The Cell Cycle

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 covers OCR A-Level Biology A specification point 2.1.6 (a) — 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?"

Lesson 7: The Cell Cycle