102209-BCH311

102209-BCH311 - Fig. 12-4, p.305 Table 12-2, p.304 I* -...

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Unformatted text preview: Fig. 12-4, p.305 Table 12-2, p.304 I* - hypoxanthine (6-oxy-purine), a modifed, less-common nucleobase Wobble Base-Pairing Combinations Codon-Anticodon Pairing and Wobble In most cases the degenerate codons for a given amino acid will only differ in the third base - that which pairs with the wobble base of the anticodon e.g. Alanine (Ala) A GC A GC C GC G GC U Fewer tRNAs are needed as a given tRNA can base-pair with several codons i. less energy required to synthesize tRNAs ii. minimizes damage that can be caused by mis-reading the code, e.g. if CUU (that encodes leucine) were to be misread as CUC, CUA, or CUG what would happen? N.B. each tRNA is specic for a single amino acid Is the Genetic Code Universal? Do all organisms use the same genetic code? The universality of the code has been observed in viruses, prokaryotes, as well as eukaryotes However, there are exceptions; 1. Mitochondrial DNA codons can be read differently 2. The marine alga Acetabularia translates the standard STOP codons UAG and UAA as glycines 3. Fungi of the genus Candida translate the codon CUG as a serine (instead of a leucine) Cricks Central Dogma, Part III: Translation Protein biosynthesis is a complex process requiring ribosomes (protein factories) , mRNA and tRNA, plus many other factors. The mRNA and tRNA which are bound to the ribosome during protein synthesis are responsible for the correct order of addition of amino acids to the growing polypeptide chain Four Stages of Translation 1. Activation - Before an amino acid is incorporated into the growing polypeptide chain, it rst must be activated - During activation the amino acid becomes covalently bonded to the tRNA forming an aminoacyl-tRNA. This step is catalyzed by an enzyme called aminoacyl-tRNA synthetase 2. Initiation - the rst aminoacyl-tRNA is bound to the start codon AUG on the mRNA (ATG on the DNA) 3. Elongation - the second aminoacyl-tRNA forms a complex with the ribosome and the mRNA and a peptide bond is formed between the rst and second amino acids. This process is repeated many times. 4. Termination - the synthesis of the polypeptide chain arrests when a STOP codon is encountered (UAG, UGA, UAA) Fig. 12-1, p.302 The four stages of Translation Amino Acid Activation The activation of the amino acid and the formation of the aminoacyl-tRNA take place in two steps, both of which are catalyzed by the enzyme aminoacyl tRNA synthetase 1. The amino acid Frst forms a covalent bond with adenosine triphosphate (ATP) producing an aminoacyl-AMP 2. The aminoacyl is then transferred to the tRNA forming an aminoacyl- tRNA Amino acid + ATP aminoacyl-AMP + PPi Aminoacyl-AMP + tRNA aminoacyl-tRNA + AMP Amino Acid Activation: Step 1 Covalent attachment of the amino acid to ATP catalyzed by aminoacyl tRNA synthetase Aminoacyl-AMP is a mixed anhydride of a carboxylic acid and a phosphoric acid As anhydrides are reactive compounds, the free energy change for the hydrolysis of aminoacyl-AMP favors the second step of the reaction...
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This note was uploaded on 12/05/2009 for the course BCH 311 taught by Professor Howlett during the Fall '08 term at Rhode Island.

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102209-BCH311 - Fig. 12-4, p.305 Table 12-2, p.304 I* -...

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