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recap4 (4.2 recap (The interpretation of Pasteur’s experiment (see Figure…
recap4
4.2 recap
The interpretation of Pasteur’s experiment (see Figure 4.6) depended on the inactivation of microorganisms by heat. We now know of microorganisms that can survive extremely high temperatures (see Chapter 26). Does this change the interpretation of Pasteur’s experiment? What experiments would you do to inactivate such microbes?
If microbes survived heat, the initial part of Pasteur’s experiment might begin with microbes already present. They would grow in both the open and closed flasks. To get the results that Pasteur did, his flasks must not have contained such microbes. An answer for the proposed experiment on heat-stable microbes might be to inactivate them using reagents, such as mercaptoethanol, that destroy proteins.
The Miller-Urey experiment (see Figure 4.7) showed that it was possible for amino acids to be formed from gases that were hypothesized to have been in Earth’s early atmosphere.These amino acids were dissolved in water. Knowing what you do about the polymerization of amino acids into proteins (see Figure 3.6), how would you set up an experiment to show that proteins can form under the conditions of early Earth?
A suggested experiment might be to dry the samples after the Miller–Urey experiment (allowing condensation reactions—polymerization) and then apply energy in the form of heat. This condition might have existed in volcanic rock on early Earth.
What conditions existing on Earth today might preclude the origin of life from the prebiotic molecules Miller and Urey used?
The presence of O2 in the atmosphere produces an oxidizing condition that prevents the reduction reactions observed in the Miller–Urey experiment.
4.1 recap
What are the key differences between purines and pyrimidines and how does this relate to the structure of DNA?
Purines contain two nitrogen–carbon rings, while pyrimidines have one ring. The double helix of DNA has uniform width because a purine on one strand is always opposite a pyrimidine on the other strand.
How can DNA molecules be very diverse, even though they appear to be structurally similar?
While DNA molecules are similar in diameter and configuration, their base sequences are different. Differences in base sequence provide the informational content of DNA.
Single-stranded nucleic acids about 25 bases long are called aptamers. Because many sequences are possible, particular aptamers can be used to bind to specific targets, either as drugs to a protein target or for quantitating a small molecule in tissues or fluids. How can aptamers have such diversity and specificity for binding?
The number of possible 25-unit sequences of four nucleotides is 425, a large number indeed. Because of internal base pairing of the single strand (as in RNA), many folded configurations are possible, which allows specific binding to target molecules.
4.3 recap
Why was the ability to both encode information and catalyze reactions important for the origin of life?
A hallmark of living systems is the ability to reproduce, and this occurs from preexisting organisms. The instructions for producing an identical organism must be passed on to the offspring. This implies informational molecules. In living systems, chemical changes constantly occur, but in ordinary chemistry they are too slow to benefit the organism. So catalysts are needed to speed up the reactions.
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In living organisms, the catalyst for the formation of the peptide bond is an RNA that does not have an informational role. How does this relate to the “RNA world” hypothesis?
Most catalysts in living systems are proteins. But the polymerization of amino acids into proteins that are catalytic must have happened before the protein catalysts were initially formed. Having an RNA, that perhaps was originally informational, act as the catalyst for protein formation solves this “chicken–egg” issue.
4.4 recap
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If you wanted to find evidence for the existence of cells more than 3 billion years ago, what would you look for?
First, examine rocks that are more than 3 billion years old. Then look at slices of rocks under microscopes for objects that look like cells or chains of cells. Finally, chemically analyze the rocks for chemical signatures for life, such as a carbon isotope ratio resulting from photosynthesis.