Protein Synthesis

Protein synthesis is the making of a protein.

It is carried out by a ribosome.

A ribosome

Protein synthesis involves three distinct stages: transcription; translation; and protein folding

1. Transcription

  • Transcription is the making of messenger RNA using a DNA template.
  • Enzymes unwind the double helix and separate the two strands by breaking the hydrogen bonds between the bases where the gene is located
  • RNA polymerase synthesises messenger RNA (mRNA) using one of the strands of DNA as a template

2. Translation

  • Translation is the making of a protein using the code in mRNA.
  • mRNA moves to cytoplasm and combines with a ribosome made up of two ribosomal RNA (rRNA) subunits
  • A codon (or a ‘triplet’) is a sequence of three bases present on mRNA or DNA.
  • Each codon ‘tells’ the ribosome to do one of three things:
    • Start making a protein (start codon)
    • Add on an amino acid to the growing chain of amino acids
    • Stop making the protein and rlease the chain of amino acids (stop codon).
  • Transfer RNA (tRNA) is another type of RNA – it is found free-floating in the cytoplasm and is responsible for carrying one amino acid. (Remember amino acids are the building blocks of proteins)
  • tRNA has a 3-base (triplet) sequence called the anti-codon that is complementary to a particular codon on the mRNA.
  • An anti-codon is a sequence of three bases present on tRNA.
  • Each tRNA in turn (with its own amino acid) lines up with the mRNA in the ribosome and an amino acids are joined together by peptide bonds in a long polypeptide line which will form the protein
  • This process continues until the stop codon on the mRNA is reached at which point all the translation machinery separates and the protein is released.
Transcription and translation

3. Protein Folding

  • The chain of amino acids leaves the ribosome and is sent to specific areas of the cell where it is packaged and folded into its functional shape.
  • It goes and carries out its specific function (e.g. enzyme action)