Objective
Outline the mitotic cell cycle, including:
- Interphase – growth in G1 and G2 phases and DNA replication in S phase
- Mitosis – the four stages (prophase, metaphase, anaphase, telophase)
- Cytokinesis – division of the cytoplasm in animal and plant cells
Assessment Objectives (Cambridge)
- AO1 – Knowledge: Define key terms, list the stages, and describe the main events.
- AO2 – Application: Predict the outcome of experimental manipulations (e.g., spindle poisons) and interpret microscope images.
- AO3 – Evaluation: Design, analyse and evaluate simple experiments that investigate cell‑division processes.
1. Chromosome Structure (required for Topic 5)
- DNA double helix wrapped around histone octamers → nucleosome.
- Linear arrays of nucleosomes form a chromatin fibre.
- During mitosis chromatin condenses into visible chromosomes:
- Each chromosome consists of two sister chromatids joined at the centromere.
- The centromere contains the kinetochore, a protein complex that attaches to spindle microtubules.
- Telomeres are repetitive DNA sequences at chromosome ends; they prevent loss of genes during replication and protect chromosome stability.
2. The Mitotic Cell Cycle
2.1 Interphase (≈ 90 % of a cell’s life)
Pre‑division period that prepares the cell for mitosis. It comprises three sub‑phases.
- G1 phase (Gap 1)
- Cell grows in size, synthesises proteins, RNA and new organelles.
- Normal metabolic activities continue.
- Checkpoint ensures DNA is intact before entry into S phase.
- S phase (Synthesis)
- Complete replication of the nuclear DNA.
- Each chromosome is duplicated, producing two sister chromatids.
- DNA content doubles: 2C → 4C (C = haploid DNA content). Chromosome number remains unchanged.
- G2 phase (Gap 2)
- Further growth and synthesis of mitotic proteins (e.g., tubulin).
- Centrioles (in animal cells) duplicate; in plant cells the microtubule‑organising centre (MTOC) is prepared.
- Checkpoints verify that DNA replication is complete and undamaged.
2.2 Mitosis – nuclear division (four consecutive stages)
| Stage | Key Events | Typical Duration (animal cells) |
|---|
| Prophase | - Chromatin condenses into discrete chromosomes (each with two sister chromatids).
- Centrioles (animal) or MTOC (plant) move to opposite poles and begin forming the mitotic spindle.
- Kinetochore plates appear on centromeres; spindle fibres start to attach.
- Nuclear envelope fragments; nucleolus disappears.
| 5–10 min |
| Metaphase | - Chromosomes line up along the metaphase plate (cell equator).
- Each kinetochore is attached to spindle fibres from opposite poles (bipolar attachment).
- Spindle‑assembly checkpoint ensures all chromosomes are correctly attached.
| 3–5 min |
| Anaphase | - Sister chromatids separate at the centromere and become individual daughter chromosomes.
- Shortening of kinetochore microtubules pulls chromosomes toward opposite poles.
- Polar (astral) microtubules lengthen, pushing the poles apart and elongating the cell.
| 1–2 min |
| Telophase | - Chromosomes reach the poles and begin to de‑condense into chromatin.
- Nuclear envelopes re‑form around each set of chromosomes.
- Spindle apparatus disassembles; nucleoli reappear.
| 5–10 min |
Spindle Overview (AO2 – application)
- Kinetochore microtubules attach to kinetochores and move chromosomes.
- Polar (astral) microtubules interact with each other to push the poles apart.
- In animal cells the spindle is organised by a pair of centrioles within the centrosome; plant cells lack centrioles and the spindle nucleates from the nuclear envelope surface (MTOC).
2.3 Cytokinesis – division of the cytoplasm
- Animal cells
- Occurs concurrently with telophase.
- A contractile ring of actin‑myosin filaments forms just beneath the plasma membrane at the cell equator.
- Ring contraction creates a cleavage furrow that deepens until the cell pinches into two daughter cells.
- Plant cells
- Occurs after telophase.
