Amino acid evolutionary favorability

Why did the evolutionary process not favor the preservation of pathways for synthesizing all of the amino acids needed in higher animals Can this be a case of too great a... 

Further studies of chirality, particularly studies focused on the hypothesis that specific environmental conditions can favor chiral selection, or on an alternative model that life with L-amino acids and D-sugars is better fit, from an evolutionary perspective, to evolve into complex organisms and... 

Fishes of the teleost suborder Notothenioidei provide another example of how relatively small differences in temperature may favor evolutionary changes in kinetic properties. A4-LDH orthologs of Antarctic members of this suborder also illustrate how different amino acid sequences can produce similar kinetic properties... 

Polypeptides would have played only a limited role early in the evolution of life because their structures are not suited to self-replication in the way that nucleic acid structures are. However, polypeptides could have been included in evolutionary processes indirectly. For example, if the properties of a particular polypeptide favored the survival and replication of a class of RNA molecules, then these RNA molecules could have evolved ribozyme activities that promoted the synthesis of that polypeptide. This method of producing polypeptides with specific amino acid sequences has several limitations. First, it seems likely that only relatively short specific polypeptides could have been produced in this manner. Second, it would have been difficult to accurately link the particular amino acids in the polypeptide in a reproducible manner. Finally, a different ribozyme would have been required for each polypeptide. A critical point in evolution was reached when an apparatus for polypeptide synthesis developed that allowed the sequence of bases in an RNA molecule to directly dictate the sequence of amino acids in a polypeptide. A code evolved that established a relation between a specific sequence of three bases in RNA and an amino acid. We now call this set of three-base combinations, each encoding an amino acid, the genetic code. A decoding, or translation, system exists today as the ribosome and associated factors that are responsible for essentially all polypeptide synthesis from RNA templates in modem organisms. The essence of this mode of polypeptide synthesis is illustrated in Figure 2.8. 

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