U C C C A A A G G G U U U U G C C C C U A G A G TA G AT C A U G A U U U C C A A

U c c c a a a g g g u u u u g c c c c u a g a g ta g

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U C C C A A A G G G U U U U G C C C C U A G A G TA G AT C A U G A U U U C C A A A G G G U U U C C C U A G DNA (Revertant allele- double mutant) DNA (Mutant allele) mRNA mRNA Protein Suppressor mutation (Single base-pair deletion) Met Ile Ser Gln Arg Val Leu A T Inserted base pair alters reading frame. C G Deletion base pair restores original reading frame. Altered amino acid sequence Mutant phenotype Wild-type phenotype Transcription Transcription Translation Translation AT G TA C A T T T A A G T A C A T C C C G G G A A A T T T G G G C C C T T T A A A C C C G G G TA G AT C A U G A U U G U A C C C A A A G G G U U U C C C U A G DNA (Gene) mRNA Protein Three base-pair insertions. Met Ile Val Pro Lys Gly Pro (term) Phe Original wild-type amino acid sequence Altered sequence with one amino acid added A T G C A T Wild-type phenotype Transcription Translation
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MBG 2040 33 directed the synthesis of a mixture of 3 homopolymers The Genetic Code - initiation and termination codons” - initiation codon: AUG - termination codons: UAA , UAG , UGA - degeneracy: partial degeneracy (pruine to pyrimidine at 3’ changes amino acids) or complete degeneracy (any of the bases at 3’ will not change amino acids) - ordered - nearly universal (exceptions: mitochondria and some protozoa) Key Points - each 20 amino acid is specified by one or more nucleotide triplets in mRNA - of the 64 possible triplets, given the 4 bases in mRNA, 61 specify amino acids and 3 signal chain termination - the code is non-overlapping, with each nucleotide part of a single codon, degenerate, with most amino acids specified by two or four codons, and ordered, with similar amino acids specified by related codons - the genetic code is nearly universal with minor exceptions, the 64 triplets have the same meaning in all organisms The Wobble Hypothesis - codons in mRNA molecules are recognized by aminoacyl-tRNAs during translation Codon-tRNA Interactions - translation of a sequence of nucleotides in mRNA into the correct sequence of amino acids in the polypeptide products requires the accurate recognition of codons by aminoacyl- tRNAs - because of degeneracy of the genetic code, either several different tRNAs must recognize the different codons specifying a given amino acid or the anticodon of a given tRNA must be able to base-pair with several different codons - both of the phenomena occur - several tRNAs exist for certain amino acids, and some tRNAs recognize more than one codon Wobble Hypothesis - hydrogen bonding between the bases in the anticodons of tRNAs and the codons of mRNAs follows strict base-pairing rules only for the first 2 bases of the codon - the base pairing involving the third base of the codon is less stringent allowing “wobble” at this site - based on molecular distances and steric (3D structure) considerations, Crick proposed that wobble would allow several types, but not all types, of base-pairing at the third codon base during the codon-anticodon interaction TABLE 12.2 Base-Pairing between the 5 Base of the Anticodons of tRNAs and the 3 Base of Codons of mRNAs According to the Wobble Hypothesis Base in Anticodon Base in Codon G U or C C G A U U A or G I A, U, or C
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MBG 2040 Base-Pairing with Inosine at the Wobble Position -
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