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Lecture.15.Fall.2013(1)

# Aaa aac aca caa cca cac acc ccc example so we have

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Unformatted text preview: ly-phenylalanin So: AAA encodes lysine CCC encodes proline GGG encodes glycine UUU encodes phenylalanine Mixed Copolymers • Mixed Copolymers – Two or more ribonuclotides used in known proportions – Probability determines the codon distribution in the mixture • Example: Mix A and C at a ratio of 1:5 • 1/6 of total is A, 5/6 is C • Possible codons: AAA, AAC, ACA, CAA, CCA, CAC, ACC, CCC Example • So we have probabilities for codon usage: – AAA = (1/6)3 = 0.4% – Any combination of 1C:2A = (5/6)(1/6)2 = 2.3% each – Any combination of 2C:1A = (5/6)2(1/6) = 11.6% each – CCC = (5/6)3 = 57.9% • We already know: – AAA = lysine – CCC = proline Results Amino Acid Percentage Probable codons Lysine < 1% AAA (0.4%) Proline 69% CCC (57.9%) + 2C:1A (11.6%) Glutamine 2% 1C:2A (2.3%) Asparagine 2% 1C:2A (2.3%) Threonine 12% 2C:1A (11.6%) Histidine 14% 2C:1A (11.6%) + 1C: 2A (2.3%) Is The Code Degenerate? • Probably - we have eight possible codons encoding only six amino acids • We still don’t know exactly which codons encode which amino acids Still More Experiments • Triplet binding assay used radioactively labeled amino acids found in tRNAs to test directly what amino acid a codon encoded • Gobel Khorana: Used repeating short RNA sequences (e.g. polyUG) to further deﬁne and ﬁnally unravel the code The code is degenerate 61 of the 64 codons encode an amino acid - the code is “degenerate” • “Start” = AUG = methionine • “Stop” =...
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