- A phragmoplast (array of microtubules) guides Golgi‑derived vesicles to the centre of the former metaphase plate.
- Vesicles fuse to form the cell plate, which expands outward and becomes a new secondary cell wall separating the daughter cells.
3. Importance of Mitosis (Growth, Repair & Asexual Reproduction)
- Supplies new cells for organismal growth (e.g., increase in body size, leaf expansion).
- Provides cells for tissue repair and regeneration (e.g., skin, blood, plant meristems).
- Enables asexual reproduction in many plants (e.g., formation of new individuals from tubers, runners, or bulbils).
4. Interpreting Photomicrographs (AO2)
When examining stained preparations (e.g., onion root tip or animal tissue), look for the following visual cues to identify each mitotic stage:
| Stage | Typical Microscopic Features |
|---|
| Prophase | Loose chromatin threads becoming distinct, thickened chromosomes; centrosomes (or MTOC) visible as small dense bodies; nuclear envelope still intact. |
| Metaphase | Chromosomes aligned in a single plane at the cell centre; each shows two clearly separated sister chromatids with a visible centromere; spindle fibres radiating toward the poles. |
| Anaphase | Sister chromatids pulled apart, moving toward opposite poles; a characteristic “V”‑shaped arrangement of chromosomes; cell elongation. |
| Telophase | Chromosomes at opposite poles beginning to de‑condense; re‑formation of nuclear envelopes; early cell plate (plant) or cleavage furrow (animal) may be visible. |
5. Practical Skills Box (AO3)
Simple Experiment – Determining the Mitotic Index in Onion Root Tips
- Objective: Estimate the proportion of cells in mitosis and identify the most common stage.
- Materials: Fresh onion, microscope, slides, cover slips, 3 % acetic acid, Feulgen stain (or acetocarmine), tweezers, forceps.
- Method (summary):
- Cut a 1‑cm segment of the root tip (≈ 1 mm from the tip) and place in 3 % acetic acid for 5 min to soften.
- Squash the tissue gently under a cover slip to obtain a single‑cell layer.
- Stain for 2–3 min, rinse, and examine under ×400 magnification.
- Count at least 500 cells, recording how many are in each mitotic stage.
- Data analysis: Mitotic index = (Number of mitotic cells ÷ Total cells counted) × 100 %.
- Safety notes: Wear gloves when handling acids and stains; dispose of waste according to school laboratory guidelines.
- Evaluation points (AO3): Discuss sources of error (e.g., uneven squashing, over‑/under‑staining) and suggest improvements such as using a calibrated counting grid or repeating the assay on multiple root tips.
6. Links to Wider Biological Context
- Stem cells – many tissues rely on mitotically active stem cells for growth and repair; they retain the ability to divide indefinitely while maintaining genomic integrity (telomere maintenance).
- Uncontrolled cell division → tumour formation – mutations that bypass cell‑cycle checkpoints (e.g., loss of p53 function) can lead to continuous proliferation, forming benign or malignant tumours. This connects Topic 5 with the later syllabus topic on cancer (Topic 10).
- Telomere shortening – each normal somatic division shortens telomeres; critically short telomeres trigger senescence (Hayflick limit). In many cancer cells telomerase is re‑activated, maintaining telomere length and allowing limitless division.
Key Points to Remember (AO1)
- Interphase (G1, S, G2) is a period of growth and DNA replication; chromosome number does not change, but DNA content doubles (2C → 4C).
- Chromosome structure: DNA + histones → chromatin → condensed chromosome (two sister chromatids, centromere, kinetochore, telomeres that prevent gene loss).
- Mitosis ensures each daughter cell receives an identical set of chromosomes through the ordered events of prophase, metaphase, anaphase, and telophase.
- Cytokinesis physically separates the two daughter cells – cleavage furrow in animal cells, cell plate & phragmoplast in plant cells.
- Mitosis supplies new cells for growth, tissue repair and asexual reproduction** in plants.
- Errors in cell‑cycle control mechanisms can lead to tumour formation; telomere maintenance is crucial for genomic stability.