The Genetic Code (DP IB Biology)

Features of the Genetic 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 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

Deducing Amino Acid Sequences

• 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

Elongation of the Polypeptide Chain

• 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