2.2.3 Lab: Modeling Meiosis Dry Lab The Living Earth Sem 1…

Modeling Meiosis

Each of your body cells is a diploid cell, which is represented by the expression 2n. This means that it has a nucleus that contains two sets of the 23 types of chromosomes found in humans. Each type of chromosome contains a unique series of genes that codes for only some of your traits. In order to produce all your genetic traits, your cells need two, or a pair, of each of the 23 types of human chromosomes. In total, your cells have 46 chromosomes, or 23 homologous pairs of chromosomes. Mitosis maintains the 2n number of chromosomes in body cells.

Why are your chromosomes in pairs? Because you received one complete set of chromosomes from your mother and one complete set from your father. The egg cell and sperm cell that fused to produce you were haploid, which means having one set of all an organism’s chromosomes. So, how can these haploid cells be made? The answer is meiosis. Cells in reproductive organs undergo meiosis as they divide to form germ cells that mature into gametes — egg cells or sperm cells. This diagram compares the processes of mitosis and meiosis.

In this descriptive lab, you will model meiosis that begins in a cell of the fruit fly (Drosophila melanogaster). You will use pipe cleaners to represent the four homologous pairs of chromosomes in the nucleus of the cell, as you did when you modeled mitosis. You will then rearrange the pipe cleaners to explore the processes of meiosis and cell division that produce sperm and egg cells.

Be sure to:
Follow safe practices during this investigation.
Pay attention to what you are modeling in each step.

Materials

• 16 pipe cleaners* (4 each of 4 different colors)
• 2 lengths of yarn or string, one 4 feet long and one 8 feet long
• Scissors
• Permanent market
• Camera or drawing utensil

*You can also use different colors and lengths of yarn or string.

Procedure

Part 1: Modeling the Preparations for Cell Division

The cells that go through cell division to make sperm and egg cells must first make a copy of each of their chromosomes. This happens in the synthesis phase, or S phase, of the cell cycle. Cells preparing to undergo meiosis go through the same cell-cycle phases as cells preparing for mitosis.

• Gap 1, or G₁ phase:The cell grows by adding cytoplasm, and its organelles are copied so there will be enough for the new cells that form.
• Synthesis, or S phase:The DNA in each chromosome is copied, and two identical new chromosomes, called chromatids, are synthesized.
• Gap 2, or G₂ phase: The DNA is checked for errors to be sure that each new cell will contain a good copy of all genetic instructions. If a problem is found, the system shuts down to prevent the formation of defective cells.
• Meiosis:The chromosomes go through two rounds of separation into new nuclei through a series of steps that eventually produces four new germ cells.
• Cytokinesis:The cytoplasm is divided, creating two cells with the formation of cell boundaries.

You will make a model of a cell of a fruit fly, focusing on the chromosomes in the nucleus. You will model some of the events in the cell cycle and then take a photo (or draw a sketch) and add labels to show what the cell looks like after key steps.

1. Build a model of a cell in the G₁ stage.

1. Use string to make a large ring (the cell membrane) with a smaller ring (the nuclear envelope) inside of it. Now you have a “cell” with a “nucleus.”
2. Cut the four pipe cleaners of each color so that each color is a different length. For instance, you might cut all four red pipe cleaners to a length of 3 inches, all four blue pipe cleaners to a length of 4 inches, etc. This shows that the chromosomes with different genes (represented by different colors) also differ in length. A pair of pipe cleaners of a certain color and length represents a homologous pair of chromosomes.
3. Place four homologous pairs of chromosomes (two pipe cleaners of the same color and length) inside the nucleus of your model cell.
4. Use the marker to mark the end of one chromosome in each pair. The marked chromosome represents the chromosome inherited from the father. Include a photo or drawing of your model for Step 1 under “Data for Part 1.”

2. Model what happens in the nucleus during the S phase.

1. Place another pipe cleaner of the same color and length beside each pipe cleaner in the nucleus. On each new pipe cleaner paired with one that is already marked, make a matching mark. This shows that these new chromosomes are identical to a chromosome from the father.
2. Twist each pair of identical pipe cleaners together once, to show they are attached. Now each chromosome is doubled and consists of two sister chromatids that are joined by a centromere. Include a photo or drawing of your model for Step 2 under “Data for Part 1.”

3. Model the G₂ phase by confirming that you have exactly double the number of pipe cleaners of each color that you had before.

Data for Part 1: Modeling the Preparations for Cell Division (6 points: 3 points for each step)

Insert photos or drawings of your model in the space below. Label each image with the following: cell membrane, nuclear envelope, nucleus, chromosome, homologous pair, sister chromatid, and centromere.

Step 1: G₁ phase

Step 2: End of S Phase

Part 2: Modeling Meiosis I

Meiosis consists of two consecutive divisions of the nucleus, which may or may not be followed immediately by cytokinesis. As human sperm cells form, cytokinesis follows both the first and second rounds of nuclear division. Each cell that begins meiosis produces four haploid sperm cells.

The cells that undergo meiosis to form human egg cells do not go through cytokinesis until a woman is mature. Then, each month, a cell that began meiosis completes the process and goes through cytokinesis. But only one of the four haploid nuclei formed becomes part of an egg cell. This happens because of unequal division of the cytoplasm. The egg cell provides almost all of the cytoplasm for the new diploid cell formed by fertilization.

