2.3Unit 2: Cells, Development, Biodiversity and Conservation

SNAB Cell Division — IAL Edexcel Linear Revision Notes & Topical Questions

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Lesson summary

2.3 Cell Division — IAL EDEXCEL LINEAR notes: Describe the cell cycle, the stages of mitosis and their significance, and how mitosis is observed and quantified in root tip cells.Cell cycle = the regulated sequence of events between one cell division and the next.It has three phas…

Interphase
Cytokinesis
DNA content
Replacement and repair
Asexual reproduction
Squash technique

Part 1 of 7The cell cycle

Cell cycle = the regulated sequence of events between one cell division and the next.
- It has three phasesinterphase, nuclear division (mitosis, the M phase) and cytokinesis.
- Its length varies with cell type, organism and conditions: onion root tip cells divide about every 20 hours and human intestinal epithelial cells about every 10 hours.
- Chemical signals called cyclins trigger the move from one phase to the next.

Part 2 of 7Interphase

Phase	Events
G1	Cell grows and makes RNA, enzymes and other proteins; a signal to divide is received
S	DNA is replicated, so each chromosome has two identical sister chromatids; S stands for synthesis and the phase is short
G2	Cell continues to grow, new DNA is checked and errors repaired, and tubulin is made for the spindle

1Interphase

G1 + S + G2, during which the cell increases in mass and carries out its normal functions.

2M phase

M phase cell growth stops during mitosis.

3Cytokinesis

Cytokinesis the whole cell divides after the nucleus has divided, giving two genetically identical daughter cells.
In animal cells the cytoplasm constricts between the nuclei.
In plant cells a new cell wall forms.

Part 3 of 7Stages of mitosis

Mitosis nuclear division producing two daughter nuclei genetically identical to the parent nucleus, with the same chromosome number.

Stage	Events
Prophase	Chromosomes condense and are visible as two sister chromatids joined at the centromere; centrosomes (replicated in G2) move to opposite poles, spindle fibres emerge and the nuclear envelope breaks into vesicles
Metaphase	Chromosomes line up at the equator (metaphase plate) with spindle fibres attached to centromeres, each sister chromatid attached to a fibre from the opposite pole
Anaphase	Centromeres divide, spindle fibres shorten and the separated chromatids (now chromosomes) are pulled to opposite poles
Telophase	Chromosomes decondense at the poles, nuclear envelopes reform and spindle fibres break down

1Order

Order prophase, metaphase, anaphase, telophase (PMAT), and cytokinesis is a separate part of the cell cycle.

2DNA content

DNA content a human cell has 46 DNA molecules before S phase and 92 after replication, so each daughter cell has 46.

Part 4 of 7Significance of mitosis

Growth

Growth daughter cells are genetically identical clones, so a zygote grows into a multicellular organism.
Growth may be across the whole body or confined to regions such as plant meristems.

Replacement and repair

Replacement and repair damaged tissues are repaired and dying cells are replaced by identical cells.
Replacement is rapid in the skin and the lining of the gut.
Some animals regenerate body parts, such as zebrafish fins and axolotl legs and tails.

Asexual reproduction

Asexual reproduction a single parent produces genetically identical offspring.
Unicellular organisms such as Amoeba divide to reproduce.
Hydra and yeast reproduce by budding, and strawberries by runners.

Part 5 of 7Practical: mitosis in root tip cells

Meristem

Meristem plant growth occurs in the root tip meristem just behind the root cap, where cells are undergoing mitosis.

Squash technique

Squash technique stained root tips are gently squashed into a thin sheet so single cells are seen clearly.

Grow roots on onion or garlic bulbs suspended over water.

Warm 1 M hydrochloric acid in a boiling tube in a 60 °C water bath for 10 minutes.

Place about 1 cm of root tip in the warm acid for 5 minutes, then rinse in cold water and blot dry.

Cut about 2 mm from the tip and place it on a slide.

Add a drop of stain such as warm acetic orcein, which stains chromosomes deep purple.

Squash the tip gently with a blunt instrument or a coverslip pressed with a rolling motion.

View under a light microscope and draw cells in the different stages of mitosis.

Result
- cells in prophase, metaphase, anaphase and telophase are seen in the stained meristem.

Part 6 of 7Mitotic index

Mitotic index

Mitotic index number of cells with visible chromosomes ÷ total number of cells.

Multiply by 100 to give a percentage.
Cells with visible chromosomes are those in prophase, metaphase, anaphase and telophase.

Use

Use it shows what proportion of a tissue is dividing, for example a high index in a root tip meristem.

Part 7 of 7More from the specification

Microtubules

Microtubules protein microtubules form the spindle that moves chromosomes during mitosis

Identifying mitosis

Identifying mitosis in a root tip squash stage cells as prophase, metaphase, anaphase or telophase according to chromosome position

Mitotic index

Mitotic index number of cells in mitosis divided by total cells observed, from a sample of several fields of view

Calculations

Calculations give results to a sensible number of decimal places and use eyepiece graticule calibration for cell size

Key terms

Term	Meaning
Cell cycle	The regulated sequence of events between one cell division and the next.
Interphase	The phase of growth and DNA replication made of G1, S and G2.
Mitosis	Nuclear division producing two genetically identical nuclei.
Sister chromatids	Two identical copies of a chromosome joined at the centromere.
Cyclin	A chemical signal that triggers progress between phases of the cell cycle.
Meristem	A plant growth region where cells divide by mitosis.
Mitotic index	The proportion of cells in a sample that are undergoing mitosis.

Avoid these mistakes

Treating cytokinesis as a stage of mitosis — it is a separate part of the cell cycle.

Saying DNA replicates in prophase — it replicates in the S phase of interphase.

Saying sister chromatids separate in metaphase — they separate in anaphase.

Counting interphase cells as dividing when calculating the mitotic index — only cells with visible chromosomes count.

Last updated: 15 October 2026

Cambridge 9700 AS Biology Core Formula

Magnification Triangle & Unit Conversion

Formula: M = I / A ⇔ A = I / M ⇔ I = M × A

Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.