# Explain how the sequence of amino acids in a polypeptide determines the three-dimensional shape of a functional protein.

> IB DP Biology Standard Level (2023 syllabus) — B1.2 Proteins · Explain · 4 marks

## Mark scheme (4 marks)

1. The sequence of amino acids (primary structure) is determined by the sequence of codons in the gene / mRNA.
2. Interactions between R groups of amino acids cause the polypeptide to fold, forming secondary structures such as alpha helices and beta pleated sheets.
3. Further folding due to interactions between R groups — including hydrogen bonds, ionic bonds, disulfide bonds, and hydrophobic interactions — produces the tertiary structure.
4. 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.

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