Genetic Code

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•  Codons of three bases on mRNA correspond to one amino acid in a polypeptide

•  The amino acid sequence of polypeptides is determined by mRNA according to the genetic code


  • The base sequence of an mRNA molecule encodes the production of a polypeptide
  • The mRNA sequence is read by the ribosome in triplets of bases called codons
  • Each codon codes for one amino acid with a polypeptide chain
  • The order of the codons in an mRNA sequence determines the order of amino acids in a polypeptide chain

Genetic Code

  • The genetic code is the set of rules by which information encoded within mRNA sequences is converted into amino acid sequences (polypeptides) by living cells
  • The genetic code identifies the corresponding amino acid for each codon combination
  • As there are four possible bases in a nucleotide sequence, and three bases per codon, there are 64 codon possibilities (43)
  • The coding region of an mRNA sequence always begins with a START codon (AUG) and terminates with a STOP codon

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•  Use a table of the genetic code to deduce which codon(s) correspond to which amino acid

Typically the genetic code shows the codon combinations expressed on an mRNA molecule

  • Tables displaying the genetic code may occasionally show the sequence on the sense strand of DNA (non-coding strand)
  • These sequences are identical to the mRNA codons with the exception of thymine (T) being present instead of uracil (U)

The Genetic Code (Wheel)

genetic code (wheel)