Translation

  • Overview of Translation Process

    • Translation occurs at the ribosomes, leading to the formation of a polypeptide chain from mRNA.

  • Ribosome Structure

    • Ribosomes are composed of two subunits:
    • Large Subunit (60S)
    • Small Subunit (40S)
    • Combined they yield a structure of 80S instead of an expected 100S due to molecular interactions.
    • Binding of mRNA: The 5' cap of modified mRNA binds tightly between the two ribosomal subunits.

  • Reading mRNA

    • Ribosomes read the mRNA in a 5' to 3' direction and in frames of three nucleotides (known as codons).
    • Start Codon: The codon AUG is recognized as the start codon, coding for methionine.

  • Example of Reading Frame

    • A correct reading frame begins with AUG. An incorrect start point (e.g., second nucleotide) would alter the entire sequence of amino acids.

  • Transfer RNA (tRNA)

    • tRNAs deliver specific amino acids to the ribosome based on their anticodons.
    • tRNA has a cloverleaf structure with the amino acid attached to the 3' end and the anticodon at the opposite end.
    • Complementary Base Pairing: Each tRNA has a unique anticodon that pairs with the mRNA codon (e.g., mRNA codon UUC pairs with anticodon AAG).

  • Ribosome Sites for tRNA

    • A Site (Acceptor Site): Where new tRNA enters.
    • P Site (Peptide Site): Where the growing polypeptide chain is formed.
    • Initiation of Protein Synthesis: The tRNA carrying methionine enters directly into the P site.

  • Peptide Bond Formation

    • As the ribosome moves (translocates) along the mRNA, tRNAs shift positions, and peptide bonds form between adjacent amino acids.
    • Example: The first tRNA brings methionine to the P site, and subsequent tRNAs enter the A site before moving to the P site for bonding.

  • Elongation and Termination

    • The elongation continues until the ribosome encounters a stop codon (UAA, UAG, UGA).
    • Stop codons lack corresponding tRNAs, leading to release factors that facilitate the detachment of the newly formed polypeptide from the ribosome.

  • Post-Translational Modifications

    • After release, polypeptides may undergo modifications such as:
    • Addition of sugar moieties.
    • Addition of phosphate groups.
    • Cleavage by enzymes.
    • The folding of proteins typically begins after 30-40 residues are synthesized; they may also associate with other polypeptides if required.