4. Model the process of meiosis I. Include a photo or drawing of your model for part of Step 4 under “Data for Part 2.”

1. Remove the string representing the nuclear envelope to show that it dissolves. This happens as the chromosomes condense and become visible inside a cell. These events make up prophase of meiosis I.
2. Arrange the two doubled pipe cleaners (sister chromatids joined by a centromere) of each color side by side so that they lie along and parallel to the equator (central line between the two poles) of the cell. Do not place all the marked chromosomes on the same side of the equator. Mix them up to model a random arrangement of the chromosomes and that either member of a homologous pair could end up on either side of the equator. Then, to represent spindle fibers, attach strings from each pole of the cell to the doubled pipe cleaners on the same side of the equator. Each pair of doubled pipe cleaners represents a homologous pair of chromosomes that has formed a tetrad, and the two members of each homologous pair are connected to opposite poles. These events make up metaphase of meiosis I.
3. Separate the two homologous chromosomes in each tetrad. Do this by moving the two homologous chromosomes in each tetrad in opposite directions. This happens inside a cell as the spindle fibers become shorter. These events make up anaphase of meiosis I.
4. Cut the string that had represented the nuclear envelope into two equal pieces. Then use each piece to make a new nuclear envelope around each group of chromosomes. You should now have two cell nuclei, but they should not be identical. Each nucleus should contain a mix of marked and unmarked chromosomes.

5. Model the process of cytokinesis.

1. Pinch in the top and bottom of the cell membrane until the two sides meet in the middle.
2. Cut the string where the top and bottom parts meet to completely separate the model into two new cells. Your fruit fly has now completed meiosis I! Include a photo or drawing of your model for Step 5 under “Data for Part 2.”

Data for Part 2: Modeling Meiosis I (15 points: 3 points for each step)

Insert photos or drawings of your model at each stage of meiosis in the spaces below. Label each image with the following: cell membrane, nuclear envelope, nucleus, chromosome, tetrad, homologous pair, sister chromatid, and centromere.

Step 4a: End of Prophase of Meiosis I

Step 4b: End of Metaphase of Meiosis I

Step 4c: End of Anaphase of Meiosis I

Step 4d: End of Telophase of Meiosis I

Step 5b: End of Cytokinesis after Meiosis I

Part 3: Modeling Meiosis II

6. Model the process of meiosis II. Include a photo or drawing of your model for part of Step 6 under “Data for Part 3.”

1. Model prophase of meiosis II by removing the string representing the nuclear envelope in each cell produced by meiosis I. You do not need to show the spindle fibers but, instead, recall that they are there by looking at your drawing or photo for Step 5a under “Data for Part 3.”
2. Model metaphase of meiosis II by moving the chromosomes (still consisting of sister chromatids joined by a centromere) to the center of the cell and aligning the centromeres across the equator.
3. Model anaphase of meiosis II by untwisting the pipe cleaners to separate each pair of sister chromatids. Bend each chromatid at its centromere (where a spindle fiber would be attached). Then place the chromatids of each pair on opposite sides of the cell, halfway between the equator and a pole, with its bend pointing toward the nearer pole. There should now be four groups, two in each cell, containing one pipe cleaner of each color. Each pipe cleaner now represents a chromosome.
4. Gather each set of separated pipe cleaners into a pile. Then cut the nuclear envelope strings in half and place a piece of string around each group. Now each cell made by meiosis I contains two identical nuclei.

7. Repeat Step 5 of Part 2 to model cytokinesis of meiosis II. Your fruit fly cell has now completed meiosis! Include a photo or drawing of your model for part of Step 7 under “Data for Part 3.”

Data for Part 3: Modeling Meiosis II (15 points: 3 points for each step)

Insert photos or drawings of your model at each stage of meiosis II in the spaces below. Label each image with the following: cell membrane, nuclear envelope, nucleus, chromosome, sister chromatid, and centromere.

Step 6a: End of Prophase of Meiosis II

Step 6b: End of Metaphase of Meiosis II

Step 6c: End of Anaphase of Meiosis II

Step 6d: End of Telophase of Meiosis II

Step 7: End of Cytokinesis after Meiosis II

Analyze Data and Draw Conclusions (14 points)

1. Mistakes sometimes happen during DNA replication. Small mistakes can cause a chromosome to produce two slightly different sister chromatids. Such mistakes can be missed when the DNA is checked for errors during the G2 phase. Look back at part 1 of the procedure.

1. How could you show in your model that such a mistake had been made but missed by the check during G₂? (1 point)
2. How could you model what would happen if the check during G₂ caused the cell cycle to shut down? Explain. (2 points)

2. A cell can begin meiosis I if it passes the check of its DNA during G₂. Did the two new cells formed after meiosis I have identical sets of chromosomes? Explain why or why not. (2 points)

3. Meiosis is a process that occurs during cell division that leads to the production of gametes. It halves the number of chromosomes that can be passed on to an offspring. It also produces new combinations (variations) of an organism’s genetic material. Use evidence you obtained from modeling meiosis to show that both statements are true. (4 points)

4. Evaluate the advantages and limitations of your models.

1. Identify and describe one benefit of modeling meiosis with pipe cleaners and string. (2 points)
2. Identify and describe three limitations of your models. (3 points)