The Universal Genetic Code (Cambridge (CIE) A Level Biology): Revision Note

The universal genetic code

• A gene is a sequence of nucleotide bases in a DNA molecule that codes for the production of a specific sequence of amino acids

  • These amino acids in turn make up a specific polypeptide (protein)

• The DNA nucleotide base code found within a gene is a three-letter, or triplet, code

• Each sequence of three bases (in other words each triplet of bases) codes for one amino acid 

  • There are 20 different amino acids that cells use to make up different proteins

• For example:

  • CAG codes for the amino acid valine

  • TTC codes for the amino acid lysine

  • GAC codes for the amino acid leucine

  • CCG codes for the amino acid glycine

• Some of these triplets of bases code for start and stop signals

  • These signals tell the cell where individual genes start and stop

  • This ensures the cell:

    • Reads the DNA correctly (the code is non-overlapping)

    • Can produce the correct sequences of amino acids (and therefore the correct protein molecules) that it requires to function properly

• The genetic code is universal, meaning that almost every organism uses the same code

• This means that the same triplets code for the same amino acids in all living things

  • This means that genetic information is transferable between species