Note 46 - Translation - Reading

Translation Overview

To manufacture proteins from DNA, mRNA must exit the nucleus and enter the cytoplasm. Here, ribosomes bind to the mRNA, initiating the translation of nucleic acid into protein.

Ribosome Structure

  • Eukaryotic Ribosomes: Consist of a large (60S) and a small (40S) subunit.

  • The designation "S" refers to sedimentation rates, which correlate with size.

  • Ribosomes clamp the mRNA between the two subunits, moving along it from the 5' to 3' direction and adding amino acids to form a polypeptide by reading codons.

Reading Frame

  • The ribosome reads the mRNA in triplets (codons), which encode specific amino acids.

  • The reading frame can change depending on the starting nucleotide, underscoring the need for proper mRNA positioning.

  • Example sequences illustrate how different starting points create different reading frames.

Role of Transfer RNA (tRNA)

  • tRNA Function: Delivers the correct amino acids to the ribosome for polypeptide synthesis.

  • Structure resembles a cloverleaf, with an anticodon that pairs with mRNA codons and an acceptor site carrying the corresponding amino acid.

  • Each tRNA is specific to one amino acid, necessitating at least 20 different tRNAs to correspond to amino acids and the flexibility (wobble hypothesis) to recognize multiple codons coding for the same amino acid.

Protein Synthesis Steps

Elongation

  1. Aminoacyl-tRNA: A tRNA with its corresponding amino acid is called aminoacyl-tRNA, produced by enzymes known as aminoacyl–tRNA synthetases.

  2. Start Codon (AUG): The ribosome identifies the start codon, ensuring the correct reading frame and starting with methionine.

  3. Ribosome Sites: Ribosomes have two sites:

    • A site (Acceptor): Incoming tRNA binds here.

    • P site (Peptide): Holds the growing peptide chain.

  4. Peptide Bond Formation: As tRNAs enter the ribosome, amino acids form peptide bonds, elongating the chain.

  5. Translocation: The ribosome shifts, positioning the next codon in the A site for further tRNA binding.

Termination

  • When the ribosome reaches a stop codon (UGA, UAG, UAA), no corresponding tRNAs are present, and translation stops.

  • A release factor recognizes the stop codon, facilitating the polypeptide’s release and disassembly of the ribosomal complex.

Final Modifications

  • Post-translation modifications may occur, including glycosylation, phosphorylation, or cleavage of the protein, which is then directed to its function in the cell.

Summary of Translation Process

  1. Ribosome binds to mRNA and reads it, starting with the AUG codon specifying methionine.

  2. tRNAs deliver amino acids to the respective sites, forming peptide bonds as the ribosome transitions along the mRNA.

  3. Translation continues until a stop codon is reached, at which point the protein is released and possibly modified for its cellular role.