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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, 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 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, which are also prokaryotes and which resemble true bacteria.

16. The passage is primarily concerned with .

A. detailing the evidence that has led most biologists to replace the trichotomous picture of living organisms with a dichotomous one

B. outlining the factors that have contributed to the current hypothesis concerning the number of basic categories of living organisms

C. evaluating experiments that have resulted in proof that the prokaryotes are more ancient than had been expected.

D. summarizing the differences in structure and function found among true bacteria, archaebacteria, and eukaryotes

17. 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

18. According to the qassage, which of the following statements about the two-category hypothesis is likely to be true?

A. It is promising because it explains the presence of true bacteria-like organisms such as organelles in eukaryotic cells.

B. It is promising because it explains why eukaryotic cells, unlike prokaryotic cells, tend to form multicellular organisms.

C. It is flawed because it fails to account for the great variety among eukaryotic organisms.

D. It is flawed because it fails to recognize an important distinction among prokaryotes.

19. It can be inferred from the passage that which of the following have recently been compared in order to clarify the fundamental classifications of living things?

A. The genetic coding in true bacteria and that in other prokaryotes.

B. The organelle structures of archaebacteria, true bacteria, and eukaryotes.

C. The cellular structures of multicellular organisms and unicellular organisms.

D. The molecular sequences in eukaryotic RNA, true bacterial RNA, and archaebacterial RNA.

20. The auihor's attitude toward the view that living things are divided into three categories is best described as one of .

A. tentative acceptance B. mild skepticism

C. limited denial D. studious criticism

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