The Genetic Code (HL IB Biology)

• The sequence of DNA nucleotide bases found within a gene is determined by a triplet (three-letter) code
• Each sequence of three bases (i.e. each triplet of bases) in a gene codes for one amino acid
• These triplets code for different amino acids – 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 start and stop signals tell the cell where individual genes start and stop
• As a result, the cell reads the DNA correctly and produces the correct sequences of amino acids (and therefore the correct protein molecules) that it requires to function properly
• The genetic code is non-overlapping
  • Each base is only read once in which codon it is part of

• There are four bases, so there are 64 different codons (triplets) possible (4³ = 64), yet there are only 20 amino acids that commonly occur in biological proteins
  • This is why the code is said to be degenerate: multiple codons can code for the same amino acids
  • The degenerate nature of the genetic code can limit the effect of mutations

• The genetic code is also universal, meaning that almost every organism uses the same code (there are a few rare and minor exceptions)
• The same triplet codes code for the same amino acids in all living things (meaning that genetic information is transferable between species)
  • The universal nature of the genetic code is why genetic engineering (the transfer of genes from one species to another) is possible

• By observing the genetic code in the mRNA it is possible to determine the sequence of amino acids that are coded for in the polypeptide

mRNA codons and amino acids table

• During translation two tRNA molecules fit onto the ribosome at any one time, bringing the amino acid they are each carrying side by side
  • The ribosome will move along the mRNA molecule, one codon at a time
• A peptide bond is then formed (by condensation) between the two amino acids
  • The formation of a peptide bond between amino acids is an anabolic reaction
  • It requires energy, in the form of ATP
  • The ATP needed for translation is provided by the mitochondria within the cell
• This process continues until a ‘stop’ codon on the mRNA molecule is reached – this acts as a signal for translation to stop and at this point the amino acid chain coded for by the mRNA molecule is complete
• This amino acid chain is then released from the ribosome and forms the final polypeptide

The process of translation diagram

The translation stage of protein synthesis – an amino acid chain is formed