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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
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
Breaking the Genetic Code
• Codon: a triplet RNA 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
The Degeneracy of the Code
• Sense codons: encoding amino acid
• Initiation codon: AUG
• Termination codon: UAA, UAG, UGA
Concept Check 2
Through wobble, a single
more than one
.
can pair with
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
more than one
.
can pair with
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
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
15.3 Amino Acid Are Assembled into a Protein
Through the Mechanism of Translation
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.
The Initiation of Translation
• Initiation factors IF-3, initiator tRNA with Nformylmethionine 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.
Elongation
• Exit site E
• Peptidyl site P
• Aminoacyl site A
• Elongation factors: Tu, Ts, and G
Concept Check 3
In elongation, the creation of peptide bonds
between amino acids is catalyzed by
a.
b.
c.
d.
rRNA
protein in the small subunit
protein in the large subunit
tRNA
.
Concept Check 3
In elongation, the creation of peptide bonds
between amino acids is catalyzed by
a.
b.
c.
d.
rRNA
protein in the small subunit
protein in the large subunit
tRNA
.
Termination
• Termination codons: UAA, UAG, and UGA
• Release factors
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
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