• Translation is where RNA is turned into proteins for the cell to use.
  • In Eukaryotes, after transcription & processing mRNA, tRNA, and ribosomal subunits are transported across the nuclear envelope and into the cytoplasm
    • In cytoplasm, a specific amino acid attaches to each of tRNAs using energy from ATP
  • In organisms that lack a nucleus/nuclear envelope, like bacteria and archaea, transcription and translation can happen at the same time → quicker
  • Proteins that will stay in the cytoplasm are translated into the cytoplasm.
  • Proteins that will be secreted out of the cell, transported to an organelle (ex: lysosome), or inserted into a membrane are translated into the endoplasmic reticulum
    • Signal peptide: translocate/guide polypeptides across ER membrane

Details Prelude

  1. Translation has three steps: initiation, elongation, termination
  • Sequence of codons on mRNA determines the sequence of amino acids in polypeptide to be synthesized
  1. Ribosome has three binding sites for tRNA
  • A site (amino acid/acceptor): accepts incoming tRNA carrying an amino acid
  • P site (polypeptide): the second position, holds tRNA with a growing chain of amino acids
  • E site (exit): in the third position, tRNA and polypeptide exit ribosome                                                                             
  1. Wobble: rules for base pairing between the third base of a codon and tRNA is flexible
  • So there are fewer tRNAs than possible amino acid codons
  1. Energy for translation is provided by GTP at each stage
  • GTP energizes the formation of the initiation complex, using initiation factors.
  1. Once polypeptide is completed, interactions among amino acids give it its secondary and tertiary structures + processes by ER or Golgi body may make final modification before the protein can function
  • A protein’s size and function is determined by the size and chemical properties of its amino acids                     

Process Steps

  1. Initiation: begins when the small ribosomal subunit recognizes & attaches to 5’ cap of mRNA
  • A tRNA (with anticodon UAC) carrying the amino acid methionine attaches to mRNA at the start codon AUG
  • Large ribosomal subunit attaches to the mRNA with the tRNA → ribosome fully assembled with tRNA occupying the P site
  1. Elongation: continues as each tRNA delivers an amino acid one by one according to mRNA sequence
  • A newly arriving tRNA attaches to the first binding site (A site)
  • rRNA molecule of the large ribosomal subunit catalyzes the formation of a peptide bond between the carboxyl end of the growing polypeptide in the P site and the amino group of the new amino acid in the A site
    • Removes the polypeptide from the tRNA in the P site and attaches it to the amino acid on the tRNA in the A site
  • Ribosome translocates the empty tRNA in the P site to the E site & tRNA in the A site to the P site
    • This leaves the A binding site empty → new tRNA can arrive
  1. Termination: occurs when ribosome encounters stop codon → A site accepts a release factor
  • Release Factor: protein shaped like a tRNA which binds to stop codon in A site instead of tRNA
  • GTP hydrolysis dissociates completed polypeptide, last tRNA, and two ribosomal subunits are released
  • Ribosomal subunits can attach to same or another mRNA and repeat the process

Process Efficiency

  • Transcription: Multiple RNA polymerase complexes can transcribe the same gene all at once, → cells can make many mRNA copies of a single gene.

Translation: Multiple ribosomes can translate the same mRNA all at once, lined up one behind the other → cell can make many copies of the same protein at a time

                  Hydrolysis of GTP increases the accuracy and efficiency


  • RNA molecules that function as an enzyme!

Structure and Functions:

  1. Have specific catalyst activity bcuz of HB and functional groups

                  Catalyze formation of peptide bonds and removal of introns