Once it was realized that the sequence of bases in a gene specified the sequence of amino acids in a protein, various possibilities for such a genetic code were considered. How many bases are necessary to specify each amino acid? Is the code overlapping or nonoverlapping? Is the code punctuated or continuous? | Chapter 32 The Genetic Code to accompany Biochemistry, 2/e by Reginald Garrett and Charles Grisham All rights reserved. Requests for permission to make copies of any part of the work should be mailed to: Permissions Department, Harcourt Brace & Company, 6277 Sea Harbor Drive, Orlando, Florida 32887-6777 Outline Elucidating the Genetic Code The Nature of the Genetic Code The Second Genetic Code Codon-Anticodon Pairing, Third-Base Degeneracy and the Wobble Hypothesis Codon Usage Nonsense Suppression Translating the Message How does the sequence of mRNA translate into the sequence of a protein? What is the genetic code? How do you translate the "four-letter code" of mRNA into the "20-letter code" of proteins? And what are the mechanics like? There is no obvious chemical affinity between the purine and pyrimidine bases and the amino acids that make protein. As a "way out" of this dilemma, Crick proposed "adapter molecules" - they are tRNAs! The Collinearity of Gene and Protein Structures Watson and Crick's structure for DNA, together with Sanger's demonstration that protein sequences were unique and specific, made it seem likely that DNA sequence specified protein sequence Yanofsky provided better evidence in 1964: he showed that the relative distances between mutations in DNA were proportional to the distances between amino acid sunstitutions in E. coli tryptophan synthase Elucidating the Genetic Code A triplet code is required: 43 = 64, but 42 = 16 - not enough for 20 amino acids But is the code overlapping? See Figure And is the code punctuated? The Nature of the Genetic Code A group of three bases codes for one amino acid The code is not overlapping The base sequence is read from a fixed starting point, with no punctuation The code is degenerate (in most cases, each amino acid can be designated by any of several triplets Biochemists Break the Code Assignment of "codons" to their respective amino acids was achieved by in vitro . | Chapter 32 The Genetic Code to accompany Biochemistry, 2/e by Reginald Garrett and Charles Grisham All rights reserved. Requests for permission to make copies of any part of the work should be mailed to: Permissions Department, Harcourt Brace & Company, 6277 Sea Harbor Drive, Orlando, Florida 32887-6777 Outline Elucidating the Genetic Code The Nature of the Genetic Code The Second Genetic Code Codon-Anticodon Pairing, Third-Base Degeneracy and the Wobble Hypothesis Codon Usage Nonsense Suppression Translating the Message How does the sequence of mRNA translate into the sequence of a protein? What is the genetic code? How do you translate the "four-letter code" of mRNA into the "20-letter code" of proteins? And what are the mechanics like? There is no obvious chemical affinity between the purine and pyrimidine bases and the amino acids that make protein. As a "way out" of this dilemma, Crick proposed "adapter molecules" - they are tRNAs! The Collinearity .
