# The Stages of Meiosis ## Overview of Meiosis - Meiosis is a form of nuclear division that produces **haploid cells from diploid cells** - It generates gametes in plants and animals for sexual reproduction - Meiosis consists of **two divisions**: meiosis I and meiosis II - Each division includes four stages: **prophase, metaphase, anaphase and telophase**

Meiosis produces four haploid daughter cells from one diploid parent cell through two successive divisions

--- ## Meiosis I: The First Division ### Prophase I - DNA condenses and becomes visible as chromosomes - DNA replication has already occurred, so each chromosome consists of **two sister chromatids** joined at the **centromere** - Chromosomes arrange themselves side by side in **homologous pairs** - A pair of homologous chromosomes is called a **bivalent** - **Crossing over** of non-sister chromatids may occur - The point where crossing over occurs is called the **chiasma** (chiasmata; plural) - Centrioles migrate to opposite poles and the spindle forms - The nuclear envelope breaks down - The nucleolus disintegrates

A bivalent is a pair of homologous chromosomes positioned side by side during prophase I

### Metaphase I - The bivalents line up along the **equator of the spindle** - Spindle fibres attach to the centromeres - The maternal and paternal chromosomes in each pair position themselves **independently of the others** - This is **independent assortment** - The proportion of paternal or maternal chromosomes on each side of the equator is due to chance

Independent assortment during metaphase I is a major source of genetic variation in gametes

### Anaphase I - Homologous pairs of chromosomes are separated - Microtubules pull **whole chromosomes** to opposite ends of the spindle - The centromeres **do not divide** (this is different from mitosis)

In anaphase I, whole chromosomes move to opposite poles, not sister chromatids

### Telophase I - Chromosomes arrive at opposite poles - Spindle fibres start to break down - Nuclear envelopes form around the two groups of chromosomes - Nucleoli reform within each nucleus - Some plant cells proceed directly into meiosis II without reformation of the nucleus in telophase I ### Cytokinesis after Meiosis I - Division of the cytoplasm occurs - Cell organelles are distributed between the two developing cells - **In animal cells:** - Cell surface membrane pinches inwards, creating a cleavage furrow in the middle of the cell - The furrow contracts, dividing the cytoplasm in half - **In plant cells:** - Vesicles from the Golgi apparatus gather along the equator of the spindle (cell plate) - Vesicles merge to form the new cell surface membrane - They secrete calcium pectate which becomes the middle lamella - Cellulose layers are deposited to form the primary and secondary walls

The end product of meiosis I is two haploid cells, each containing one chromosome from each homologous pair

--- ## Meiosis II: The Second Division ### Key Features of Meiosis II - There is **no interphase** between meiosis I and meiosis II - DNA is **not replicated** between the two divisions - The second division is almost identical to mitosis - Each cell from meiosis I divides to produce two more cells ### Prophase II - Nuclear envelope breaks down - Chromosomes condense and become visible - A spindle forms at a **right angle to the previous one** ### Metaphase II - Chromosomes line up in a **single file** along the equator of the spindle - Spindle fibres attach to the centromeres ### Anaphase II - Centromeres divide - Individual **chromatids** are pulled to opposite poles - This creates four groups of chromosomes - Each group has **half the number of chromosomes** compared to the original parent cell

During anaphase II, sister chromatids separate and move to opposite poles as individual chromosomes

### Telophase II - Nuclear membranes form around each group of chromosomes - Chromosomes begin to decondense - Nucleoli reform - Spindle fibres break down ### Cytokinesis after Meiosis II - Cytoplasm divides as new cell surface membranes form - **Four haploid cells** are produced - Each cell is genetically unique due to crossing over and independent assortment --- ## Summary Table: Stages of Meiosis | **Division** | **Stage** | **Key Events** | |--------------|-----------|----------------| | **Meiosis I** | Prophase I | Chromosomes condense; homologous pairs form bivalents; crossing over occurs; nuclear envelope breaks down | | | Metaphase I | Bivalents line up at equator; independent assortment occurs; spindle fibres attach to centromeres | | | Anaphase I | Homologous chromosomes separate and move to opposite poles; centromeres do not divide | | | Telophase I | Chromosomes reach poles; nuclear envelopes reform; cytokinesis produces two haploid cells | | **Meiosis II** | Prophase II | Nuclear envelope breaks down; spindle forms at right angle to previous spindle | | | Metaphase II | Chromosomes line up in single file at equator | | | Anaphase II | Centromeres divide; sister chromatids separate and move to opposite poles | | | Telophase II | Nuclear membranes reform; cytokinesis produces four haploid cells | --- ## Understanding Chromosomes vs Chromatids - We count chromosomes by the **number of centromeres present** - When chromosomes duplicate during S phase, the amount of DNA doubles - However, there are still the same number of chromosomes because there are the same number of centromeres - After replication, each chromosome consists of two **chromatids** (strands of replicated chromosomes) - For example, in humans: - Before S phase: 46 chromosomes, 46 DNA molecules - After S phase: 46 chromosomes, 92 DNA molecules (92 chromatids) - After meiosis I: 23 chromosomes (each consisting of 2 chromatids) - After meiosis II: 23 chromosomes (each consisting of 1 chromatid)

Think of it like this: a chromosome is like a book, and when it replicates, you get two identical copies attached together - still one 'book title' but two copies

--- ## Key Differences Between Meiosis I and Meiosis II | **Feature** | **Meiosis I** | **Meiosis II** | |-------------|---------------|----------------| | DNA replication | Occurs before meiosis I | Does not occur | | Pairing of homologous chromosomes | Yes (bivalent formation) | No | | Crossing over | May occur | Does not occur | | What separates | Homologous chromosomes | Sister chromatids | | Centromere division | No | Yes | | Number of cells produced | 2 haploid cells | 4 haploid cells (total) | | Genetic variation | Created by crossing over and independent assortment | Further distributed between cells | --- ## Visualising the Process While diagrams cannot be included here, imagine the process like this: **Meiosis I:** - Think of sorting a deck of cards where you have pairs (one red, one black of each number) - In prophase I, the pairs line up together and might swap some cards (crossing over) - In metaphase I, each pair stands at the centre line - In anaphase I, one card from each pair goes left, one goes right - Now you have two half-decks, each with one card from every number **Meiosis II:** - Each half-deck now splits its cards down the middle - Remember each 'card' is actually two cards stuck together (sister chromatids) - In anaphase II, these stuck-together cards finally separate - You end up with four quarter-decks, each genetically unique

A common mistake is to confuse what separates in meiosis I (homologous chromosomes) versus meiosis II (sister chromatids)

--- ## Practical Considerations - The stages of meiosis can be observed in reproductive tissues - In plant cells, meiosis occurs in anthers (to produce pollen) and ovaries (to produce egg cells) - Preparing slides of meiotic tissue requires careful timing as cells pass through stages quickly - Staining techniques similar to those used for mitosis can reveal chromosomes during meiosis - The presence of bivalents during prophase I and metaphase I distinguishes meiosis from mitosis