Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- The cell cycle is a tightly regulated sequence that controls how eukaryotic cells grow and divide: interphase (G1, S, G2), then mitosis, then cytokinesis.
- G1: the cell is metabolically active, grows and copies its organelles and cytoplasm.
- S: DNA (as chromatin) replicates. Each chromosome becomes two sister chromatids joined at a centromere.
- G2: protein synthesis, lots of ATP made, centrosome duplication completed.
- G0: a non-dividing state outside the cycle. Many G0 cells can re-enter with the right cues; other non-dividing cells are held at one stage.
- Mitosis passes a complete genome to two genetically identical daughter cells. Used for growth, tissue repair and asexual reproduction.
Key relationships
| Step | Main events |
|---|---|
| Prophase | chromosomes condense; spindle forms; centrosomes move to opposite poles; nuclear envelope breaks down |
| Metaphase | spindle fibres line chromosomes up on the equator |
| Anaphase | sister chromatids separate and are pulled to opposite poles |
| Telophase | spindle breaks down; nuclear envelopes re-form; cytoplasm starts to divide |
| Cytokinesis | animal: cleavage furrow; plant: cell plate from Golgi vesicles |
| Stage (cell with 2n chromosomes) | Chromosomes | Chromatids | DNA |
|---|---|---|---|
| G1 | 2n | 2n | x |
| G2 to metaphase | 2n | 4n | 2x |
| Anaphase (whole cell) | 4n | 4n | 2x |
| Each daughter | 2n | 2n | x |
| Calculation | How |
|---|---|
| Mitotic index | (cells in mitosis ÷ total cells) × 100 |
| Time in a stage | (cells in stage ÷ total cells) × cycle length |
Assumptions behind the numbers
- Timing from cell counts assumes cells are spread at random through the cycle, not dividing in step.
- Large counts give better estimates; misidentifying early prophase as interphase lowers the mitotic index.
Mistakes to avoid
- Saying the chromosome number doubles in S phase (the DNA doubles).
- Placing DNA replication in prophase.
- Calling interphase a resting stage.
- Giving plant cells a cleavage furrow.
- Saying mitosis halves the chromosome number.
- Forgetting to include interphase cells in the total when calculating a mitotic index.
Quick self-check
- A cell has 12 chromosomes in G1. How many chromatids in G2, and how many chromosomes in anaphase? (24 chromatids; 24 chromosomes in anaphase, 12 going to each pole.)
- 30 of 240 cells counted are in mitosis. Mitotic index? (30 ÷ 240 × 100 = 12.5%)
- Why are the two daughter cells genetically identical? (S phase makes exact copies of every chromosome, and anaphase sends one sister chromatid of each chromosome to each pole.)
Next: practice questions.