Sequence Decoding


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•  Deducing the DNA base sequence for the mRNA strand


mRNA is a complementary copy of a DNA segment (gene) and consequently can be used to deduce the gene sequence

For converting a sequence from mRNA to the original DNA code, apply the rules of complementary base pairing:

  • Cytosine (C) is replaced with Guanine (G) – and vice versa
  • Uracil (U) is replaced by Adenine (A)
  • Adenine (A) is replaced by Thymine (T)


Answer:        (DNA)   TAC  GGT  CAC  TGA  AGT  CCC  TGC  TTA  CTG  AAT

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•  Use a table of mRNA codons and their corresponding amino acids to deduce the sequence of amino acids

   coded by a short mRNA strand of known base sequence

mRNA → Polypeptide

In order to translate an mRNA sequence into a polypeptide chain, it is important to establish the correct reading frame

The mRNA transcript is organised into triplets of bases called codons, and as such three different reading frames exists

  • An open reading frame starts with AUG and will continue in triplets to a termination codon
  • A blocked reading frame may be frequently interrupted by termination codons

Once the start codon (AUG) has been located and reading frame established, the corresponding amino acid sequence can be deduced using the genetic code 


Answer:    (codons)  GU  AUG  CAC  GUG  ACU  UUC  CUC  AUG  AGC  UGA  U

Answer:   (amino acid)     Met    His     Val     Thr    Phe    Leu    Met    Ser   STOP

The Genetic Code (Grid)

genetic code (grid)