Mitosis and cytokinesis
CIE A-Level BiologyΒ· Unit 5: The Mitotic Cell CycleΒ· 45 min read
1. Mitosis: Definition and Pre-Division Preparationβ β ββββ± 10 min
Mitosis
A type of nuclear division that produces two genetically identical daughter nuclei, each containing the same number of chromosomes as the original parent cell
Example:
A diploid parent cell (2n = 4) produces two diploid daughter cells, each with 4 complete chromosomes
Mitosis follows the G2 phase of interphase, the stage of the cell cycle between divisions. During interphase before mitosis: DNA replicates during S phase, the cell produces extra organelles, and synthesizes proteins needed for division. Chromosomes are decondensed and not visible as individual structures at this point.
A diploid dog cell has 2n = 78 chromosomes. How many sister chromatids are present at the start of prophase of mitosis?
- 1
Every chromosome replicates during S phase of interphase to form two identical sister chromatids, held together at the centromere.
- 2
For 2n = 78, there are 78 individual chromosomes in the cell.
- 3
Total number of sister chromatids = 78 Γ 2 = 156
Exam tip:
CIE examiners frequently test the common misconception that interphase is a stage of mitosis. Interphase is not part of mitosis.
2. The Four Stages of Mitosisβ β β βββ± 15 min
Mitosis proceeds in four continuous, sequential stages, each defined by the position and state of chromosomes within the cell.
Stage | Key Events |
|---|---|
Prophase | Chromosomes condense and become visible; nucleolus disappears; nuclear envelope breaks down; spindle fibres form from centrosomes |
Metaphase | Chromosomes align at the equator of the cell; spindle fibres attach to centromeres of each chromosome |
Anaphase | Centromeres split; sister chromatids separate and are pulled to opposite poles by shortening spindle fibres |
Telophase | Chromosomes decondense; nuclear envelope reforms around each chromosome set; nucleoli reappear; spindle fibres break down |
How would you distinguish between metaphase and anaphase in a photomicrograph of dividing cells?
- 1
In metaphase, all chromosomes are arranged in a single flat plane at the centre (equator) of the cell, and centromeres are still intact.
- 2
In anaphase, centromeres have split and sister chromatids have separated, so two distinct groups of chromosomes are visible at opposite poles of the cell.
Exam tip:
When describing mitotic stages in exam answers, always mention changes to chromosomes, the nuclear envelope, and the spindle to get full marks.
3. Cytokinesis: Cytoplasmic Divisionβ β β βββ± 10 min
Cytokinesis
Division of the cytoplasm that follows mitosis, producing two separate daughter cells each containing one daughter nucleus
Cytokinesis differs significantly between animal and plant cells, due to the presence of a rigid cell wall in plant cells:
Animal cells: A cleavage furrow forms around the middle of the cell, pulled inwards by a ring of actin contractile fibres, until the cytoplasm is pinched into two separate cells.
Plant cells: Vesicles from the Golgi apparatus assemble at the equator of the cell, then fuse to form a new cell plate. The cell plate matures into a new cell wall and cell membrane, separating the two daughter cells.
Explain why cytokinesis occurs via different mechanisms in plant and animal cells.
- 1
Animal cells only have a flexible cell membrane, no outer rigid cell wall.
- 2
This allows the membrane to be pulled inwards to form a cleavage furrow, splitting the cell into two.
- 3
Plant cells have a rigid cell wall that cannot be pulled inwards. Instead, a new cell wall must be built between the two new nuclei to separate them.
Exam tip:
Comparing cytokinesis in plants and animals is a common 3-4 mark question in CIE papers, always mention the cell wall as the root cause of the difference.
4. Biological Significance of Mitosisβ β ββββ± 8 min
Because mitosis produces genetically identical daughter cells, it is essential for three core biological functions in living organisms:
Growth: Multicellular organisms grow by producing more genetically identical cells via mitosis, increasing body size from a zygote to an adult.
Repair and replacement: Dead or damaged cells are replaced by identical new cells via mitosis, for example replacement of skin and gut lining cells.
Asexual reproduction: Many organisms produce genetically identical offspring via mitosis, for example vegetative reproduction in potato plants and budding in yeast.
Check your understanding:
Which of the following is not a function of mitosis in humans?
Growth of the fetus
Production of sperm and eggs
Repair of a broken bone
Replacement of skin cells
Reveal answer
Production of sperm and eggs βGametes (sperm and eggs) are produced by meiosis, not mitosis. All other options are core functions of mitosis in humans.
5. Common Pitfalls
Wrong move:
Claiming interphase is a stage of mitosis
Why:
Interphase is the part of the cell cycle before mitosis begins, not a stage of mitosis itself
Correct move:
Mitosis only consists of prophase, metaphase, anaphase and telophase
Wrong move:
Stating chromosome number halves during anaphase of mitosis
Why:
Separated sister chromatids are each full chromosomes, so the total number of chromosomes temporarily doubles; ploidy is maintained
Correct move:
Mitosis produces daughter cells with the same chromosome number as the parent cell
Wrong move:
Claiming plant cells use a cleavage furrow for cytokinesis
Why:
Plant cells have a rigid cell wall that cannot be pulled inwards to form a furrow
Correct move:
Plant cells form a cell plate from Golgi vesicles; only animal cells form a cleavage furrow
Wrong move:
Saying spindle fibres pull chromatids to the poles in metaphase
Why:
Spindle fibres only attach to centromeres in metaphase; separation of chromatids occurs in anaphase
Correct move:
Alignment at the equator occurs in metaphase; separation to opposite poles occurs in anaphase
Wrong move:
Claiming mitosis produces genetically different daughter cells
Why:
Mitosis separates identical sister chromatids, so daughter cells are genetic clones of the parent cell
Correct move:
Meiosis produces genetically different daughter cells; mitosis produces genetically identical daughter cells
6. Quick Reference Cheatsheet
Stage/Process | Key Exam Features |
|---|---|
Interphase (pre-mitosis) | DNA replicated, chromosomes invisible, ATP increased |
Prophase | Chromosomes condense, nuclear envelope breaks down, spindle forms |
Metaphase | Chromosomes line up at equator, spindle attached to centromeres |
Anaphase | Centromeres split, sister chromatids pulled to opposite poles |
Telophase | Nuclear envelope reforms, chromosomes decondense |
Animal Cytokinesis | Cleavage furrow, splits from outside in |
Plant Cytokinesis | Cell plate from Golgi vesicles, new cell wall built from centre out |
Significance of mitosis | Growth, repair, asexual reproduction; produces genetically identical cells |
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 Β· 1
Identify stage of mitosis from diagram
- 2023 Β· 2
Compare cytokinesis in plants/animals
- 2021 Β· 3
Describe events of anaphase
What's Next
Understanding mitosis and cytokinesis provides the foundation for learning about meiosis, the nuclear division that produces gametes for sexual reproduction. Comparing these two processes is a common extended exam question, so mastering mitotic events first will help you clearly highlight key differences between the two. This knowledge also forms the base for studying the control of the cell cycle and cancer, where uncontrolled mitosis leads to tumour formation, another frequently tested topic in CIE A-Level Biology. Mastering mitotic stages is also essential for practical exam questions involving observation of dividing cells.