(ii) how gene sequencing has allowed for the sequences of amino acids in…

The idea

Gene sequencing determines the order of nucleotide bases in a length of DNA. Because the genetic code is a triplet code, non-overlapping and degenerate, each codon of three bases corresponds to one amino acid (or a start/stop signal). Once the base sequence of a gene is known, the reading frame is identified and the codons are translated using a codon table to give the primary sequence of amino acids in the polypeptide. Degeneracy means several codons can specify the same amino acid, so the amino acid sequence is often more reliably predicted than the exact base sequence is from the protein. Comparing predicted sequences between organisms also reveals conserved regions and possible mutations.

What earns the marks

• Gene sequencing establishes the linear order of nucleotide bases in DNA, which is the information used to deduce codons.
• The genetic code is a triplet code read in a fixed reading frame, so each group of three bases specifies one amino acid or a stop signal.
• A codon table is applied to the sequenced bases to predict the order of amino acids in the polypeptide.
• Degeneracy of the code means more than one codon can code for the same amino acid, so prediction of amino acid sequence is generally more reliable than reverse prediction.
• Predicted amino acid sequences can be compared between organisms or with a reference to identify conserved regions or the effect of a mutation.

Common mistakes and how to put them right

• Assuming one base codes for one amino acid.

  Put it right by stating that three bases form a codon specifying one amino acid.

• Ignoring the reading frame and grouping bases arbitrarily.

  Put it right by starting from the correct start codon and reading in non-overlapping triplets.

• Claiming the amino acid sequence can be predicted with certainty from any DNA fragment.

  Put it right by noting that only coding sequence with a known reading frame gives a reliable prediction, and degeneracy limits reverse prediction.

Exam tips

• State explicitly that the code is a triplet code and that codons are read in a fixed frame before applying a codon table.
• Use the term degenerate correctly to explain why amino acid sequence is more reliably predicted than base sequence.
• Link the prediction to a practical use, such as comparing sequences to find conserved regions or identifying a mutation's effect.