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

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?

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

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

Sequence of a Protein

Breaking the Genetic Code

• Codon: a triplet RNA code

Breaking the Genetic 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

The Degeneracy of the Code

• Sense codons: encoding amino acid

• Initiation codon: AUG

• Termination codon: UAA, UAG, UGA

The Degeneracy of the Code

Concept Check 2

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

a. codon, anticodon

b. group of three nucleotides in DNA, codon in mRNA

c. tRNA, amino acid

d. anticodon, codon

Concept Check 2

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

a. codon, anticodon

b. group of three nucleotides in DNA, codon in mRNA

c. tRNA, amino acid

d. 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

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

Sequence of a Protein

• 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

The Reading Frame and Initiation Codons

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

• 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.

The Binding of Amino Acids to Transfer RNAs

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

• Energy molecule: GTP

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.

The Initiation of Translation

• Exit site E

• Peptidyl site P

• Aminoacyl site A

• Elongation factors: Tu, Ts, and G

Elongation

Concept Check 3

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

a. rRNA

b. protein in the small subunit

c. protein in the large subunit

d. tRNA

Concept Check 3

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

a. rRNA

b. protein in the small subunit

c. protein in the large subunit

d. tRNA

• Termination codons: UAA, UAG, and UGA

• Release factors

Termination

• The three-dimensional structure of the ribosome

• Polyribosomes:– An mRNA with several ribosomes attached

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

15.4 Additional Properties of RNA and Ribosomes Affect Protein Synthesis