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

The code has “no” punctuation.

The code has directionality and is degenerate.

(5’-UUC-3’)(5’-UUU-3’)

The Genetic Code is Nearly Universal

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’)

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 + 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

Val-tRNAThr does not happen.However, Ser-tRNAThr can occur 1/100 or 1/1000 times.

Too Small

Too Large

Proofreading by Aminoacyl-tRNASynthetases

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’