Science, Part 65
Question 1: Explain the RNA world hypothesis.
Answer 1: The RNA world hypothesis attempts to explain the development of protocells from simple organic molecules. This hypothesis is a “gene first” hypothesis, meaning that it represents the view that nucleic acids formed before the biochemical reactions and pathways required for self-replication existed. The RNA world hypothesis suggests that nucleotides, the units from which RNA strands are formed, floated in the primordial soup along with other chemicals. Eventually, some of these nucleotides were synthesized into chains, which are said to be the first forms of life. These primitive life forms would have then engaged in competition for free nucleotides; this competition operated as a process of natural selection. The RNA chains then developed the ability to link amino acids into proteins and continued to evolve into the first prokaryotic cells. The discovery that RNA could behave as an enzyme (a ribozyme) lent credence to the idea that RNA could catalyze protein synthesis. Some discredit this hypothesis because there is little evidence that nucleotides could have been formed in the primordial soup.
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Question 2: Explain the iron-sulfur world theory.
Answer 2: The iron-sulfur world theory is a “metabolism first” model of the evolution of organic molecules into primitive cells—it suggests that the formation of primitive metabolism (a cycle of energy-producing chemical reactions) created an environment in which RNA could later form. The energy produced by these reactions could in turn fuel the development of increasingly complex cells. This hypothesis states that organic molecules were created on the surfaces of iron-bearing minerals near hydrothermal vents on the ocean floor, rather than in the “soup” of the ocean itself. Redox reactions of metal sulfides may have provided the energy necessary for development of self-replicating organisms. This model is attractive because it provides for a succession of developmental steps within a single structure. Critics, however, have pointed out that the spontaneous development of a closed metabolic cycle such as that included in the iron-sulfur theory is unlikely.
Question 3: Describe the Earth’s atmosphere.
Answer 3: Earth’s atmosphere is a mixture of molecules and particles that envelop the planet. Today, this gaseous conglomeration is approximately 78 percent nitrogen and 21 percent oxygen, with trace gases such as the greenhouse gas carbon dioxide and the noble gas argon making up the remaining one percent. The majority of these elements are concentrated in the lowest ten kilometers of the atmosphere. The atmosphere’s lower boundary is the Earth’s surface, while its hazy upper boundary is 10,000 kilometers above that. The atmosphere shields the planet’s surface from harmful cosmic rays and absorbs much of the ultraviolet radiation beamed towards the planet from the sun. The motion of air within the atmosphere helps to regulate the Earth’s temperature through convection and by allowing certain amounts of thermal energy in from, and out to, space.
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