# The Stages of Mitosis ## Overview of Mitosis - Mitosis is the process of nuclear division producing **two genetically identical daughter nuclei** - The daughter nuclei are also genetically identical to the parent cell nucleus - They contain the same number of chromosomes as the parent cell

Mitosis is nuclear division resulting in two genetically identical daughter nuclei with the same chromosome number as the parent cell

- Although mitosis is one continuous process in reality, it can be divided into **four main stages** for study purposes ## The Four Stages - Mitosis consists of four distinct stages: - **Prophase** - **Metaphase** - **Anaphase** - **Telophase**

Use the mnemonic **PMAT** to remember the order: Prophase, Metaphase, Anaphase, Telophase

- Most organisms have many chromosomes (humans have 46), but for clarity these notes describe mitosis in an animal cell with only four chromosomes - The different chromosome colours represent maternal and paternal origin (half from each parent) ## Prophase ### Key Events - **Chromosomes condense** and become visible when stained - Each chromosome consists of **two identical chromatids** called **sister chromatids** - Each chromatid contains one DNA molecule - Sister chromatids are joined at the **centromere** - The two **centrosomes** (replicated during $G_2$ phase) move towards **opposite poles** of the cell - **Spindle fibres** (protein microtubules) begin to emerge from the centrosomes - In animal cells, centrosomes consist of two centrioles - The **nuclear envelope** (nuclear membrane) **breaks down** into small vesicles - The **nucleolus disappears** ### Summary Table | Structure | Change during Prophase | |-----------|----------------------| | Chromosomes | Condense and become visible as pairs of sister chromatids | | Centrosomes | Move to opposite poles of the cell | | Spindle fibres | Begin to form from centrosomes | | Nuclear envelope | Breaks down into vesicles | | Nucleolus | Disappears | ## Metaphase ### Key Events - **Centrosomes reach opposite poles** of the cell - **Spindle fibres** continue to extend from the centrosomes - Chromosomes **line up at the equator** of the spindle (also called the **metaphase plate**) - They are positioned equidistant from the two centrosome poles - Spindle fibres reach the chromosomes and **attach to the centromeres** - This attachment involves specific proteins called **kinetochores** - Each **sister chromatid** is attached to a spindle fibre originating from **opposite poles**

At metaphase, chromosomes are aligned in the middle - think "M for Middle"

### Summary Table | Structure | Position/State at Metaphase | |-----------|---------------------------| | Centrosomes | At opposite poles | | Chromosomes | Lined up at cell equator | | Spindle fibres | Attached to centromeres via kinetochores | | Sister chromatids | Each attached to opposite poles | ## Anaphase ### Key Events - The **sister chromatids separate** at the centromere - The centromere divides in two - **Spindle fibres begin to shorten** - The separated sister chromatids (now called **chromosomes**) are **pulled to opposite poles** - They are pulled by the shortening spindle fibres (protein microtubules)

Sister chromatids become individual chromosomes once they separate at the centromere

### V-Shape Formation - As chromosomes are pulled to opposite poles, they often form a characteristic **V-shape** - The centromere leads (at the point of the V) as the chromosome arms trail behind - This creates a distinctive appearance that helps identify anaphase in micrographs ### Summary Table | Event | Description | |-------|-------------| | Centromere division | Sister chromatids separate | | Spindle fibre action | Shorten, pulling chromosomes apart | | Chromosome movement | Move to opposite poles of the cell | | Chromosome number | Doubles at each pole (temporarily) | ## Telophase ### Key Events - **Chromosomes arrive at opposite poles** and begin to **decondense** - They start to uncoil and become less visible - **Nuclear envelopes** (nuclear membranes) begin to **reform** around each set of chromosomes - This creates two separate nuclei - The **spindle fibres break down** - New **nucleoli reform** within each nucleus - Each chromatid now contains one DNA molecule that will replicate during the next interphase ### Following Mitosis - Telophase is followed by **cytokinesis** (division of the cytoplasm) - The centrosome will replicate during interphase before the next nuclear division ### Summary Table | Structure | Change during Telophase | |-----------|------------------------| | Chromosomes | Arrive at poles and decondense | | Nuclear envelope | Reforms around each chromosome set | | Spindle fibres | Break down and disappear | | Nucleolus | Reforms in each nucleus | ## Understanding DNA and Chromosome Numbers - After interphase but before mitosis, a human parent cell nucleus contains **92 DNA molecules** - This is because the 46 DNA molecules have replicated during S phase to form sister chromatids - Sister chromatids = 46 chromosomes × 2 chromatids each = 92 DNA molecules - During mitosis, these sister chromatids separate so each daughter cell receives **46 chromosomes** - Each chromosome contains one DNA molecule - This maintains the diploid number of 46 in human cells

Count chromosomes by counting centromeres: 46 centromeres = 46 chromosomes, even if each has 2 sister chromatids

### Example: Human Cell Through Mitosis | Stage | Chromosomes | Chromatids | DNA Molecules | |-------|-------------|-----------|---------------| | After S phase (before mitosis) | 46 | 92 | 92 | | Prophase to Metaphase | 46 | 92 | 92 | | After Anaphase | 92 (46 at each pole) | 92 | 92 | | After Telophase (each nucleus) | 46 | 46 | 46 | - Remember: different organisms have different diploid numbers - Always check the organism mentioned in exam questions - Only human diploid cells have 46 chromosomes ## Key Terminology | Term | Definition | |------|------------| | Sister chromatids | Two identical copies of a chromosome joined at the centromere | | Centromere | The region where sister chromatids are joined together | | Spindle fibres | Protein microtubules that move chromosomes during mitosis | | Centrosome | Structure that organizes spindle fibres (contains centrioles in animal cells) | | Kinetochore | Protein structure on the centromere where spindle fibres attach | | Metaphase plate | The equator of the spindle where chromosomes align | ## Practical Tips for Identification - **Prophase**: Look for condensed chromosomes as pairs, nuclear envelope disappearing - **Metaphase**: Chromosomes aligned at the cell equator in a straight line - **Anaphase**: Chromatids being pulled apart in a V-shape towards opposite poles - **Telophase**: Two groups of chromosomes at opposite ends, nuclear envelopes reforming

Read questions carefully - chromosome numbers vary between organisms and only human cells have a diploid number of 46