AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
The relationship between primary, secondary, tertiary and quaternary…
Full specification wording
General properties of proteins: The relationship between primary, secondary, tertiary and quaternary structure, and protein function.
Primary structure is the sequence of amino acids in a polypeptide.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 7 of 9
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Primary structure is the sequence of amino acids in a polypeptide. Secondary structure is the folding of the backbone into an alpha helix or beta pleated sheet, held by hydrogen bonds. Tertiary structure is the 3D folding of the chain, held by hydrogen bonds, ionic bonds, and disulfide bridges between R groups. Quaternary structure involves two or more polypeptides, and sometimes prosthetic groups, assembled together. Structure determines function: primary structure dictates R group positions, which determines tertiary folding. This specific 3D structure forms functional regions like an enzyme's active site. Quaternary structure also affects function, such as haemoglobin's cooperative binding of oxygen.
What earns the marks
- Primary structure as the sequence of amino acids in the polypeptide.
- Secondary structure as the alpha helix or beta pleated sheet formed by hydrogen bonding.
- Tertiary structure as the three-dimensional folding formed by interactions between R groups.
- Quaternary structure as two or more polypeptide chains held together.
- Explaining that specific tertiary or quaternary structures create functional regions, such as active sites.
Common mistakes and how to put them right
Using the vague term 'shape' instead of specifying 'tertiary structure' when describing 3D folding.
Saying secondary structure is held by ionic bonds or disulfide bridges.
Claiming every protein has all four levels of structure (many lack quaternary structure).
Describing primary structure as the number of amino acids rather than their sequence.
Stating that a mutation changes the tertiary structure directly, without mentioning the change to the primary structure first.
Exam tips
- Use the specific term 'tertiary structure' rather than just '3D shape' to ensure clarity in your answers.
- Set out the causal chain in order: primary sequence, R group interactions, tertiary structure, and resulting function.
- Remember that quaternary structure can involve non-protein prosthetic groups, like the haem group in haemoglobin, which are essential for function.
Try the lesson
The start of the General properties of proteins: The relationship between primary, secondary, tertiary and quaternary structure, and protein function. lesson, made from checked teaching notes for this specification point.
Have a go first
Which level of protein structure is defined as the specific sequence of amino acids joined by peptide bonds?
Listen
Primary structure is the sequence of amino acids in a polypeptide. Secondary structure is the folding of the backbone into an alpha helix or beta pleated sheet, held by hydrogen bonds. Tertiary structure is the 3D folding of the chain, held by hydrogen bonds, ionic bonds, and disulfide bridges between R groups. Quaternary structure involves two or more polypeptides, and sometimes prosthetic groups, assembled together. Structure determines function: primary structure dictates R group positions, which determines tertiary folding. This specific 3D structure forms functional regions like an enzyme's active site. Quaternary structure also affects function, such as haemoglobin's cooperative binding of oxygen. A strong answer earns marks for these points. primary structure as the sequence of amino acids in the polypeptide. secondary structure as the alpha helix or beta pleated sheet formed by hydrogen bonding. tertiary structure as the three-dimensional folding formed by interactions between R groups. quaternary structure as two or more polypeptide chains held together. Exam tip: Use the specific term 'tertiary structure' rather than just '3D shape' to ensure clarity in your answers.
Key words
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primary structure
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secondary structure
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tertiary structure
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quaternary structure
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See every definition
- primary structure
- The sequence of amino acids in a polypeptide, which dictates R group positions and therefore determines tertiary folding.
- secondary structure
- The folding of the backbone into an alpha helix or beta pleated sheet, held by hydrogen bonds.
- tertiary structure
- The three-dimensional folding of the chain, held by hydrogen bonds, ionic bonds and disulfide bridges between R groups.
- quaternary structure
- Two or more polypeptides, and sometimes prosthetic groups, assembled together, which also affects function.
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