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B1.2.7 (HL)—Primary structure impact

Primary structure is the amino-acid sequence of a polypeptide, and sequence changes can alter folding, properties and protein safety outcomes.

Syllabus
First assessment 2025
Objective
B1.2.7
Level
HL

Exam analysis

Chance of appearing2%of analysed past papers
Latest appearanceNovember 2018
Most common paperPaper2
Typical marks1–3

Common command terms

  • Analyse
  • Outline

Recent exam appearances

November 2018Paper2 ["HL"] · TZ02(a)(ii)[ 1 ]B1.2.7 (HL)—Primary structure impact
May 2018Paper3 ["HL"] · TZ212(b)[ 3 ]B1.2.7 (HL)—Primary structure impact
Practice this objective

Coverage 2018–2018 · Updated 15 Jul 2026

Primary Structure Sets the Folding Possibilities

HL only

Primary structure is the exact amino-acid sequence of a polypeptide, and that sequence constrains every later level of folding.

The order places particular R-groups at particular positions. A substitution can create or remove an interaction, alter a bend or change an active site, so sequence is the starting information for conformation.

Trace a sequence change by checking:

  • which residue changed
  • what chemistry the new R-group adds or removes
  • which later interaction or function could shift

Replacing one non-polar residue with a charged residue in a buried region can destabilize folding because the new charge is poorly suited to the hydrophobic interior.

Primary structure means sequence, not the first stage in time only. It remains part of the molecule even after secondary and tertiary folding occur.

Primary structure impact

HL only

Assessment in practice

1–3 marks
How it is assessed

This objective is assessed through multiple choice, commonly using Analyse / Outline.

Command terms

Analyse / Outline

What earns marks

Build the answer around this relationship: Primary structure is the ordered amino-acid sequence of a polypeptide.

Representative question

Question 1

[Maximum number: 1]

Hemoglobin is a protein made up of two alpha and two beta polypeptide chains. In sickle cell anemia, a mutation causes one glutamic acid in each beta chain to be replaced by valine, as shown in the image.

Normal beta chain

Sickle beta chain

How does this mutation in hemoglobin cause sickle cell anemia?

A

It prevents the beta chains from forming a protein.

B

It replaces an amino acid with a fatty acid in the beta chain.

C

It changes the three-dimensional conformation of hemoglobin.

D

The polypeptide produced in sickle hemoglobin is shorter than in normal hemoglobin.

Folding Levels

HL only

HL protein questions are level-control questions. R-group chemistry predicts solubility and interactions. Primary structure is the DNA-coded amino acid sequence. Secondary structure is local alpha helix or beta-sheet stabilized by backbone hydrogen bonds. Tertiary structure is one polypeptide’s 3D fold stabilized by R-group interactions. Quaternary structure joins multiple chains. Examples such as haemoglobin, insulin, and collagen anchor these levels in real proteins.

  • R-group chemistry controls folding interactions and solubility.
  • Primary = amino acid sequence controlled by DNA via mRNA.
  • Secondary = local alpha helices and beta-sheets stabilized by backbone hydrogen bonds.
  • Tertiary = one polypeptide folded by R-group interactions.
  • Quaternary = two or more polypeptide chains in one functional protein.
  • Globular/fibrous comparison depends on shape, solubility, and function.

Concept essentials

  • Primary structure is the ordered amino-acid sequence of a polypeptide.
  • DNA and mRNA sequences determine the amino-acid order during protein synthesis.
  • Amino-acid sequence influences later folding and protein properties.
  • Sequence comparison can be used to evaluate similarity between proteins.