Lecture 19 - Genetic code, translation, virus&prok diversity

Lecture 19 - Genetic code, translation, virus&prok diversity

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1 Lecture 19.1.Origins of Life, con't IV. Evolution of the genetic code The "universal" genetic code apparently was present in the MRCA Recall translation: * tRNA is an intermediate between the information-containing molecule (mRNA) and protein * RNA is the catalytic component of translation, so translation machinery plausibly predates catalytic proteins * each tRNA carries a specific amino acid (20 different tRNAs) * The genetic code is redundant (most amino acids encoded by > 1 triplet codon), but the redundancy is not random. Codons usually variable only in the third position. It is hypothesized that amino acids can be divided into "early" and "late"; early AAs were initially derived by pre-biotic synthesis and "late" AAs emerged after evolution of biosynthetic pathways. Interestingly, "early" AAs as inferred from biosynthetic pathways are exactly the same as those AAs produced experimentally by abiogenic synthesis (e.g., Miller and Urey exp't). Further hypothesized that originally, only the first two bases in the triplet were coding. Three classes of theories of origin of genetic code: (1) error-minimization, from selection to reduce mis-translation, (2) stereochemical, which posits stereochemical affinities between coding triplets (codons and/or antidocons) and the associated AA (3) "coevolution", which posits that the structure of the code reflects the biosynthetic pathways of AA formation There is a strong correlation between the first bases of codons and the biosynthetic pathway of the AAs they encode. e.g., codons beginning with C encode AAs synthesized from a-ketoglutarate, codons beginning with A encode AAs synthesized from oxaloacetate, and codons beginning with U encode AAs synthesized from pyruvate. Codons beginning with G encode AAs that can form by a direct reductive amination of a simple a-keto acid.
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2 Also a strong correlation between the second base of the codon and the hydrophobicity of the encoded AA: codons w/ U as the second base encode the most hydrophobic AAs, those with A as the second base encode the most hydrophilic. These facts led to the suggestion that prior to the emergence of macromolecules, simple AAs were synthesized in covalent complexes with dinucleotides with a-keto acids, originating either from the reductive tricarboxylic acid cycle or the reductive acetate pathway. The bases and phosphates of the dinucleotides are proposed to have catalytically enhanced the rates of AA synthesis. figures Moreover, it can be shown that the code is optimized for robustness to mis-translation (or mutation). V. Origin of Translation
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Lecture 19 - Genetic code, translation, virus&prok diversity

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