Download CHAPTER 39: The Genetic Code

CHAPTER 39: The Genetic Code
Problems 1,3,5,7-9,12,13,15,18-19,21,23,25-26
39:1 The Genetic Code Links Nucleic Acid and
Protein Information
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.
Three nucleotides encode an amino acid.
•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
The code is nonoverlapping
The code has “no” punctuation.
The code has directionality and is degenerate.
(5’-UUU-3’)
(5’-UUC-3’)
The Genetic Code is Nearly Universal
Transfer RNA Molecules Have a Common Design
1.
2.
3.
4.
73-93 ribonucleotides.
L-shaped 3-D structure.
Unusual bases.
“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 BasePairing
• 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 (AU, G-C)
• However, the 5’
anticodon position has
some flexibility in
base pairing (the
“wobble” position)
3’
5’
(5’)
(3’)
tRNAPhe
AAG
5’
UUC
mRNA
(Phe)
Wobble position
3’
Variable position
The Synthesis of Long Proteins Requires a Low
Error Frequency
39:2 Amino Acids Are Activated by
Attachment to Transfer RNA
Amino Acid + ATP
aminoacyl-AMP + tRNA
PPi + H2O
Amino Acid + tRNA + H2O
aminoacyl-AMP + PPi
aminoacyl-tRNA + AMP
2 Pi
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
Val-tRNAThr does not happen.
However, Ser-tRNAThr can occur 1/100 or 1/1000 times.
Proofreading by Aminoacyl-tRNA
Synthetases
Too Small
Too Large
Aminoacyl-tRNA Synthetases Recognize
Anticodon Loop and Acceptor Stem
39:3 A Ribosome is a Ribonucleoprotein
DNA Replication: 5’→3’
Transcription: 5’→3’
Translation: 5’→3’