Biology: Chap. 10

Protein Synthesis: Translation

Overview

• Translation occurs after initiation.
• The tRNA is associated with the start codon, and the large subunit has come.
• The p site fits in this tRNA amino acid complex, containing the first amino acid.

Elongation Process

• Codon Recognition: A tRNA comes and associates with the codon.
  • The A site is empty, awaiting the next tRNA.
  • Example: If the codon is UUU, a tRNA with the anticodon AAA will associate with it.
  • This tRNA carries the amino acid phenylalanine.
• Peptide Bond Formation: The ribosome breaks the covalent bond between the tRNA and the first amino acid.
  • It catalyzes the formation of a covalent bond between the two amino acids.
  • Example: Methionine is attached to phenylalanine, forming the start of the protein.
• Translocation: The ribosome moves down the messenger RNA by one codon (three nucleotides).
  • The first tRNA moves to the E site (exit site).
  • The second tRNA moves to the P site, carrying the first two amino acids (e.g., phenylalanine and methionine).
  • The tRNAs in the E site exits; in eukaryotes, tRNAs are potentially one-time use only.
  • This process repeats with a new tRNA entering the A site.
  • Example: A third tRNA with anticodon ACG comes in. The corresponding codon being UGC codes for the amino acid cysteine.
  • The bond is broken, and a covalent bond forms between the existing chain and the new amino acid, then translocation occurs again.

Components and Their Roles

• Messenger RNA (mRNA): Contains codons, which are triplets of nucleotides.
  • Serves as the template for protein synthesis.
• Ribosome: Facilitates the process; has a small and large subunit.
  • E site: Exit site for tRNA.
  • P site: Holds the tRNA carrying the growing polypeptide chain.
  • A site: Entry site for the next tRNA.
• tRNA: Transfers amino acids to the ribosome corresponding to each codon.

Codons and the Genetic Code

• Codons are mRNA sequences. The chart provided typically shows mRNA codons.
• The coding sequence of DNA matches the mRNA sequence but with T instead of U.
• Example:
  • If the DNA non-coding sequence is ATC AAA, the coding sequence is ATG TTT.
• The genetic code dictates which codons correspond to which amino acids.

Translocation Defined

• Codon recognition: new tRNA enters A site.
• Peptide bond formation: bond is formed.
• Translocation: Ribosome moves down a notch.

Biological Significance

• Proteins are polymers of amino acids; the order of amino acids determines protein shape and function.
  • Example: Methionine, phenylalanine, cysteine, and lysine are amino acids.
• The messenger RNA sequence determines the order of amino acids in the protein.
• The mRNA sequence is based on the DNA sequence of the gene.

Termination

• Translation continues until the ribosome reaches one of three stop codons: UAA, UAG, and UGA.
• These codons do not code for an amino acid.
• A special protein inserts itself into the A site, causing the ribosome to dissociate and the protein to be released.
• In eukaryotes, mRNA is generally for one-time use only.

Genetic Code Universality and Mutations

• Genetic code is universal across nearly all living things.
  • Implications for genetic modifications.
  • Bacteria can make human proteins because they recognize the same genetic code.

Mutations

• Changes in DNA sequence can occur in various locations.

Location of Mutation and Effect

• Between Genes: Changes in these regions typically have no effect.
• Within a Gene: Changes in UTRs (untranslated regions) or introns have no effect on protein structure.
  • The vast majority of DNA changes occur in these regions, with no impact on biology.
• Within Exons (Coding Regions):
  • Silent Mutation: A change in the DNA sequence does not alter the amino acid coded for.
    • Example: UGC and UGU both code for cysteine.
    • No effect on protein shape or function.
  • Missense Mutation: A change in the DNA sequence alters the amino acid coded for.
    • Example: Changing ACG to ACC changes the codon from UGC (cysteine) to UGG (tryptophan).
    • Potential to change the organism's biology, leading to variable effects: no impact, protein breaking, or altered protein function.
  • Nonsense Mutation: A change in the DNA sequence results in a stop codon.
    • Example: Changing ACG to ACU changes the codon to UGA (stop codon).
    • Results in a broken or truncated protein with loss of function.
  • Indels/Frameshift Mutation: Insertion or deletion of a nucleotide shifts the reading frame.
    • Results in a completely different amino acid sequence after the mutation, often leading to non-functional proteins.