• 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