The Cell Cycle

Spec Mapping: This lesson is mapped to OCR H420 Module 2.1.6 — Cell division, diversity and cellular organisation (refer to the official OCR H420 specification document for exact wording). It develops the phases of the cell cycle (G₁, S, G₂, M), the roles of cell-cycle checkpoints, and the consequences of loss of control.

All cells arise from other cells (the third tenet of cell theory), and the sequence of events by which one cell becomes two is the cell cycle. At A-Level you need to know the phases of the cycle, the control checkpoints that keep division tightly regulated, and the consequences of loss of control. This lesson develops the OCR H420 Module 2.1.6 content on the cell cycle and prepares for Lesson 8 (mitosis).

1. Overview of the Cell Cycle

Key Definition — Cell Cycle: The ordered sequence of events that takes place in a eukaryotic cell, leading to the growth, DNA replication and division of the cell into two genetically identical daughter cells.

The cell cycle has two major parts:

Interphase — a long period of growth and DNA replication, subdivided into G₁, S and G₂.
Mitotic (M) phase — a short period of nuclear division (mitosis) and cell division (cytokinesis).

For a typical human cell in culture, interphase lasts roughly 20 hours and M phase only about 1 hour — so the majority of the cycle is actually interphase, not division.

graph LR
  A["G1 phase<br/>cell growth<br/>protein synthesis"] --> B["S phase<br/>DNA replication"]
  B --> C["G2 phase<br/>further growth<br/>organelle replication"]
  C --> D["M phase<br/>mitosis + cytokinesis"]
  D --> A

2. Interphase in Detail

Interphase is often misunderstood as "resting". It is in fact the most metabolically active part of the cycle — a cell in interphase is extremely busy. It has three sub-phases.

2.1 G₁ — First Gap Phase

In G₁ (first gap) the cell:

Grows in size.
Synthesises proteins and enzymes required for DNA replication.
Duplicates many cell organelles (mitochondria, chloroplasts, ribosomes).
Synthesises structural proteins and increases cytoplasmic volume.
Monitors its environment; if conditions are unfavourable or the cell has detected damage, it will pause or exit the cycle entirely into a quiescent state called G₀.

G₁ is the most variable phase of the cycle — it can last from a few hours in rapidly dividing cells to years or decades in cells that exit to G₀ (e.g. neurones).

2.2 S Phase — Synthesis

In S (synthesis) phase the cell replicates its entire nuclear DNA in a single, tightly controlled event. Each chromosome is duplicated so that by the end of S phase it consists of two identical sister chromatids joined at a centromere. A human cell replicates its 6 × 10⁹ base pairs in about 6–8 hours from thousands of origins of replication firing simultaneously.

The centrosome (which organises spindle microtubules) also begins to replicate during S phase in animal cells.

2.3 G₂ — Second Gap Phase

In G₂ (second gap) the cell:

Continues to grow.
Completes replication of any remaining organelles (particularly mitochondria and centrosomes).
Synthesises proteins needed for mitosis — including tubulin for the spindle and condensins for chromosome condensation.
Checks for DNA damage and repairs it before committing to division.

2.4 Summary Table

Phase	Main events	Typical duration (cultured human cell)
G₁	Growth, protein synthesis, organelle duplication	9 h (very variable)
S	DNA replication	6 h
G₂	Further growth, preparation for mitosis	4 h
M	Mitosis and cytokinesis	~1 h

3. M Phase — Mitosis and Cytokinesis

M phase is when the nucleus divides by mitosis (covered in Lesson 8) and the cell divides by cytokinesis to produce two genetically identical daughter cells. Mitosis itself comprises four substages: prophase, metaphase, anaphase and telophase.

It is the shortest phase of the cycle because the longer the cell spends with condensed chromosomes and a disassembled nuclear envelope, the more vulnerable it is to DNA damage.

