Explain how the sequence of amino acids in a polypeptide determines the three-dimensional shape of a functional protein.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The primary structure of a protein is the linear sequence of amino acids, which is set by the codon sequence in the gene/mRNA. The side‑chains (R groups) of these amino acids interact with each other; these interactions drive the polypeptide to fold into regular secondary structures such as α‑helices and β‑pleated sheets. Further folding occurs as the R groups form hydrogen bonds, ionic bonds, disulfide bridges and hydrophobic interactions, giving the protein its tertiary structure. Because the type and position of the R groups are fixed by the amino‑acid sequence, a unique sequence produces a unique three‑dimensional shape, and this shape determines the protein’s function.
Examiner tips
- Use the phrase "primary, secondary, tertiary structure" in order
- Explain the role of R‑group interactions in each folding step
- Show that the sequence fixes the R‑group pattern, leading to a unique shape
Common mistakes
- Confusing the sequence with the final 3D shape, or omitting the role of R‑group interactions
- Failing to mention the specific types of bonds (hydrogen, ionic, disulfide, hydrophobic) that stabilise tertiary structure
Mark scheme (4 marks)
- The sequence of amino acids (primary structure) is determined by the sequence of codons in the gene / mRNA.
- Interactions between R groups of amino acids cause the polypeptide to fold, forming secondary structures such as alpha helices and beta pleated sheets.
- Further folding due to interactions between R groups — including hydrogen bonds, ionic bonds, disulfide bonds, and hydrophobic interactions — produces the tertiary structure.
- Because the type and position of R groups depend entirely on the amino acid sequence, a unique sequence produces a unique three-dimensional shape, which determines the protein's function.
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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