Molecular Basis of Genetic Code and Mutation

The genetic code is a triplet nucleotide sequence (codon) which encodes a specific amino acid during translation.

Figure 1: Genetic code

The three-letter codons consisting of four nucleotides found in mRNA (A, U, G, C) which produce a total of 64 different combinations. Of these 64 codons, 61 code for amino acids, the remaining three represent stop signals which trigger the end of protein synthesis (see Figure 1).

Properties of Genetic Code 

• Universal: A particular codon will encode the same amino acid in all living beings
• Multiple codons: Different codons code for the same amino acid e.g. UUC and UUU code for Phe (Phenylalanine)
• Non-ambiguous: Each triplet specifies only a single amino acid
• Start codon (AUG): Codes for Met (Methionine) and marks the beginning of translation
• Stop codons (UAG, UAA, and UGA): Terminate protein synthesis as they do not code for any amino acid

Mutations

• Any alterations in the DNA sequence will result in an amino acid change e.g., GAA (Glutamic Acid) changes to GAC (Aspartic Acid)
• Changes in the amino acid sequence will alter the structure of the protein e.g., GAA (Glutamic Acid) changes to UAA (Stop codon)
• The structural change in the protein will alter its function e.g., for extracellular protein mutations in ACG (Cysteine) to TAT (Tyrosine) leads to loss of disulfide bond and protein function
• Silent mutations alter the DNA structure, but do not result in amino acid and functional changes e.g., GAA (Glutamic Acid) change to GAG (Glutamic Acid)

Resources

• GLI Training package on LPA 2012

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