Purpose: The purpose of this exercise is to investigate the process of meiosis and how this process compares

to mitosis.

Objectives:

  1. What type of reproduction uses the process of meiosis?
  2. What are the phases of meiosis and demonstrate how reduction of the DNA occurs to produce haploid cells.
  3. How does meiosis lead to greater genetic variation?
  4. How do the processes of mitosis and meiosis compare?

Background:

Most cells in organisms that reproduce sexually are diploid. They have two sets of chromosomes and two complete sets of genes. Gametes are an exception. Gametes are the cells that combine during sexual reproduction. In animals, these cells are called sperm and eggs. Gametes are haploid cells with only one set of chromosomes. Meiosis is the process in which haploid cells form from diploid cells.

In this lab, you will model the steps in meiosis. You will make drawings of your models. You will also identify points in the process that can lead to greater genetic variation.

Pre-Lab Questions:

  1. Define the term homologous chromosomes. ______

______

  1. What is a tetrad? ______
  2. The process of mitosis in multicellular organisms creates 2 daughter cells identical to the parent cell. In the process of meiosis how many cells will be produced from one parent cell? ______

Describe how the DNA in the new daughter cells will compare to the original parent cell. ______

______

  1. In mitosis there is one division of the DNA and one division of the cytoplasm. In the process of meiosis how many divisions must occur? ______
  2. If a diploid cell has two pairs of homologous chromosomes. How many different combinations of chromosomes could there be in the gametes at the end of meiosis? ______

______

Part 1 – Procedures: Virtual Meiosis

In this investigation, you will view sites that illustrate the process of meiosis. For each site answer the questions associated. Remember that url's must be typed in exactly as they appear.

Sumanas Inc., Animation of Meiosis
---> go to animation gallery --> go to general biology --> Meiosis

  1. Read the introduction. Explain how sexual reproduction results in unique offspring. ______

______

(Click the "STEP THROUGH" button)

  1. DNA replication takes place when? ______
  2. Meiosis consists of two cell divisions: ______& ______
  3. Centrosomes (aka centrioles) migrate to ______
  4. The pairing of homologous chromosomes is called: ______
  5. Crossing over points are called ______
  6. What happens in metaphase I ______
  7. What happens during anaphase I ______
  8. What is interkinesis? ______
  9. In prophase II, each cells is [ diploid / haploid ] (circle)
  10. In metaphase II, chromosomes line up in [ single | double ] file.
  11. What happens during telophase II? ______
  12. (Click to Conclusion). Explain how the DNA compares in each of the four daughter cells produced by meiosis. ______

(Click the Back Button)  (Click Meiosis Again)  (Click on Quiz)

  1. With respect to meiosis, when does DNA replication occur? ______
  2. When does crossing over occur? ______
  3. During which phase do chromosomes line up along the equator? ______
  4. During which phase does the nuclear membrane form around the chromosomes? ______

Gene Segregation

There are two ways in which the chromosomes can end up after meiosis. Sketch the two ways and indicate by color the chromosomes (use the following color codes: Blue, Purple & Red, Orange)

Internet Website 2: PBS: Mitosis vs. Meiosis
--> Click "How Cells Divide" -->"Mitosis vs. Meiosis"

After viewing the animation, fill out the chart below, by placing a check in the box or boxes to indicate which the event occurs in (some events might have checks for both mitosis and meiosis).

Meiosis / Mitosis
Two cell divisions
Centrioles appear
Chromosomes pair up
Spindle fibers form
Two cell divisions
Cytokinesis
Four daughter cells

Part 2 – Procedures: - Modeling Meiosis and DNA Structures

The diploid cell in your model will have two pairs of homologous chromosomes. In order to keep track of the pairs, you will use 2 different colors of beads. The beads will represent genes. Use the large sheet of paper to represent the cell.

Part A: Interphase – In this model the organism will have 2 different chromosomes.

Just before meiosis begins, the chromosomes are replicated.

  1. Use 10 beads of the same color and a white centromere to construct the first single chromosome. (Represents chromosome #1 from Mom)
  1. Now use ten beads of the other color and a white centromere to construct a second single chromosome. (Represents chromosome #1 from Dad)
  2. Now construct the chromosome #2 from mom using only 5 beads of the same color as you used in step 1.
  3. Then construct the chromosome #2 from dad using only 5 beads of the same color as you used in step 2.
  4. Make a drawing of the chromosomes in the space below. Use colored pencils.
  1. Now replicate your single chromosomes using the different colored beads.
  2. Attach the 2 single chromosomes of the same color together at the centromeres
  3. Make a drawing of the chromosomes in the space below.

Part B: Meiosis I - During meiosis I, the cell divides into two diploid daughter cells.

  1. Pair up the chromosomes to form tetrads. Use the longer tetrad to model crossing-over.
  1. Make a drawing of the tetrads in the space below. This would be during PROPHASE I.
  1. Line up the tetrads down the center of your “cell.”
  2. Make a drawing of the tetrads in the space below. This would be during Metaphase I.
  1. Then model what happens to the chromosomes during ANAPHASE I, and draw the tetrads splitting and the sister chromatids moving to opposite sides of the cell.
  1. Divide the cell into two daughter cells. Use the space below to make a drawing of the result. End of TELOPHASE I

Part C: Meiosis II - During meiosis II, the daughter cells divide again.

  1. Line up the chromosomes at the center of the first cell, one above the other. Separate the chromatids in each chromosome and move them to opposite sides of the cell.
  1. Repeat Step 15 for the second cell.
  1. Divide each cell into two daughter cells. Use the space below to make a drawing of the four haploid cells.

Analyze and Conclude

  1. How does crossing-over increase variation in genes? ______

______

______

2. Suppose no crossing-over takes place. Use Step 5 to explain why meiosis will still increase genetic variation. ______

______

3. Compare your models of the four haploid cells with those of other groups. Describe and explain any variation that you observe. ______

______

4. What would happen to the possible variation in gametes if the number of chromosome pairs increased from two to three? What if the number increased again from three to four? ______

______

______

5. How can independent assortment help explain genetic diversity in humans? Hint: How many pairs of chromosomes are in a human diploid cell? ______

______

______

Cell Division Cross word vocab