2.3Unit 2: Cells, Development, Biodiversity and Conservation
SNAB Cell Division — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 9 parts Lesson 5 of 26 1 quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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.
1
Grow roots on onion or garlic bulbs suspended over water.
2
Warm 1 M hydrochloric acid in a boiling tube in a 60 °C water bath for 10 minutes.
3
Place about 1 cm of root tip in the warm acid for 5 minutes, then rinse in cold water and blot dry.
4
Cut about 2 mm from the tip and place it on a slide.
5
Add a drop of stain such as warm acetic orcein, which stains chromosomes deep purple.
6
Squash the tip gently with a blunt instrument or a coverslip pressed with a rolling motion.
7
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
1
Treating cytokinesis as a stage of mitosis — it is a separate part of the cell cycle.
2
Saying DNA replicates in prophase — it replicates in the S phase of interphase.
3
Saying sister chromatids separate in metaphase — they separate in anaphase.
4
Counting interphase cells as dividing when calculating the mitotic index — only cells with visible chromosomes count.
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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.