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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
– 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
– 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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