Translation

Genetic Code and Protein Synthesis

• Learning Objectives: Use the genetic code to write the amino acid sequence for a segment of mRNA.

Overview of the Genetic Code

• mRNA carries a genetic code in triplet code words known as codons.
• There are 64 codons that correspond to 20 different amino acids necessary for protein synthesis.
• Start Codon: AUG
  • Signals the beginning of protein synthesis.
• Stop Codons: UGA, UAA, UAG
  • Signifies the end of the protein synthesis process.

Codon Table and Corresponding Amino Acids

• The codon-to-amino acid mapping is crucial for protein synthesis:
  • Example codons:
  • UCU → Serine (Ser)
  • UUU → Phenylalanine (Phe)
  • GGA → Glycine (Gly)
  • AUG → Methionine (Met)
• Codon table includes various combinations of nucleotides, where the first and second letter are key in determining the amino acid coded from the third letter in the triplet.

Example Calculation of Amino Acids

• Example mRNA Segment: 5’—CCU—AGC—GGA—CUU—3’
  • Break down of codons:
  • CCU = Proline (Pro)
  • AGC = Serine (Ser)
  • GGA = Glycine (Gly)
  • CUU = Leucine (Leu)
• Amino Acid Sequence: Proline - Serine - Glycine - Leucine

Advantages of the Genetic Code

• Degeneracy: Genetic code is highly degenerate; most amino acids can be coded by multiple codons.
  • This redundancy helps reduce errors during translation.
• Mutation Resistance: Changes in the third nucleotide have minimal effects on the amino acid produced, providing robustness against mutations.
• Error Impact: If the first base in a codon is altered, it is more likely to change the corresponding amino acid than if the third base is changed.

Transfer RNA (tRNA)

• Role of tRNA: During translation, tRNA molecules convert the genetic code from mRNA into protein.
  • Each tRNA carries a specific amino acid and has an anticodon that pairs with the mRNA codon.

Protein Synthesis Process

1. Activation

• tRNA Activation: tRNA molecules pick up specific amino acids matching their anticodons.
  • Occurs when an amino acid is attached to the tRNA molecule.

2. Initiation and Elongation

• Initiation: A start codon (AUG) binds the first tRNA carrying methionine.
• As the ribosome moves along the mRNA:
  • New amino acids are added to the growing polypeptide chain via peptide bonds.

3. Termination

• The ribosome encounters a stop codon (UGA, UAA, UAG) that does not have a corresponding tRNA.
• This signals the end of polypeptide synthesis, causing the synthesis to terminate and the polypeptide to be released.
• tRNA Recycling: Used tRNA molecules re-enter the free tRNA pool to pick up new amino acids for subsequent translations.