15.1 Many Genes Encode Proteins

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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

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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?
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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

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

• Codon a triplet RNA code

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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

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The Degeneracy of the Code

• Sense codons encoding amino acid
• Initiation codon AUG
• Termination codon UAA, UAG, UGA

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Concept Check 2
Through wobble, a single can
pair with more than one .

1. codon, anticodon
2. group of three nucleotides in DNA, codon in mRNA
3. tRNA, amino acid
4. anticodon, codon

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Concept Check 2
Through wobble, a single can
pair with more than one .

1. codon, anticodon
2. group of three nucleotides in DNA, codon in mRNA
3. tRNA, amino acid
4. anticodon, codon

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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

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

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

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The Initiation of Translation

• Initiation factors IF-3, initiator tRNA with
  N-formylmethionine attached to form fmet-tRNA
• Energy molecule GTP

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The Initiation of Translation

• The ShineDalgarno 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.

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Elongation

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

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Concept Check 3
In elongation, the creation of peptide bonds
between amino acids is catalyzed by
.

1. rRNA
2. protein in the small subunit
3. protein in the large subunit
4. tRNA

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Concept Check 3
In elongation, the creation of peptide bonds
between amino acids is catalyzed by
.

1. rRNA
2. protein in the small subunit
3. protein in the large subunit
4. tRNA

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Termination

• Termination codons UAA, UAG, and UGA
• Release factors

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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

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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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