2.3.3 Nature of the Genetic Code

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 (AUG – 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

• The non-overlapping nature of the genetic code means that each base is only read once 
• The adjacent codons do not overlap
• A non-overlapping code means that the same letter is not used for two different codons; in other words, no single base can take part in the formation of more than one codon

The genetic code is degenerate

• 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 mRNA codons and their amino acids, showing the degenerate nature of the genetic code

The genetic code is universal

• 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

• 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)