Cambridge A-Level Biology 9700 – Replication and Division of Nuclei and Cells
Objective
Outline the mitotic cell cycle, including:
Interphase – growth in G1 and G2 phases and DNA replication in S phase
Mitosis – the four stages
Cytokinesis – division of the cytoplasm
The Mitotic Cell Cycle
1. Interphase
Interphase occupies about 90 % of the cell’s life and prepares the cell for division. It consists of three sub‑phases:
G1 phase (Gap 1) – cell grows in size, synthesises proteins and organelles, and carries out its normal functions.
S phase (Synthesis) – DNA replication occurs. Each chromosome is duplicated to form two sister chromatids joined at the centromere. The amount of DNA doubles: \$2C \rightarrow 4C\$ (where \$C\$ is the haploid DNA content).
G2 phase (Gap 2) – further growth, synthesis of microtubule proteins, and preparation of the mitotic spindle. The cell checks that DNA replication is complete and undamaged.
2. Mitosis
Mitosis is the process of nuclear division and is divided into four consecutive stages.
Stage
Key Events
Duration (typical)
Prophase
Chromatin condenses into visible chromosomes.
Each chromosome consists of two sister chromatids.
Centrosomes migrate to opposite poles and begin forming the mitotic spindle.
Nuclear envelope starts to break down.
5–10 min
Metaphase
Chromosomes align at the metaphase plate (cell equator).
Spindle fibers attach to kinetochores on each centromere.
Checkpoint ensures all chromosomes are correctly attached.
3–5 min
Anaphase
Sister chromatids separate as spindle fibers shorten.
Each chromatid (now a daughter chromosome) moves toward opposite poles.
Cell elongates due to pushing forces of polar microtubules.
1–2 min
Telophase
Chromosomes de‑condense back into chromatin.
Nuclear envelopes re‑form around each set of chromosomes.
Spindle apparatus disassembles.
5–10 min
3. Cytokinesis
Cytokinesis is the physical division of the cytoplasm, occurring concurrently with telophase in animal cells and after mitosis in plant cells.
Animal cells: A contractile ring of actin‑myosin filaments forms beneath the plasma membrane, creating a cleavage furrow that deepens until the cell splits into two daughter cells.
Plant cells: A cell plate forms along the former metaphase plate. Vesicles from the Golgi fuse to build a new cell wall, eventually separating the two daughter cells.
Suggested diagram: A schematic showing the sequence of interphase, the four stages of mitosis, and cytokinesis in an animal cell.
Key Points to Remember
Interphase is a period of growth and DNA replication; it does not involve chromosome segregation.
During S phase the DNA content doubles, but the chromosome number remains the same because each chromosome now has two sister chromatids.
Mitosis ensures that each daughter cell receives an identical set of chromosomes.
Cytokinesis physically separates the two daughter cells, completing cell division.