The Universal Genetic Code (CIE A Level Biology)

• A gene is a sequence of nucleotide bases in a DNA molecule that codes for the production of a specific sequence of amino acids, that 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 (TAC – methionine) 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) and can produce the correct sequences of amino acids (and therefore the correct protein molecules) that it requires to function properly
• There are four bases so there are 64 different triplets possible (4³), yet there are only 20 amino acids that commonly occur in biological proteins.
  • This results in multiple triplets coding for the same amino acids thus the code is said to be degenerate
  • The advantage of a degenerate genetic code is that it can limit the effect of mutations
• The genetic code is universal, meaning that almost every organism uses the same code (there are a few rare and minor exceptions)
• This means that the same triplets code for the same amino acids in all living things (meaning that genetic information is transferable between species)

Genetic Code Diagram

A DNA molecule with the triplet code for the start amino acid (methionine) and valine