How is the cell cycle controlled, and what happens when that control fails?
Topic 4.6 Regulation of the Cell Cycle: explain how checkpoints and regulatory molecules control progression through the cell cycle, and how loss of control leads to cancer.
A focused answer to AP Biology Topic 4.6, covering cell-cycle checkpoints, cyclins and cyclin-dependent kinases, growth factors, the link to signal transduction, and how loss of regulation causes cancer.
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What this topic is asking
The College Board (Topic 4.6) wants you to explain how the cell cycle is regulated: the role of checkpoints, the regulatory molecules (cyclins and cyclin-dependent kinases), external signals such as growth factors, and how loss of this control leads to cancer. This topic ties the unit together, because cell-cycle control is itself driven by signal transduction.
Checkpoints
Checkpoints prevent damaged or incompletely prepared cells from dividing, which protects the next generation of cells.
Cyclins and cyclin-dependent kinases
Because cyclin levels change in a regular pattern, the complexes form and break apart at the right times, driving the cycle forward in sequence. This is a phosphorylation-based control, the same mechanism as signal transduction (Topic 4.3).
External signals
The decision to divide is not made by the cell alone. Growth factors are external signals that bind receptors and, through signal transduction, tell a cell whether to pass the G1 checkpoint and divide. This links Unit 4 together: cell communication and signal transduction control the cell cycle. Cells also stop dividing when crowded (density-dependent inhibition) or when not anchored to a surface.
When regulation fails: cancer
Why control matters
A multicellular organism needs the right cells to divide at the right time and place: for growth, for repair of wounds, and to replace worn-out cells. Both too little division (failure to repair) and too much (cancer) are harmful, which is why several independent checkpoints and signals all have to agree before a cell divides.
Try this
Q1. Identify what the G2 checkpoint verifies before the cell enters mitosis. [1 point]
- Cue. That DNA has been completely and correctly replicated.
Q2. Explain how cyclins and cyclin-dependent kinases drive the cell past a checkpoint. [2 points]
- Cue. A cyclin binds a CDK to form an active complex that phosphorylates target proteins, switching on the events that move the cell into the next phase.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2019 (style)4 marksSection II (long FRQ excerpt). A mutation disables the checkpoint that normally stops the cycle when DNA is damaged. (a) Predict the effect on the cell. (b) Explain how this can contribute to cancer.Show worked answer →
A 4-point predict-and-explain FRQ on checkpoints and cancer.
(a) Predict (1 point): the cell continues through the cycle and divides even when its DNA is damaged.
(b) Explain (3 points): (1 point) checkpoints normally halt the cycle so damage can be repaired before division; (1 point) without the checkpoint, damaged or mutated DNA is copied and passed to daughter cells; (1 point) the accumulation of mutations can lead to uncontrolled division (cancer), because the controls that normally limit division are lost.
Markers reward explaining the checkpoint's normal stop-and-repair role and linking its loss to passing on mutations and uncontrolled division.
AP 2022 (style)1 marksSection I (multiple choice). What is the role of cyclins in the cell cycle? (A) They replicate DNA. (B) They bind cyclin-dependent kinases to drive the cell into the next phase. (C) They split the cytoplasm. (D) They are the spindle fibers.Show worked answer →
The correct answer is (B).
Cyclins are regulatory proteins that rise and fall in concentration through the cycle and bind cyclin-dependent kinases (CDKs); the active cyclin-CDK complex phosphorylates targets that drive the cell past a checkpoint into the next phase. (A), (C) and (D) describe other processes.
Related dot points
- Topic 4.5 Cell Cycle: describe the phases of the cell cycle, including interphase and mitosis, and explain how the events of each phase produce two genetically identical cells.
A focused answer to AP Biology Topic 4.5, covering G1, S, G2, the phases of mitosis, cytokinesis and G0, and how the cycle produces two genetically identical daughter cells, with a worked timing calculation.
- Topic 4.3 Signal Transduction Pathways: explain how signalling pathways relay and amplify a signal to produce a response, and how mutations or chemicals that change the pathway affect the cell.
A focused answer to AP Biology Topic 4.3, covering relay molecules, phosphorylation cascades, signal amplification, the variety of cellular responses, and how mutations and chemicals alter pathways.
- Topic 4.4 Feedback: explain how negative feedback maintains homeostasis and how positive feedback amplifies a response, using examples from cellular and organismal systems.
A focused answer to AP Biology Topic 4.4, covering negative feedback and homeostasis, positive feedback and amplification, set points, and how feedback data are analyzed, with a worked chi-square example.
- Topic 4.2 Introduction to Signal Transduction: describe the reception, transduction and response stages of a signalling pathway, and the roles of receptors, ligands and second messengers.
A focused answer to AP Biology Topic 4.2, covering the three stages of signal transduction (reception, transduction, response), membrane and intracellular receptors, ligands, relay molecules and second messengers.
- Topic 1.6 Nucleic Acids: describe the structural similarities and differences between DNA and RNA and explain how the directionality and base pairing of nucleic acids support their function.
A focused answer to AP Biology Topic 1.6, covering nucleotide structure, the antiparallel double helix, base pairing, the 5' to 3' directionality, and the structural differences between DNA and RNA.
Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)