4. G₀ — Exit from the Cycle

Some cells leave the cycle during G₁ and enter G₀, a non-dividing state. Possible fates include:

Terminal differentiation — neurones and cardiac muscle cells do not divide after they differentiate. They remain in G₀ for life.
Temporary quiescence — lymphocytes can re-enter the cycle rapidly when activated.
Senescence — cells that have divided many times (reached the Hayflick limit) lose the ability to divide further.

5. Regulation of the Cell Cycle — Checkpoints

The cell cycle is tightly controlled by checkpoints that verify the cell is ready to proceed. If a checkpoint detects a problem — DNA damage, incomplete replication, unfavourable conditions — the cycle pauses or is abandoned (apoptosis).

Three main checkpoints are examinable:

5.1 The G₁/S Checkpoint (Restriction Point)

At the end of G₁, the cell asks: "Am I ready to replicate DNA?"

Is the cell large enough?
Are nutrients and growth factors present?
Is the DNA intact (no damage)?

If all answers are yes, the cell commits to division and enters S phase. If not, it pauses in G₁ or exits to G₀. This is sometimes called the restriction point and is the most important control point — once past it, the cell is committed to a full cycle.

5.2 The G₂/M Checkpoint

At the end of G₂, the cell asks: "Is replication complete and error-free?"

Has all DNA been replicated?
Is any DNA damage repaired?
Is the cell large enough to divide?

If problems are detected, the cycle pauses in G₂ while repairs are made. If repairs cannot be made, the cell may enter apoptosis (programmed cell death).

5.3 The Spindle Assembly Checkpoint (Metaphase Checkpoint)

During mitosis, the cell asks: "Are all chromosomes correctly attached to the spindle?"

The cycle cannot proceed to anaphase until every chromosome is attached by kinetochores to microtubules from both poles of the spindle. A single unattached chromosome is enough to halt progression. This prevents the unequal distribution of chromosomes to daughter cells (non-disjunction, which would lead to aneuploidy — for example, Down syndrome is trisomy 21 caused by non-disjunction).

graph TD
  A["G1/S checkpoint<br/>Am I ready to replicate?"] --> B["G2/M checkpoint<br/>Is replication complete?"]
  B --> C["Spindle assembly checkpoint<br/>Are chromosomes attached?"]
  C --> D[Proceed to anaphase]
  A -->|No| E[Arrest or G0]
  B -->|No| F[DNA repair or apoptosis]
  C -->|No| G[Wait for attachment]

5.4 Molecular Machinery

At A-Level you do not need the molecular detail, but it is worth knowing that checkpoints are enforced by proteins called cyclins and cyclin-dependent kinases (CDKs). Cyclin concentrations rise and fall through the cycle, binding to CDKs to activate them; active CDKs phosphorylate downstream proteins that drive the next stage. p53 is a famous "guardian" protein that stops the cycle in response to DNA damage. Mutations in TP53 are found in more than half of human cancers.

6. Cancer and Loss of Cell Cycle Control

Cancer is fundamentally a disease of uncontrolled cell division. Mutations in cell cycle regulator genes allow cells to:

Divide even when they should not (mutations activating accelerator genes called proto-oncogenes, turning them into oncogenes).
Ignore signals to stop (mutations inactivating tumour suppressor genes such as TP53 and RB1).
Escape apoptosis.
Ignore damage at checkpoints.

The result is a mass of cells (a tumour) that grows without regulation. Malignant tumours can invade surrounding tissues and metastasise via blood and lymph.

This is why understanding checkpoints matters medically: many chemotherapy drugs work by exploiting the fact that cancer cells have lost checkpoint control and cannot repair DNA damage, making them more vulnerable than healthy cells.

7. Common Exam Mistakes

Saying interphase is "resting". It is the most metabolically active part of the cycle.
Forgetting that DNA replicates in S phase, not during mitosis.
Confusing G₁ and G₂. G₁ is before S; G₂ is after S.
Writing that checkpoints are "optional". They are obligatory and prevent faulty division.
Saying cancer is "fast division". It is uncontrolled division — cancer cells often divide at similar rates to normal dividing cells.
Claiming all cells are always cycling. Many (neurones, skeletal muscle) are permanently in G₀.
Writing "the cell cycle makes gametes". Gametes are made by meiosis, not mitosis.

