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15.1 Many Genes Encode Proteins

This article explores the fundamental concepts of genes encoding proteins through the one gene-one polypeptide hypothesis. It discusses how the genetic code determines the amino acid sequence of proteins, including the roles of codons, wobble hypothesis, and initiation and termination mechanisms in translation. Additionally, it examines the importance of transfer RNA (tRNA) in interpreting the code, challenges in mRNA surveillance, and the role of ribosome structure in protein synthesis. The article highlights the intricate processes involved in the translation of genetic information into functional proteins.

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15.1 Many Genes Encode Proteins

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  1. 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

  2. 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?

  3. 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? B  A  C

  4. 15.2 The Genetic Code Determines How the Nucleotide Sequence Specifies the Amino Acid Sequence of a Protein

  5. Breaking the Genetic Code

  6. Breaking the Genetic Code • Codon: a triplet RNA code

  7. 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

  8. The Degeneracy of the Code • Sense codons: encoding amino acid • Initiation codon: AUG • Termination codon: UAA, UAG, UGA

  9. Concept Check 2 Through wobble, a single can pair with more than one . • codon, anticodon • group of three nucleotides in DNA, codon in mRNA • tRNA, amino acid • anticodon, codon

  10. Concept Check 2 Through wobble, a single can pair with more than one . • codon, anticodon • group of three nucleotides in DNA, codon in mRNA • tRNA, amino acid • anticodon, codon

  11. 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

  12. 15.2 The Genetic Code Determines How the Nucleotide Sequence Specifies the Amino Acid Sequence of a Protein

  13. 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. • Nonoverlapping: A single nucleotide may not be included in more than one codon. • The universality of the code: near universal, with some exceptions

  14. 15.3 Amino Acid Are Assembled into a Protein Through the Mechanism of Translation

  15. The Binding of Amino Acids to Transfer RNAs • Aminoacyl-tRNA syntheses and tRNA charging • The specificity between an amino acid and its tRNA is determined by each individual aminoacyl-tRNA synthesis. There are exactly 20 different aminoacylt-tRNA syntheses in a cell.

  16. The Initiation of Translation • Initiation factors IF-3, initiator tRNA with N-formylmethionine attached to form fmet-tRNA • Energy molecule: GTP

  17. The Initiation of Translation • The Shine–Dalgarno consensus sequence in bacterial cells is recognized by the small unit of ribosome. • The Kozak sequence in eukaryotic cells facilitates the identification of the start codon.

  18. Elongation • Exit site E • Peptidyl site P • Aminoacyl site A • Elongation factors: Tu, Ts, and G

  19. Concept Check 3 In elongation, the creation of peptide bonds between amino acids is catalyzed by . • rRNA • protein in the small subunit • protein in the large subunit • tRNA

  20. Concept Check 3 In elongation, the creation of peptide bonds between amino acids is catalyzed by . • rRNA • protein in the small subunit • protein in the large subunit • tRNA

  21. Termination • Termination codons: UAA, UAG, and UGA • Release factors

  22. 15.4 Additional Properties of RNA and Ribosomes Affect Protein Synthesis • The three-dimensional structure of the ribosome • Polyribosomes: • An mRNA with several ribosomes attached

  23. 15.4 Additional Properties of RNA and Ribosomes Affect Protein Synthesis • Messenger RNA surveillance: • Detect and deal with errors in mRNA • Nonsense – mediated mRNA decay: eliminating mRNA containing premature termination codons • The posttranslational modifications of proteins

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