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U C CC A AA G GG U UUU GCC C C U AGA GTA GAT CA U GA U UU C CA A AG G G U U UC C CU A GDNA(Revertant allele-double mutant)DNA(Mutant allele)mRNAmRNAProteinSuppressormutation(Singlebase-pairdeletion)MetIleSerGlnArgValLeuATInserted base pair alters reading frame.CGDeletion base pair restores original reading frame.Altered amino acid sequenceMutantphenotypeWild-typephenotypeTranscriptionTranscriptionTranslationTranslationAT GTA CAT TTA AGTACATC C CG G GA A AT T TG G GC C CT T TA A AC C CG G GTA GAT CA U GA U UG U A C C CA A AG G GU U UC C CU A GDNA(Gene)mRNAProteinThree base-pair insertions.MetIleValProLysGlyPro(term)PheOriginal wild-type aminoacid sequenceAltered sequencewith one aminoacid addedATGCATWild-typephenotypeTranscriptionTranslation
MBG 204033directed 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 the64 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 12.2Base-Pairing between the 5′Base of the Anticodons of tRNAs and the 3′Base of Codons of mRNAs According to the Wobble HypothesisBase in Anticodon Base in CodonG U or CC GA UU A or GI A, U, or C
MBG 2040Base-Pairing with Inosine at the Wobble Position -