8. Exam-Style Questions

Describe the main events of interphase. (5)
Explain the role of checkpoints in the cell cycle. (4)
Suggest why cancerous cells are often characterised by mutations in the TP53 gene. (3)
Distinguish between G₀ and G₁ phases. (2)

Model answer for (2): "Checkpoints are control points during the cell cycle where the cell monitors whether it is ready to proceed to the next stage. The G₁/S checkpoint checks for adequate size, nutrients and undamaged DNA before committing to DNA replication. The G₂/M checkpoint ensures replication is complete and DNA damage has been repaired before mitosis begins. The spindle assembly checkpoint verifies that all chromosomes are attached to spindle microtubules from both poles before anaphase. Checkpoints ensure that only intact, correctly prepared cells divide, preventing the production of faulty daughter cells and helping to prevent cancer."

Summary

The cell cycle is the sequence of events from one cell division to the next: G₁ → S → G₂ → M.
Interphase (G₁, S, G₂) is a period of active growth, DNA replication, and preparation.
M phase comprises mitosis and cytokinesis.
The cycle is controlled by checkpoints at G₁/S, G₂/M and the spindle assembly point.
Cells that stop dividing enter G₀ — permanently (neurones) or temporarily (lymphocytes).
Checkpoints are enforced by cyclins, CDKs and guardian proteins such as p53.
Loss of checkpoint control underlies cancer — a disease of uncontrolled division.

9. The Cell Cycle as a Mermaid Diagram with Checkpoint Annotations

flowchart LR
  G1["G1 (gap 1)<br/>cell growth<br/>protein synthesis<br/>organelle duplication"] -->|G1/S checkpoint<br/>(restriction point)| S["S phase<br/>DNA replication<br/>centrosome duplication"]
  S --> G2["G2 (gap 2)<br/>further growth<br/>preparation for mitosis"]
  G2 -->|G2/M checkpoint<br/>(DNA damage / replication complete?)| M["M phase<br/>mitosis + cytokinesis"]
  M -->|Spindle assembly checkpoint<br/>(metaphase)| Anaphase["Anaphase + telophase<br/>+ cytokinesis"]
  Anaphase --> G1
  G1 -.->|exit| G0["G0 (quiescent)<br/>terminal differentiation<br/>or temporary quiescence"]
  G0 -.->|reactivation| G1

The checkpoint annotations make the regulatory logic explicit: each transition is gated by a quality-control signal that, if it fails, either pauses the cycle (for repair) or triggers apoptosis (if repair fails).

10. Historical Context — Cell Theory and Cell-Cycle Discovery

The third tenet of cell theory — omnis cellula e cellula ("every cell from a cell"), in paraphrase from Rudolf Virchow's 1855 framing — established that division, not spontaneous generation, is the origin of all cells. Two pioneers of cell-cycle observation deserve naming at A-Level depth:

Walther Flemming (1880s) named the term chromatin and described the stages of nuclear division by direct observation of salamander cells stained with aniline dyes. His paraphrased framing of "indirect nuclear division" was the seed of what we now call mitosis.
Theodor Boveri and Walter Sutton (1902) — independently proposed the chromosome theory of inheritance, that chromosomes are the physical carriers of Mendel's "factors". Their convergence is a textbook example of independent discovery driven by improved microscopy and the rediscovery of Mendel.
Leonard Hayflick (1961) — discovered that normal somatic cells in culture divide only a finite number of times (the Hayflick limit, ~40–60 divisions for human fibroblasts) before entering replicative senescence. The Hayflick limit is the natural cell-cycle counterpart of telomere shortening and a major theme of ageing research.

Lesson 7: The Cell Cycle