3.7B Explain how the order of bases in a section of DNA decides the order…

The idea

A gene is a section of DNA whose base sequence is read in triplets. Each triplet, or codon, codes for one amino acid, so the linear order of bases dictates the linear order of amino acids in the polypeptide. Because the sequence of amino acids determines where hydrogen bonds, ionic interactions and hydrophobic contacts form, the chain folds into a precise three-dimensional shape. That shape creates a specific active site in an enzyme, so only a complementary substrate binds. Change one base and a different amino acid may be inserted, altering folding and possibly destroying the active site. The base order therefore links directly to protein function.

What earns the marks

• A gene is a section of DNA; the sequence of bases within it is read as triplets (codons).
• Each triplet codes for one specific amino acid, so base order determines amino acid order in the polypeptide.
• The order of amino acids determines how the chain folds, through bonds and interactions between amino acids.
• Folding produces a specific three-dimensional shape, giving enzymes a specifically shaped active site complementary to their substrate.
• A change in base sequence can change amino acid order, altering folding and shape and so affecting function.

Common mistakes and how to put them right

• Thinking one base codes for one amino acid.

  Put it right by stating that a triplet of three bases codes for one amino acid.

• Believing the protein's shape is fixed regardless of amino acid order.

  Put it right by explaining that amino acid order determines folding and therefore shape.

• Assuming all enzymes have the same active site shape.

  Put it right by stating each enzyme's active site is specifically shaped and complementary to its substrate.

Exam tips

• Use the word triplet or codon explicitly when linking bases to amino acids.
• Chain the logic: base order → amino acid order → folding → specific shape → active site function.
• When asked about a mutation, state the consequence for amino acid order, folding and active site shape in sequence.