Nucleic Acid & Amino Acid Sequence Comparison (AQA A Level Biology): Revision Note

Linking DNA to polypeptides

• When comparing the genetic code to amino acid sequences, mRNA codons are often used

• The four bases found in RNA molecules (adenine, uracil, cytosine and guanine) can form 64 different codons, yet there are only 20 amino acids that are coded for

• Therefore, the genetic code is degenerate

  • Multiple mRNA codons can encode the same amino acid; e.g., UGU and UGC both code for the amino acid cysteine

  • This means that a change in the genetic code doesn’t necessarily result in a change in the amino acid sequence

• Some codons send important signals to the translation machinery

  • The START codon marks the start of the protein and therefore initiates the process of translation from the right location (this is always the amino acid methionine in eukaryotic cells, coded for by the codon AUG)

  • STOP codons cause translation to terminate at the end of the protein and do not code for any amino acids, e.g. UAA

• The genetic code is non-overlapping

  • Each base is only read once in the codon that it is part of

• The number of amino acids in a protein can be calculated using the number of coding nucleotides in the mRNA molecule and vice versa:

  • Each amino acid is coded for by a triplet (codon) of 3 nucleotides

  • The last codon is a STOP codon, which does not code for an amino acid

  • When given the number of amino acids, multiply by 3 and add three (for the STOP codon)

Working out an amino acid sequence

• An amino acid sequence can be derived when given a genetic code table (or codon table)

• The bases in the codon are read left to right across the table, and the corresponding amino acids can be found