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1.10: Meiosis and Gametogenesis

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  • Page ID
    222596
    • Sharbani Banerjee, Casey Barr, Joe Frantz, Melissa Graham, Tyler Wilson
    • Oxnard College

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    In the previous laboratory exercise, you examined mitosis, the process of cell division used for growth, repair, and replacement of cells. Mitosis produces two genetically identical daughter cells and helps multicellular organisms maintain their tissues throughout life.

    In this laboratory, you will investigate meiosis, a different type of cell division found in organisms that reproduce sexually. Meiosis produces haploid cells containing half the chromosome number of the original cell. In animals, these cells develop into gametes, such as sperm and eggs. In plants, meiosis produces spores that develop into structures that later produce gametes. During meiosis, chromosomes are shuffled and recombined, creating genetic variation among offspring.

    In this exercise, you will compare mitosis and meiosis, model the process of meiosis using chromosome beads, explore how independent assortment and crossing over increase genetic variation, and observe cells undergoing meiosis in the lily anther.

    Comparing Mitosis and Meiosis:

    In both meiosis and mitosis, the original parent cell is a diploid cell. Remember, this means the cell has two copies of each chromosome. Meiosis and mitosis are both nuclear divisions that result in new daughter cells. However, the two processes have significant differences. Using the diagram of mitosis and meiosis below, fill out the chart below comparing the two forms of nuclear division.

    This illustration compares meiosis and mitosis. In meiosis, there are two rounds of cell division, whereas there is only one round of cell division in mitosis. In both mitosis and meiosis, DNA synthesis occurs during S phase. Synapsis of homologous chromosomes occurs in prophase I of meiosis, but does not occur in mitosis. Crossover of chromosomes occurs in prophase I of meiosis, but does not occur in mitosis. Homologous pairs of chromosomes line up at the metaphase plate during metaphase I of meiosis, but not during mitosis. Sister chromatids line up at the metaphase plate during metaphase II of meiosis and metaphase of mitosis. The result of meiosis is four haploid daughter cells, and the result of mitosis is two diploid daughter cells.

      Mitosis
    (begins with a single cell)    		 Meiosis
    (begins with a single cell)
    # Chromosomes in parent cells    
    # DNA replications    
    # Nuclear divisions    
    # Daughter cells produced    
    Purpose    

    Part 1: Meiosis Bead Simulation

    Materials:

    • 12 magnets (=centromeres)
    • 48 beads of one color
    • 48 beads of another color

    Procedure:

    1. Set up half of the beads exactly as follows, representing genes on the chromosome of a hypothetical critter. We will assume that the critter is diploid (2N) and has three different chromosomes. Because the critter has two copies of each of the three chromosomes, the diploid number is 6 (2 × 3 = 6).

      This is what your critter's chromosomes look like in the unreplicated form. Note that there are six chromosomes here consisting of three homologous pairs. Each chromosome pair consists of a maternal and paternal version of the chromosome. The maternal and paternal versions are represented by the respective bead color.

      This is what your critter’s chromosomes look like in the unreplicated form. Note that there are six chromosomes here consisting of three homologous pairs. Each chromosome pair consists of a maternal and paternal version of the chromosome. The maternal and paternal versions are represented by the respective bead color.

    2. Replicate your chromosomes! Make enough copies of each chromosome to represent both paternal and maternal chromosomes in a replicated form, as shown below. Note that the sister chromatids are identical in color. Be sure you can identify the sister chromatids, chromosomes, and the difference between a replicated and non-replicated form.

    Chromosome diagram illustrating homologous pairs and centromere positions during chromosome organization. White and gray bead-like chromatids connect at black centromeres, while brackets group corresponding chromosomes and show duplicated structures.

    3. Using your maternal and paternal sets of replicated chromosomes and this lab (or the text) as a reference, practice the process of meiosis until you are very comfortable with it. Each person in the group should practice the entire process.

    Part 1: Simulate crossing over by exchanging beads on non-identical sister chromatids as illustrated below. Do this for all the three chromosome pairs.

    Schematic chromosome diagram showing a before-and-after arrangement of white and gray bead-like chromatids, black centromere bars, and crossed strands suggesting chromosomal recombination. Arrow points to four aligned chromatids on right, grouped by color with matching centromere markers.

    Do NOT proceed until until your instructor directs you to do so.

    Part 2: Independent Assortment

    There are two possible ways pairs of homologs (also known as tetrads) can line up on the metaphase plate during Metaphase I. This possible number of alignments equals 2n, where n is the number of chromosomes per set. In humans, n=23, so there are 223 possible ways the homologous pairs can line up on the metaphase plate!

    Procedure:

    1. Use the beads from the last simulation. This time, demonstrate the principle of independent assortment by determining how many different gametes you can form with three homologous pairs.
    2. Use the chromosomes to demonstrate 4 possible ways chromosomes can line up on the metaphase plate.
    3. Using simple chromosome drawings like those demonstrated by your instructor, draw a picture of each possible way of lining up. Use different colors to represent maternal and paternal chromosomes.
    4. Then draw a picture of each possible gamete formed when the chromosomes line up like that.

    Diagram illustrating two possible alignments of three homologous chromosomes during meiosis I and one resulting gamete. White and gray chromosomes show alternative parental origins, black bands mark centromeres, and brackets group homologues to highlight independent assortment.

    Lab Questions:

    1. How many possible gametes can be formed following meiosis (excluding crossing over events) from an original cell that contains a diploid number of six (2n = 6)? [The number of possible gametes = 2n where n is the number of chromosomes per set.]

