4.4.1Additional lessons
Cell Division — IB Biology HL Revision Notes & Topical Questions
5 min read 6 parts Lesson 41 of 101
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.
Section 1 of 6Why Cells Divide
A dividing cell is the parent cell, and the cells it produces are daughter cells.
Growth
- Growth Repeated divisions of a zygote produce the many cells of a multicellular organism, which then specialise into tissues and organs.
Tissue repair
- Tissue repair Damaged or worn out cells such as skin or gut lining are replaced by division of neighbouring cells.
Asexual reproduction
- Asexual reproduction Single-celled organisms and some multicellular ones make genetically identical offspring by dividing.
Gamete formation
- Gamete formation A different kind of division, meiosis, halves the chromosome number so that fertilisation restores the diploid number.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Daughter nuclei | 2 | 4 |
| Chromosome number | Same as parent (2n) | Halved (n) |
| Genetically identical? | Yes | No |
| Purpose | Growth, repair, asexual reproduction | Gamete production |
Section 2 of 6Nuclear Division and Cytokinesis
Division of the cytoplasm is separate from division of the nucleus and normally follows it.
Nuclear division
- Nuclear division The nucleus divides first, so that each future daughter cell receives a complete copy of the genetic material.
Cytokinesis in animals
- Cytokinesis in animals A ring of actin and myosin contracts beneath the plasma membrane at the equator, pinching the cell in two.
Cytokinesis in plants
- Cytokinesis in plants Vesicles from the Golgi apparatus fuse across the equator to form a cell plate that develops into new cell walls.
Unequal cytokinesis
- Unequal cytokinesis In oogenesis one large egg and small polar bodies form, and in yeast budding makes a small bud, so cytoplasm is shared unequally.
Section 3 of 6Preparing the DNA
Replication first
- Replication first DNA is copied before division, so each chromosome consists of two identical sister chromatids joined at the centromere.
Histones
- Histones DNA winds around histone proteins to form nucleosomes, and this chromatin then coils and supercoils into short, thick chromosomes.
Why condense
- Why condense Tightly packed chromosomes are less likely to tangle or break when they are pulled apart.
Anucleate cells
- Anucleate cells Some cells, such as red blood cells, lose the nucleus and cannot divide, which shows why a nucleus is needed for mitosis.
Section 4 of 6Moving the Chromosomes
1
Microtubules are built by joining many tubulin dimers, each made of two protein subunits, and they extend from the centrosomes.
2
Motor proteins attached to the chromosomes walk along the microtubules, using energy from ATP.
3
Microtubules attach to the centromere regions and align the chromosomes at the cell equator.
4
Shortening of the microtubules pulls sister chromatids apart towards opposite poles, giving each pole a full set.
Section 5 of 6Worked Example
Question
- A human cell has 46 chromosomes.
- How many chromosomes and chromatids are there before division, and in each daughter cell after mitosis?
- Before division, DNA replication has occurred, so each of the 46 chromosomes has 2 chromatids.
- Chromatids = 46 x 2 = 92.
- Mitosis separates the sister chromatids, which then count as individual chromosomes.
- Each daughter nucleus receives 46 chromosomes.
Answer
- 46 chromosomes (92 chromatids) before division;
- each daughter cell has 46 chromosomes, each with one chromatid.
Section 6 of 6Exam Checklist & Tips
4.4.1 – Key Points for the Exam
- Cytokinesis The division of the cytoplasm that follows nuclear division and forms two cells.
- Sister chromatids Two identical DNA copies produced by replication and joined at a centromere.
- Nucleosome A length of DNA wound around a core of eight histone proteins.
- Diploid A cell with two complete sets of chromosomes, written 2n.
Common Mistakes
- Saying that mitosis and cytokinesis are the same event; one divides the nucleus and the other the cytoplasm.
- Describing chromatids as chromosomes only after they separate; a joined pair is still one chromosome.
- Believing the chromosome number halves in mitosis; it stays the same.
- Writing that plant cells form a cleavage furrow; they build a cell plate.
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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.