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D2.1.15 (HL)—Cell cycle control by cyclins

Cyclins control cell-cycle timing by rising and falling, activating CDKs that phosphorylate proteins at specific stages of division in cells.

Syllabus
First assessment 2025
Objective
D2.1.15
Level
HL

Exam analysis

Chance of appearing4%of analysed past papers
Latest appearanceMay 2022
Most common paperPaper1
Typical marks1–4

Common command terms

  • Compare
  • Explain
  • State

Scoring notes

Common mistake
Saying cyclins are constant instead of rising and falling during stages.

Recent exam appearances

May 2022Paper2 ["HL"] · TZ22(a)(iii)[ 1 ]D2.1.15 (HL)—Cell cycle control by cyclins
May 2018Paper1 ["HL"] · TZ14[ 1 ]D2.1.15 (HL)—Cell cycle control by cyclins
May 2017Paper1 ["HL"] · TZ14[ 1 ]D2.1.15 (HL)—Cell cycle control by cyclins
November 2016Paper2 ["HL"] · TZ02(b)[ 4 ]D2.1.15 (HL)—Cell cycle control by cyclins
Practice this objective

Coverage 2016–2022 · Updated 16 Jul 2026

Cyclin Thresholds Control Cell-Cycle Checkpoints

HL only

Different cyclins rise and fall in concentration during the cell cycle, and a cell passes a checkpoint only when the relevant cyclin reaches its threshold level.

Cyclin synthesis raises concentration before a transition; cyclin breakdown lowers it afterwards. This repeating pattern coordinates each checkpoint with the correct stage of the cycle.

Cyclin below threshold → checkpoint is not passed. Cyclin reaches threshold → transition can be triggered. Cyclin is degraded → its signal falls before the next cycle stage.

A graph may show one cyclin accumulating before mitosis, crossing a threshold as mitosis begins, then falling rapidly after it is degraded.

The syllabus requires changing cyclin concentrations and threshold control, not names or detailed roles of specific cyclins.

Cell cycle control by cyclins

HL only

Assessment in practice

1–4 marks
How it is assessed

This objective is assessed through structured response, commonly using Compare / Explain / State.

Command terms

Compare / Explain / State

What earns marks

Build the answer around this relationship: Cyclin concentrations fluctuate through the cell cycle.

Watch for

Saying cyclins are constant instead of rising and falling during stages.

Representative question

Question 1

[Maximum number: 4]

Explain how the cell cycle is controlled.

HL Cell Cycle and Cancer

HL only

Cell proliferation increases cell number by repeated mitosis, as in plant meristems, early embryos, skin replacement, and wound healing. The cell cycle includes interphase, mitosis, and cytokinesis; interphase has G1 growth, S-phase DNA replication, and G2 preparation. Interphase is metabolically active, not resting; cells synthesize proteins, replicate DNA, grow cytoplasm, and increase organelles. Cyclin concentrations rise and fall to activate cyclin-dependent kinases; CDK-cyclin complexes such as MPF control checkpoints and mitosis entry. Proto-oncogene activation and tumour suppressor loss disrupt checkpoints; accumulated mutations can cause uncontrolled proliferation and cancer. Benign tumours grow locally, malignant tumours invade neighbouring tissues, and metastasis spreads cancer cells to form secondary tumours.

Concept essentials

  • Cyclin concentrations fluctuate through the cell cycle.
  • Cyclins activate cyclin-dependent kinases.
  • CDK-cyclin complexes phosphorylate proteins that drive cell-cycle events.
  • Different cyclins regulate different stages or checkpoints.