4.2.6 Nucleic Acid & Amino Acid Sequence Comparison

• 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 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 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 three (for the stop codon)

mRNA Codons and Amino Acids Table