Overlap for example if aug encodes met then only cys

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Unformatted text preview: two - Frameshift No Three + or three - No change or small changes Yes Overlap? • If the code is overlapping, then this sequence: # # # # AUG CAA # could encode four amino acids (AUG-UGC-GCA-CAA) • However, this predicts that the next amino acid in a protein sequence is constrained by the one that came before it. Overlap? • For example: If AUG encodes met, then only cys and trp can follow met, since they are the only two amino acids encoded by UG_. • No such constraints are observed in protein sequence • In addition, a single point mutation changes only one, not three, amino acids Is the code degenerate? • Early evidence: – frameshift studies showed that you can revert a protein back to wildtype sequence by combining two mutations – 64 codons, but only 20 amino acids - are the other 44 “nonsense” codons? Cracking the Code • Depended on the development of an in vitro protein synthesizing system – All of the cellular components of protein synthesis, plus artificially synthesized RNA template molecules – Polynucleotide phosphorylase - requires no template and allows random connecting of nucleotides Cracking the Code – RNA molecules produced by polynucleotide phosphorylase contain nucleotides in the proportions they were provided in the test tube – All 20 amino acids are present in the tube, but only one of them is radioactively labeled Homopolymers RNA polymer Polypeptide encoded poly-A poly-lysine poly-C poly-proline poly-G poly-glycine poly-U po...
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This document was uploaded on 02/25/2014.

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