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4/19/2013
Translation
C483 Spring 2013
1. Use Figure 22.3 to determine which template strand DNA sequence (written in the 5' → 3' direc on) specifies the tripeptide with the sequence gly‐ala‐leu.
A) GGGGCTCTC B) CTCTCGGGG C) CCCCGAGAG D) GAGAGCCCC 2. Which is not true about the genetic code? A) Some amino acids share the same codon. B) The first two nucleotides of a codon are often enough to specify a given amino acid. C) Some codons do not specify an amino acid. D) Nearly all organisms use the same genetic code. 3. The unambiguity and degeneracy of the genetic code can best be respectively exemplified by which of the following?
A) UUU and UUC both code for Phe; UUU codes only for Phe.
B) UUU codes only for Phe; UUU and UUC both code for Phe.
C) UUU codes for both Phe and Ser; UUU and UUC both code for Phe and Ser.
D) UUU and UUC both code for Phe and Ser; UUU codes for both Phe and Ser.
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4. ________ carries the anticodon. A) rRNA
C) tRNA
B) mRNA D) DNA 5. Different tRNA molecules that bind the same amino acid are called ________ tRNA
molecules. A) synonymous C) homologous B) complementary D) isoacceptor
6. Which statement is false concerning aminoacyl‐tRNA synthetase? A) It recognizes only one amino acid, but may recognize more than one tRNA. B) It requires ATP. C) It catalyzes formation of a high‐energy amide bond. D) Aminoacyl‐tRNA synthetases are capable of proofreading mechanisms. 7. At the ribosome the template mRNA is translated in the ________ direction, while the protein is synthesized in the ________ direction. A) 5' → 3'; N‐terminal to C‐terminal B) 5' → 3'; C‐terminal to N‐terminal C) 3' → 5'; N‐terminal to C‐terminal D) 3' → 5'; C‐terminal to N‐terminal Information Flow
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Overview
• Genetic Code
• tRNA structure
• Aminoacyl‐tRNA
synthetase
• Ribosome structure
• Next lecture: initiation, elongation, termination
Codons
• Three letter codons
– Why 3?
• Overlapping vs
nonoverlapping
• 5’3’
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Reading Frame
Standard Genetic Code
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Genetic Code
• Unambiguous: Each codon has one and only one amino acid
• Degenerate: most amino acids have multiple codons (Leu has 6 codons)
• First two nucleotides (5’) often contain all information for amino acid (Gly—any others?)
• Similar codons often have similar AA
– Thr/Ser and Glu/Asp minimize mutations
• Stop codons (termination codon)
• Initiation codon
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tRNA structure
• Tertiary structure
• Anticodon arm
• Acceptor stem
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Primary and Secondary Structure
• Anticodon
• Antiparallel base pairing with mRNA
tRNAPhe
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Wobble
• 5’ site of anticodon is less demanding in complementarity
• Remember‐‐3’ of codon
• Inosinate often found at 5’
– Capable of more H‐
bonding
– Less selective
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Aminoacyl‐tRNA Synthetase
• tRNA serves to match the amino acid to the codon
• Requires covalent attachment of amino acid to tRNA
• A key step in fidelity of message even prior to initiation, elongation, termination!
• 20 synthetases serve to recognize correct AA and covalently attach isoacceptor tRNAs
Aminoacyl tRNA Synthetase Reaction
•
•
•
•
Covalently attach amino acid to acceptor arm
High energy bond
“activated’ amino acid
Costs 2 ATP equivalents
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Proofreading
•
•
•
•
Generally, very few mistakes
Some mistakes are more common
Require proofreading
1 in 105 error
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Translation • Translation complex made up of
– Ribosome
– Ribosome accessory proteins
– mRNA
– Aminoacyl tRNA
• mRNA read 5’3’
• Protein made N‐terminus to C‐terminus
Ribosome
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A site and P site
Summary
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Answers
1.
2.
3.
4.
5.
6.
7.
D
A
B
C
D
C
A
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