The Search for Life's Origins Progress and Future Directions in Planetary Biology and Chemical Evolution (1990) / Chapter Skim
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4. The Origin of Life
4. The Origin of Life
Pages 78-90
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From page 78... ...
the phylogenetic history recorded in the nucleotide and amino acid sequences found in living cells—molecular phylogeny. According to the geological record, the Earth appears to be 4.5 billion years old.
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From page 79... ...
Evaluation of these alternative models necessitates new conjectures and experiments as to how the biologically important monomers were formed and what substituted for hydrogen as a reductant. · Comparative molecular biology: Here, an attempt is made to deduce the characteristics of the earliest cells and cellular mechanisms by inspecting contemporary organisms for features that are common to the three primary lines of descent: eubacteria, archpebacteria, and eukaryotes.
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From page 80... ...
The condensation of hydrogen cyanide in the presence of ammonia produces amino acids as well as the purine nucleotide bases, adenine and guanine, components of all nucleic acids. Cytosine, a base found in nucleic acids, can be readily synthesized from cyanoacetylene.
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From page 81... ...
These questions require careful study, including detailed comparative analysis of contemporary metabolic pathways. Sulfide may have been abundant on the early Earth, yet it has received little experimental attention with regard to its possible involvement in prebiotic syntheses.
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From page 82... ...
It is widely believed that reactions simpler than, but similar to, nucleic acid replication and protein synthesis appeared very early in the history of life on Earth. Any attempt to provide a chemical model of the evolution of these coupled processes must grapple with a fundamental problem: nucleic acids are molecules that seem ideally suited for replication, whereas polypeptides seem similarly suited for function.
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From page 83... ...
On the primitive Earth, "RNA life," in which RNA molecules catalyzed a limited set of metabolic reactions in addition to RNA replication, may have preceded life as we know it. However, it remains problematic because · as yet there is no known route from a simple prebiotic environment to a self-replicating RNA; · no prebiotic synthesis of ribose has yet been found that does not also produce a wide range of other sugars; · the condensation of ribose with bases would give complex mixtures of products, including L- as well as D-nucleosides, and nucleosides with a- as well as those with p-glycosidic linkages; and · presently known region-specific and efficient syntheses of internucleotide bonds require special conditions.
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From page 84... ...
RNA molecules have been demonstrated to have specific hydrolytic and ligating activities, and they can act as simple polymerases by extending preexisting RNA chains at the expense of other preexisting ribooligonucleotides. RNA can also act as a phosphate monoester transfer catalyst and phosphomonoesterase.
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From page 85... ...
He considered that monomers such as nucleotides and amino acids could be adsorbed from dilute solution onto a clay surface and there polymerized to give proteins and nucleic acids. Clays are made up of various ions embedded in a two-dimensional silicate lattice.
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From page 86... ...
To understand the origin of the translation apparatus and the genetic code, comparative molecular studies of extant systems are necessary to gain detailed insight into the essential workings of these processes in modern organisms. This information should allow the construction of meaningful models for primitive versions of the processes that are best tested by a direct study with synthetic polymers, in the tradition of prebiotic chemical studies.
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From page 87... ...
Because a hydrophobic amino acid tends to have a hydrophobic anticodon, a mutual interaction of the esterified amino acid with its anticodon might stabilize the amino acid ester bond against hydrolysis. The result would be a preference for a hydrophobic amino acid to remain in the site adjacent to a hydrophobic anticodon.
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From page 88... ...
"Ratchet" models propose that tRNA enters the ribosome and remains at one location but subsequently undergoes conformational changes that result in the movement of mRNA relative to tRNA. In both models, the actual synthetic step occurs at the 3'-end of the tRNA, which is a considerable distance from the site of the codon-anticodon interaction.
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From page 89... ...
Now, macrosequencing projects involving large eukaryotic genomes seem inevitable; the technology is at hand for the detailed mapping of bacterial genomes (the entire Escherichia cold genomic DNA sequence will soon be available) ; and several mitochondrial and chloroplast genome sequences have been published.
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From page 90... ...
It is clear that a major commitment for genome analysis will require support from many federal agencies besides NASA. However, important aspects of this major undertaking are within the purview of NASA's program in planetary biology and chemical evolution: for example, analysis of sequence data from the standpoint of the essential elements of genome structure and its fluidity and the implications of such studies for the origin of life.
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