AQA AS Biology

Revision Notes

4.2.6 Nucleic Acid & Amino Acid Sequence Comparison

Relating Nucleic Acid & Amino Acid Sequences

  • A triplet is a sequence of three DNA bases that codes for a specific amino acid
  • A codon is a sequence of three mRNA bases that codes for a specific amino acid
  • A codon is transcribed from the triplet and is complementary to it
  • An anticodon is a sequence of three tRNA bases that are complementary to a codon
  • 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) have the ability to form 64 different codons
  • The genetic code is degenerate
    • Multiple mRNA codons can encode the same amino acid
    • This means that a change in the genetic code doesn’t necessarily result in a change in the amino acid sequence
    • UGU and UGC both code for the amino acid cysteine
  • Some send important signals to the transcription machinery
    • The START codon initiates the process of transcription and ensure it starts in the right location (this is always the amino acid methionine in eukaryotic cells, coded for by the codon AUG)
    • STOP codons cause transcription to terminate and do not code for an amino acid e.g. UAA
  • The genetic code is non-overlapping
    • Each base is only read once in the codon 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:
    • When given the number of coding mRNA nucleotides, divide by 3 and minus one (for the stop codon – it is best to state this in your answer too)
    • When given the number of amino acids, multiply by 3 and add one (for the stop codon)

Exam Tip

In the exam, you may be asked to predict the effect of specific mutations in the genetic code. Remember that the genetic code is degenerate and non-overlapping! Also if stop and start codons are inserted into a coding sequence they can have major effects.

You will not be required to memorise specific codons and the amino acids for which they code.

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