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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 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.
1. 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
(E) formulating a hypothesis about the mechanisms of
evolution that resulted in the ancestors of the
prokaryotes
2. 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 cukaryotes form two fundamental
categories
(D) subcellular structures are visible with a microscope
(E) prokaryotic and eukaryotic cells have similar
enzymes
3. According to the passage, 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 account for the
similarity between prokaryotes and eukaryotes.
(E) It is flawed because it fails to recognize an important
distinction among prokaryotes.
4. 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
(E) The amino acid sequences in enzymes of various
eukaryotic species and those of enzymes in
archaebecterial species
5. If the “new techniques” mentioned in line 31 were
applied in studies of biological classifications other than
bacteria, which of the following is most likely?
(A) Some of those classifications will have to be
reevaluated.
(B) Many species of bacteria will be reclassified
(C) It will be determined that there are four main
categories of living things rather than three.
(D) It will be found that true bacteria are much older
than eukaryotes.
(E) It will be found that there is a common ancestor of
the eukaryotes, archaebacteria, and true bacteria.
6. 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 counterparts.
7. 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.
8. The author’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 oriticism
(E) whole hearted endorsement