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CHAPTER 39: The Genetic CodeProblems 1,3,5,7-9,12,13,15,18-19,21,23,25-26
1. Three nucleotides encode an amino acid.
2. The code is nonoverlapping.
3. The code has “no” punctuation.
4. The code has directionality.
5. The code is degenerate.
39:1 The Genetic Code Links Nucleic Acid and Protein Information
•DNA – only four bases (A,T,G,C)•Must code for 20 amino acids•Two-base code: 42 = 16 combinations•Four-base code: 44 = 256 combinations•Three-base code: 43 = 64 combinations
Three nucleotides encode an amino acid.
The code is nonoverlapping
Transfer RNA Molecules Have a Common Design
1. 73-93 ribonucleotides.2. L-shaped 3-D structure.3. Unusual bases.4. “Stems”: 3’-CCA acceptor
stem, TΨC loop, DHU loop, and anticodon loop.
5. 5’-Phosphorylation.6. Amino acid attached to 3’-
CCA.7. Anticodon near center of
sequence.
Some Transfer RNA Molecules Recognize More Than One Codon Because of Wobble in Base-
Pairing• tRNA molecules are
named for the amino acid that they carry (e.g. tRNAPhe)
• Base pairing between codon and anticodon is governed by rules of Watson-Crick (A-U, G-C)
• However, the 5’ anticodon position has some flexibility in base pairing (the “wobble” position)
AAG
3’ 5’
UUC 3’5’
mRNA(Phe)
Wobble position
Variable position
tRNAPhe
(5’) (3’)
39:2 Amino Acids Are Activated by Attachment to Transfer RNA
Amino Acid + ATP aminoacyl-AMP + PPi
aminoacyl-AMP + tRNA aminoacyl-tRNA + AMPPPi + H2O 2 Pi
Amino Acid + tRNA + H2O aminoacyl-tRNA + AMP + 2Pi
Synthetases Have Highly Discriminating Amino Acid Activation Sites
• Attachment of the correct amino acid to the corresponding tRNA is a critical step
• Synthetase binds ATP and the correct amino acid (based on size, charge, hydrophobicity)
• Synthetase then selectively binds specific tRNA molecule based on structural features
• Synthetase may recognize the anticodon as well as the acceptor stem