37-38 support the idea that “true bacteria indeed form a large coherent group” of the kind postulated by the two-category hypothesis.