# Mitosis and cytokinesis

> CIE A-Level Biology · 9700
> Source: https://www.owlsprep.com/study/cie-9700-u5-mitosis-and-cytokinesis/

This subtopic covers the process of nuclear division (mitosis) and cytoplasmic division (cytokinesis) that completes the mitotic phase of the cell cycle. You will learn stage-specific events and key differences between plant and animal cell division.

**Prerequisites:** [The mitotic cell cycle overview](https://www.owlsprep.com/study/cie-9700-u5-mitotic-cell-cycle-intro/); [Chromosome structure](https://www.owlsprep.com/study/cie-9700-u5-chromosome-structure/)

## Learning objectives

- Identify the four main stages of mitosis and describe their key events
- Distinguish between cytokinesis in plant and animal cells
- Explain the biological significance of mitosis
- Recognise mitotic stages from diagrams and photomicrographs

## Mitosis: Definition and Pre-Division Preparation

**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.

**Worked example:** 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 × 2 = 156

> **Exam tip:** CIE examiners frequently test the common misconception that interphase is a stage of mitosis. Interphase is not part of mitosis.

## The Four Stages of Mitosis

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 |

**Worked example:** 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.

## Cytokinesis: Cytoplasmic Division

**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.

**Worked example:** 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.

## Biological Significance of Mitosis

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**

Check your understanding:

1. 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

   *Why:* Gametes (sperm and eggs) are produced by meiosis, not mitosis. All other options are core functions of mitosis in humans.

## Common pitfalls

- **Wrong:** Claiming interphase is a stage of mitosis
  - Why it fails: Interphase is the part of the cell cycle before mitosis begins, not a stage of mitosis itself
  - Correct: Mitosis only consists of prophase, metaphase, anaphase and telophase
- **Wrong:** Stating chromosome number halves during anaphase of mitosis
  - Why it fails: Separated sister chromatids are each full chromosomes, so the total number of chromosomes temporarily doubles; ploidy is maintained
  - Correct: Mitosis produces daughter cells with the same chromosome number as the parent cell
- **Wrong:** Claiming plant cells use a cleavage furrow for cytokinesis
  - Why it fails: Plant cells have a rigid cell wall that cannot be pulled inwards to form a furrow
  - Correct: Plant cells form a cell plate from Golgi vesicles; only animal cells form a cleavage furrow
- **Wrong:** Saying spindle fibres pull chromatids to the poles in metaphase
  - Why it fails: Spindle fibres only attach to centromeres in metaphase; separation of chromatids occurs in anaphase
  - Correct: Alignment at the equator occurs in metaphase; separation to opposite poles occurs in anaphase
- **Wrong:** Claiming mitosis produces genetically different daughter cells
  - Why it fails: Mitosis separates identical sister chromatids, so daughter cells are genetic clones of the parent cell
  - Correct: Meiosis produces genetically different daughter cells; mitosis produces genetically identical daughter cells

## 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 |

## 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.

- [Cell cycle control and cancer](https://www.owlsprep.com/study/cie-9700-u5-cell-cycle-control-and-cancer/)
- [Stem cells](https://www.owlsprep.com/study/cie-9700-u5-stem-cells/)
- [Nucleic Acids and Protein Synthesis](https://www.owlsprep.com/study/cie-9700-u6-overview/)

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