    2. How many possible gametes can be formed following meiosis (excluding crossing over events) from an original cell that contains a diploid number of 46 (2n = 46)?

    3. Explain why crossing over can result in a much larger number of possible gamete types than independent assortment alone.


    4. Based upon the processes of independent assortment, crossing over, and random fertilization, what is one important difference you would expect to see between a sexually reproducing population of organisms and an asexually reproducing population of organisms?

    Part 3: Gametogenesis in the Lily Anther

    The formation of gametes, or gametogenesis, is the first stage in sexual reproduction. In single-celled organisms, e.g., many Protista, the vegetative cell acts as a gamete. In more complex organisms specialized regions of the organism are responsible for creating gametes.

    In flowering plants, meiosis occurs in specialized reproductive structures. Female reproductive cells develop within the ovary, while male reproductive cells develop within the pollen sacs (microsporangia) of the anther. In this exercise, you will examine meiosis in the lily anther, a tissue that contains many cells undergoing division at the same time. Because the cells do not separate after each division, the stages of meiosis are relatively easy to observe.

    Illustration A shows cross section of an anther, which has four lobes each containing a pollen sac, or microsporangium. Inside the pollen sac is a layer called the tapetum, and within this ring are the microspore mother cells. As the microsporangium matures, two pollen sacs merge and an opening forms between them so that the pollen can be released. Micrographs in part B show pollen sacs with a visible opening between them. b: A micrograph of an immature lily anther shows four pollen sacs containing pollen grains.
    Figure 1: A cross section of an anther at two developmental stages. The immature anther (top) contains pollen sacs. The sacs are filled with immature male gametes undergoing meiosis. When the gametes mature and become pollen grains, the pollen sac walls split open (bottom), and the pollen grains are released.
    Illustration shows the formation of pollen from a microspore mother cell. The mother cell undergoes meiosis to form a tetrad of cells, which separate to form the pollen grains. The pollen grains undergo mitosis without cytokinesis, resulting in four mature pollen grains with two nuclei each. One is called the generative nucleus, and the other is called the pollen tube nucleus. Two projective layers form around the mature pollen grain, the inner intine and the outer exine. Micrograph shows a pollen grain, which looks like puffed wheat.
    Figure 2: Meiosis in the lily anther. Notice that the cells remain together after each division until they become fully mature

    Stages of Meiosis in the Lily Anther

    Prophase I

    Metaphase I

    Anaphase 1

    Telophase 1

    Microscopic photograph shows blue-stained cells with visible nuclei and condensed chromosomes, illustrating cell division. Black arrow highlights a prominent cell containing clustered chromosomes, while surrounding cells display varying division stages.

    Microscopic photograph shows multiple blue-stained cells with red-purple chromosomes at different stages of cell division. Black arrow highlights a central cell containing condensed chromosomes, while surrounding cells display varied chromosome arrangements and shapes.

    Microscopy photograph shows stained cells at different stages of cell division, likely for observing meiosis. Purple-blue cells surround a central enlarged cell marked by a black arrow, with visible condensed chromosomes and varied nuclear structures.

    Microscope photograph showing purple-stained cells at different stages of division. Black arrow highlights a large central cell with condensed chromosome-like structures, surrounded by similarly stained rounded cells.

    Homologues pair

    Homologues line up

    Homologues separate

    Nuclear membrane reforms

    Prophase II

    Metaphase II

    Anaphase II

    Telophase II

    Microscopic photograph shows turquoise-stained cells with purple internal structures, illustrating a biological sample during cellular development or division. Black arrow highlights a central cell, while additional cells appear scattered across a pale background.

    clipboard_e5eb02eb2ec744460f3b52ff2c0fe7f93.png

    Microscope photograph showing onion root-tip cells at various stages of mitosis. Blue-stained cells contain condensed purple chromosomes, with black arrow marking a cell in anaphase as chromosome groups separate toward opposite poles.

    clipboard_ed2841d81e2cda5056cdbdfefc9cb1438.png

    Chromosomes condense

    Replicated chromosomes line up

    Sister chromatids separate

    Nuclear membrane reforms

    Attribution: Lily anther meiosis photographs courtesy of Dr. Derek Zelmer (University of South Carolina Aiken), used with permission.

    Procedure:

    1. Examine prepared slides of lily anther microsporangium under low power (10X). Note the bilobed structure. Immature gametes undergoing meiosis are found in the center of each lobe.
    2. With the 40X objective (400X magnification), find one or two cells in each stage of meiosis and draw one in the provided space below. Use the photographs above to identify the stages. Remember that if you see two cells together, the cells are in meiosis II.

    Square Square

    Prophase I Metaphase I

    Square Square

    Anaphase I Telophase I

    Square Square

    Prophase II Metaphase II

    Square Square

    Anaphase II Telophase II

    Licensing and Attributions

    Adapted from Biology Labs. Authored by: Wendy Riggs. Provided by: College of the Redwoods. Located at: http://www.redwoods.edu. License: CC BY: Attribution

    CC licensed content, Original

    Module 8: Meiosis and Gametogenesis is shared under a CC BY license and was authored, remixed, and/or curated by LibreTexts.

    This page titled 1.10: Meiosis and Gametogenesis was last modified on Sat, 31 Oct 2026 00:05:53 GMT and is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by Sharbani Banerjee, Casey Barr, Joe Frantz, Melissa Graham, Tyler Wilson.