Passage Reading and English Comprehension

Nearly twenty years ago, biochemists found that a separable constituent of deoxyribonucleic acid (or DNA) appeared to guide the cell's protein-synthesizing machinery. The internal structure of DNA seemed to represent a set of coded instructions which dictated the pattern of protein-synthesis. Experiments indicated that in the presence of appropriate enzymes each DNA molecule could form a replica, a new DNA molecule, containing the specific guiding message present in the original. This idea, when added to what was already known about the cellular mechanisms of heredity (especially the knowledge that DNA is localized in chromosomes), appeared to establish a molecular basis for inheritance.

Proponents of the theory that DNA was a "self-duplicating" molecule, containing a code that by itself determined biological inheritance, introduced the term “central dogma” into scientific literature. They did so in order to describe the principles that could explain DNA's governing role. The dogma originally involved an admittedly unproven assumption that, whereas nucleic acids can guide the synthesis of other nucleic acids and of proteins, the reverse effect is impossible; that is, proteins cannot guide the synthesis of nucleic acids. But actual experimental observations deny the second and crucial part of this assumption. Other test-tube experiments show that agents besides DNA have a guiding influence. The kind of protein that is made may depend on the specific organism from which the necessary enzyme is obtained. It also depends on the test tube's temperature, the degree of acidity, and the amount of metallic salts present.

The central dogma banishes from consideration the interactions among the numerous molecular processes that have been discovered in cells or in their extracted fluids. In the living cell, molecular processes - the synthesis of nucleic acids and proteins or the oxidation of food substance - are not separate but interact in exceedingly complex ways. No matter how many ingredients the biochemists' test tubes may contain, the mixtures are nonliving. However, these same ingredients, organized by the subtle structure of the cell, constitute a system, which is alive.

Consider an example from another field. At ordinary temperatures, electricity flows only so long as a driving force from a battery or generator is imposed upon the circuit. At temperatures near absolute zero, metals exhibit superconductivity: a unique property that causes an electric current to flow for months after the voltage is cut off. Although independent electrons exist in a metal at ordinary temperatures, at very low temperatures they interact with the metal's atomic structure in such a way as to lose their individual identities and form a coordinated, collective system which gives rise to superconductivity.

Such discoveries of modern physics show that the unique properties of a complex system are not necessarily explicable solely by the properties that can be observed in its isolated parts. We can expect to find a similar situation in the complex chemical system of the living cells.

1827. The author is primarily concerned with

	(a) Proposing that a new philosophical foundation for modern biochemistry be developed
	(b) Describing the various processes that take place in a living cell
	(c) Drawing analogies between different scientific fields
	(d) Revealing a discrepancy between a scientific theory and some experimental results

1828. The author provides information that would answer which of the following questions?

I. What have test-tube experiments revealed about the role of DNA?
II. What viruses interfere with DNA replication?
III. What methods have been developed to allow scientists to observe a living cell?

	(a) I only
	(b) II only
	(c) I and III only
	(d) II and III only

1829. The author's argument is directed against which of the following?

I. The use of test-tube experimentation alone to establish the validity of scientific theories
II. The exclusion of experimental facts from the formation of scientific theories
III. The observation of certain cellular components in isolation

	(a) I only
	(b) I and II only
	(c) I and III only
	(d) I, II and III

1830. The author refers to the results of test-tube experiments involving the replication of DNA primarily in order to

	(a) Question the validity of experimental results that describe the structure of DNA
	(b) Provide evidence to contradict the theory that DNA alone governs protein synthesis
	(c) Show the way in which DNA acts as a self-duplicating molecule
	(d) Explain the internal structure of DNA

1831. According to the passage, a metal cannot become super conductive unless

	(a) The voltage through the metal is increased.
	(b) The metal's atomic structure has been destroyed
	(c) The metal is exposed to very low temperatures.
	(d) Electrons in the metal interact with one another at ordinary temperatures

1832. The author suggests that the most important difference observed between a dead cell and a living cell results primarily from the

	(a) Differences in the chemical elements present in each
	(b) Differences in the degree of acidity present in each
	(c) Biochemical procedures used to examine each cell
	(d) Integrating mechanism thought to exist within the structure of the living cell

1833. The experimental results mentioned in the passage suggest that biological inheritance depends on

	(a) Instructions contained in a single molecule within the cell.
	(b) Processes that are guided by specific enzymes.
	(c) Genetic information contained in metallic salts.
	(d) Interactions among several molecular processes in the cell

1834. The author presents his argument primarily by

	(a) Contrasting two fields of science.
	(b) Providing experimental evidence against a point of view
	(c) Criticizing proponents of other theories
	(d) Stating a new theory and its important implications.

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