4.4.1Additional lessons
Cell Division — IB Biology SL Revision Notes & Topical Questions
5 min read 4 parts Lesson 109 of 142
Part 1 of 4Why Cells Divide
Cell division involves two linked eventsnuclear division and cytokinesis.
Cell theory
- New cells arise only from pre-existing cells, so a multicellular organism develops from one cell by repeated divisions.
Parent and daughter
- The dividing cell is the parent cell, and each division gives two daughter cells.
Nuclear division
- The division of the nucleus, by mitosis or meiosis, distributes the genetic material between daughter nuclei.
Cytokinesis
- The division of the cytoplasm, which usually follows nuclear division and separates the daughter cells.
Importance
- Division allows growth, replacement of lost cells and, in some organisms, reproduction.
Part 2 of 4Cytokinesis
Cytokinesis differs between animal and plant cells because plant cells have a rigid cell wall.
1
After the nuclei have separated, the cell begins to divide the cytoplasm between them.
2
In an animal cell, the plasma membrane pinches inward around the cell equator to form a cleavage furrow.
3
The furrow deepens until the cell is split into two daughter cells.
4
In a plant cell, vesicles gather at the cell equator and fuse to form a cell plate.
5
The plate grows outwards to the cell walls and becomes a new wall and membranes between the two cells.
| Feature | Animal cell | Plant cell |
|---|---|---|
| Method | Cleavage furrow | Cell plate |
| Starts at | Outside edge | Centre of cell |
| Cell wall | Absent | New wall forms |
Part 3 of 4Mitosis, Meiosis and Equal Division
The type of nuclear division determines whether the daughter cells are identical to the parent or different from it.
Mitosis, Meiosis and Equal Division
Mitosis
- Produces two genetically identical diploid daughter nuclei, used for growth, tissue repair and asexual reproduction.
Meiosis
- Produces four genetically different haploid nuclei, used for making gametes in sexual life cycles.
Equal cytokinesis
- Cytoplasm is divided evenly, as in most body cells and in the formation of sperm.
Unequal cytokinesis
- Cytoplasm is divided unequally, as in human oogenesis, where one cell receives most of the cytoplasm and yeast budding.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Daughter nuclei | 2 | 4 |
| Chromosome number | Same as parent | Half of parent |
| Genetic identity | Identical | Different |
| Role | Growth and repair | Gamete formation |
Part 4 of 4Exam Checklist & Tips
4.4.1 – Key Points for the Exam
- Cytokinesis Division of the cytoplasm following nuclear division.
- Cleavage furrow The inward pinching of the plasma membrane in animal cytokinesis.
- Cell plate A structure formed from vesicles that becomes the new wall in plant cytokinesis.
- Haploid Having one set of chromosomes in the nucleus.
- Diploid Having two sets of chromosomes in the nucleus.
- Parent cell A cell that divides to give daughter cells.
Common Mistakes
- Using nuclear division and cytokinesis as the same event.
- Stating that plant cells pinch in two like animal cells.
- Saying meiosis produces genetically identical cells.
- Assuming cytokinesis is always equal.
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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.