Transcript Chapter 15
LECTURE CONNECTIONS 15 | The Genetic Code and Translation © 2009 W. H. Freeman and Company 15.1 Many Genes Encode Proteins • The One Gene One Enzyme Hypothesis: • Genes function by encoding enzymes, and each gene encodes a separate enzyme. • More specific: one gene one polypeptide hypothesis. • In 1940s, Beadle and Tatum performed experiments in Neurospora (fungus). Precursor Group I Group II Group III mutations mutations mutations Ornithine Citrulline Arginine Concept Check 1 Auxotrophic mutation 103 grows on minimal medium supplemented with A, B, or C. Mutation 106 grows on medium supplemented with A and C, but not B; and mutation 102 grows only on medium supplemented with C. What is the order of A, B, C in a biochemical pathway? Proteins 15.2 The Genetic Code Determines How the Nucleotide Sequence Specifies the Amino Acid Sequence of a Protein • Codon: the set of bases that encode an aa. (The genetic code is a triplet code). • Which group of three nucleotides specify which amino acids? • In 1964, Nirenberg and Matthaei created a synthetic RNAs by using an enzyme called polynucleotide phosphorylase. • Nirenberg and Leder used ribosome-bound tRNAs to provide additional information about the genetic code. Short sequences of mRNA would bind to a ribosome. The codon on the short mRNA would then base pair with the matching anticodon on a tRNA that carried the aa specified by the codon. The Degeneracy of the Code • Each nucleotide can have one of four possible bases (A, G, C and U) at each nucleotide position, thus permitting 43 = 64 possible codons. • Sense codons: encoding amino acid (61 codons) • Initiation codon: AUG • Termination codon: UAA, UAG, UGA • 61 sense codons and 20 aa- the code contains more information than is needed to specify the aa: degenerate code The Degeneracy of the Code • Degenerate code: Amino acid may be specified by more than one codon. • Synonymous codons: codons that specify the same amino acid • Isoaccepting tRNAs: different tRNAs that accept the same amino acid but have different anticodons • Wobble hypothesis There are still more codons than anticodons, because different codons can sometimes pair with the same anticodon through flexibility in base pairing at the third position of the codon. The mRNA and tRNA pair in an antiparallel fashion. Pairing at the first and second codon positions is in accord with the Watson and Crick pairing rules (A with U, G with C); however, pairing rules are relaxed at the third position of the codon, and G on the anticodon can pair with either U or C on the codon in this example. • Wobble allows some tRNAs to pair with more than one codon on an mRNA; thus from 30 to 50 tRNAs can pair with 61 sense codons. Concept Check 2 Through wobble, a single than one . a. b. c. d. can pair with more codon, anticodon group of three nucleotides in DNA, codon in mRNA tRNA, amino acid anticodon, codon 15.1 Many Genes Encode Proteins • The One Gene One Enzyme Hypothesis: • Genes function by encoding enzymes, and each gene encodes a separate enzyme. • More specific: one gene one polypeptide hypothesis The Reading Frame and Initiation Codons • Reading frame: three ways in which the sequence can be read in groups of three. Each different way of reading encodes a different amino acid sequence. Initiation code AUG Termination code UAA, UAG, UGA • Nonoverlapping: A single nucleotide may not be included in more than one codon. • The universality of the code: near universal, with some exceptions