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Sometimes students struggle with some concepts that relate to evolution. Meiosis is a somewhat complicated process, but necessary to mix up genetics of offspring so natural selection can work on a population by choosing the most desirable traits to be passed down to the next generation.
Hands-on activities can help some students grasp the concepts. Especially in cellular processes when it is difficult to imagine something so small. The materials in this activity are common and easily found. The procedure does not rely on expensive equipment like microscopes or take up a lot of space.
Preparing for Modeling Meiosis Classroom Lab Activity
Before starting the lab, make sure students can define the following terms:
- Crossing Over
- Homologous Pair
Purpose of the Lesson
To understand and describe the process of meiosis and its purpose using models.
Most cells in multicellular organisms like plants and animals are diploid. A diploid cell has two sets of chromosomes that form homologous pairs. A cell with only one set of chromosomes are considered haploid. Gametes, like the egg and sperm in humans, are examples haploid. Gametes fuse during sexual reproduction to form a zygote which is once again diploid with one set of chromosomes from each parent.
Meiosis is a process that starts with one diploid cell and creates four haploid cells. Meiosis is similar to mitosis and must have the cell's DNA replicate before it can begin. This creates chromosomes that are made up of two sister chromatids connected by a centromere. Unlike mitosis, meiosis requires two rounds of division to get half the number of chromosomes into all of the daughter cells.
Meiosis begins with meiosis 1 when homologous pairs of chromosomes will be split. The stages of meiosis 1 are similarly named to the stages in mitosis and also have similar milestones:
- prophase 1: homologous pairs come together to form tetrads, nuclear envelope disappears, spindle forms (crossing over may also happen during this phase)
- metaphase 1: tetrads line up at the equator following the law of independent assortment
- anaphase 1: homologous pairs are pulled apart
- telophase 1: cytoplasm divides, nuclear envelope may or may not reform
The nuceli now only have 1 set of (duplicated) chromosomes.
Meiosis 2 will see the sister chromatids split apart. This process is just like mitosis. The names of the stages are the same as mitosis, but they have the number 2 after them (prophase 2, metaphase 2, anaphase 2, telophase 2). The main difference is that the DNA does not go through replication before the start of meiosis 2.
Materials and Procedure
You will need the following materials:
- 4 different colors of paper (preferably light blue, dark blue, light green, dark green)
- Ruler or Meter Stick
- 4 paper clips
- Using 1 m piece of string, make a circle on your desk to represent the cell membrane. Using a 40 cm piece of string, make another circle inside the cell for the nuclear membrane.
- Cut 1 strip of paper that is 6 cm long, and 4 cm wide from each color of paper (one light blue, one dark blue, one light green, and one dark green) Fold each of the four strips of paper in half, lengthwise. Then place the folded strips of each color inside the nucleus to represent a chromosome before replication. The light and dark strips of the same color represent homologous chromosomes. At one end of the dark blue strip write a large B (brown eyes) on the light blue make a lower case b (blue eyes). On the dark green at a tip write T (for tall) and on the light green write a lower case t (short)
- Modeling interphase: to represent DNA replication, unfold each paper strip and cut in half lengthwise. The two pieces that result from cutting each strip represent the chromatids. Attach the two identical chromatid strips at the center with a paperclip, so an X is formed. Each paper clip represents a centromere.4
- Modeling prophase 1: remove the nuclear envelope and put it aside. Place the light and dark blue chromosomes side by side and the light and dark green chromosomes side by side. Simulate crossing over by measuring and cutting a 2 cm tip for a light blue strip that includes the letters you drew on them earlier. Do the same with a dark blue strip. Tape the light blue tip to the dark blue strip and vice versa. Repeat this process for the light and dark green chromosomes.
- Modeling metaphase 1: Place four 10 cm strings inside the cell, so that two strings extend from one side into the center of the cell and two strings extend from the opposite side into the center of the cell. The string represents the spindle fibers. Tape a string to the centromere of each chromosome with tape. Move the chromosomes to the center of the cell. Make sure that the strings attached to the two blue chromosomes come from opposite sides of the cell (same for the two green chromosomes).
- Modeling anaphase 1: Grab onto the ends of the strings on both sides of the cell, and slowly pull the strings in opposite directions, so the chromosomes move to opposite ends of the cell.
- Modeling telophase 1: Remove the string from each centromere. Place a 40 cm piece of string around each group of chromatids, forming two nuclei. Place a 1 m piece of string around each cell, forming two membranes. You now have 2 different daughter cells.
- Modeling prophase 2: Remove the strings that represent the nuclear membrane in both cells. Attach a 10 cm piece of string to each chromatid.
- Modeling metaphase 2: Move the chromosomes to the center of each cell, so they are lined up at the equator. Make sure the strings attached to the two strips in each chromosome come from opposite sides of the cell.
- Modeling anaphase 2: Grab onto the strings on both sides of each cell, and pull them slowly in opposite directions. The strips should separate. Only one of the chromatids should have the paper clip still attached to it.
- Modeling telophase 2: Remove the strings and paper clips. Each strip of paper now represents a chromosome. Place a 40 cm. piece of string around each group of chromosomes, forming four nuclei. Place a 1m string around each cell, forming four separate cells with only one chromosome in each.
Have students answer the following questions to understand the concepts explored in this activity.
- What process did you model when you cut the strips in half in interphase?
- What is the function of your paper clip? Why is it used to represent a centromere?
- What is the purpose of placing the light and dark strips of the same color side by side?
- How many chromosomes are in each cell at the end of meiosis 1? Describe what each part of your model represents.
- What is the diploid chromosome number of the original cell in your model? How many homologous pairs did you make?
- If a cell with a diploid number of 8 chromosomes undergoes meiosis, draw what the cell looks like after Telophase 1.
- What would happen to an offspring if cells did not undergo meiosis before sexual reproduction?
- How does crossing over change diversity of traits in a population?
- Predict what would happen if homologous chromosomes did not pair in prophase 1. Use your model to show this.