Protein Synthesis:

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Lecture 15 Protein Synthesis Translation of
the Genetic Message Chapter 12
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Translating the Genetic Message

• General steps in protein synthesis.
• The genetic code.
• Chain initiation.
• Chain elongation.
• Chanin termination.

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Aminoacyl-tRNA
mRNA aminoacyl-tRNA bind to ribosome
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The genetic code Triplet Nonoverlapping Commaless
Degenerate Universal
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Why 3 bases? 20 amino acids 1 base ? 4 amino
acids
2 bases ? 42 16 amino acids
3 bases ? 43 4 x 4 x 464
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Nonoverlapping
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Commaless
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Degenerate 1 amino acid may have more than
1 codon. Leucine has 6
However, a particulr codon only codes for 1
amino acid not ambiguous CUU only codes for
leucine.
3 stop codons
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Universal all organisms have the same code.
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(No Transcript)
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How do we know which triplet codes for which
amino acid?
Poly-U UUUUU ? phenylalanine
ACACACA ? alternating threonine and histidine.
Reading frames.
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General Characteristics of the Code

• Certain protections against mutaition.
• For 32 codons coding for 8 amno acids, the
• third base is irrelevant.
• A U in the second place means it codes for a
• hydrophobic amino acid.

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Formation of an Amino-acyl tRNA
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t-RNA
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Amino acid Attached here.
tRNA
Anticodon loop
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Formation of aminoacyl-tRNA
Why is the ATP needed?
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ATP adenine-ribose-phosphate-phosphate-phosphat
e
Break bonds release energy.
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Role of ATP in supplying energy for reactions.
Typical A B ? A-B
A ATP ? AP ADP AP ADP P ? A-B P
Adenine-ribose-PPP
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Amino acid activation.
aa tRNA ? aa-tRNA
aa ATP ? aa-AMP PPi
aa-AMP tRNA ? aa-tRNA
Adenine-ribose-PPP
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Amino acid ATP?aminoacyl-AMP PPi
aminoacyl-AMP tRNA ? aminoacyl-tRNA

AMP
Amino acid ATP ? aminoacyl-tRNA
AMP PPi
Both steps catalyzed by An aminoacyl-tRNA
synthetase 1 per aa.
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Protein synthesis occurs on ribosomes. The
prokaryotic ribosome has two subunits 30s and
50s
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30s
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50s ribosomal subunit
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30s 50s form the 70s ribosome
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Chain initiation formyl methionine Two
methionine tRNAs tRNAmet all methionines
inside the protein. met tRNAmet ?
met-tRNAmet
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tRNAfmet
fmet formyl methionine
makes
fmet-tRNAfmet N-terminal amino acid
in prokaryotes. Made in two steps.
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Chain initiation formalmethionine
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Chain initiation formalmethionine
fmet-tRNAfmet
tetrahydrofolate
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What happens to met-tRNAmet? Nothing. No formal
group added.