2.3.6 The Genetic Code

• A gene is a sequence of nucleotides that forms part of a DNA molecule (one DNA molecule contains many genes)
• This sequence of nucleotides (the gene) codes for the production of a specific polypeptide (protein)
• Protein molecules are made up of a series of amino acids bonded together
• The shape and behaviour of a protein molecule depends on the exact sequence of these amino acids (the initial sequence of amino acids is known as the primary structure of the protein molecule)
• The genes in DNA molecules, therefore, control protein structure (and as a result, protein function) as they determine the exact sequence in which the amino acids join together when proteins are synthesised in a cell

A gene is a sequence of nucleotides that codes for the production of a specific protein molecule (polypeptide)

The triplet code

• 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 codes 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

Codons and anticodons

• Once mRNA has been formed and left the nucleus it moves to the ribosomes where it can as a template for protein synthesis
• Each triplet within the mRNA code is described as a codon
• The tRNA molecules that transfer amino acids possess anticodons which are complementary to the codons on mRNA

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