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?
Directions: Read the passage and answer the question.
Nearly twenty years ago, biochemists found that a separable constituent of deoxyribonucleic acid (or DNA) appeared to guide the cell’s protein-synthesising 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 localised 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 the 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, organised 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.