Lab Activity No. 7

Meiosis

General Biology Lab/Conference

A. Objectives

Upon completion of this lab activity, you should be able to:

1. Define/explain each of the following terms: DNA, chromosomes, sister chromatids, homologous chromosomes, centromere, meiosis (and all of its stages), diploid, haploid, gene, allelele, gametes, ovum, sperm, zygote, tetrad, crossing over, and recombination.

2. Demonstrate and explain meiosis using chromosome models.

B. Background:

In lecture, we have/will discuss concepts relating to meiosis. The purpose of today’s activity is to introduce you to these concepts using models and group work/discussion. It is important that all group members participate and understand each aspect of the activities. You should refer to the relevant chapters in your text book for assistance in meiosis and answering the questions.

Like mitosis, meiosis is a process of nuclear division. However, the outcome of meiosis is radically different. Meiosis is the basis for sexual reproduction. While mitosis provides diploid daughter cells, meiosis yields haploid daughter cells called gametes. Gametes are produced in reproductive organs of organisms, thus meiosis is restricted to those organs only.

C. Activity: Simulation ofMeiosis

Build four pairs of chromosomes (eight total) by assembling strings of pop beads as follows:

1. Build the first pair of chromosomes by putting 8 yellow beads on each end of two yellow magnetic centromeres.

2. Build the second pair by putting 8 red beads on each end of two red magnetic centromeres.

3. Build the third and fourth pairs by repeating steps 1 and 2 but using only 3 beads on the ends of the centromeres. You should now have four long strings (two red and two yellow) and four short strings (two red and two yellow).

These eight strings of pop beads will be used to represent the chromosomes in the nucleus of a dividing cell. A single string represents one unduplicated (unreplicated) chromosome. When it is matched with its identical partner by joining the magnetic centromeres, the two strands will represent one duplicated (replicated) chromosome. Remember each replicated chromosome consists of two sister chromatids.

4. During the simulation, you will keep track of alleles on the chromosomes. Before beginning the simulation, mark the location of alleles using a black marking pen. Mark “A” on one end of each long red chromatid, mark “a” on one end of each long yellow chromatid. (You should have written the letter “A” twice and the letter “a” twice.) Repeat the procedure for the short chromatids using “F” and “f’. A, a, F, and f represent alleles for two genes.

5. Place one of each kind of strand in the center of the table, which will represent the nucleus of a dividing cell. Your nucleus should have four chromosomes, two long (one red and one yellow) and two short (one red and one yellow).

6. As outlined below, manipulate the model chromosomes through all the phases of meiosis (steps following), starting with G1 of interphase and ending with cytokinesis. Make certain that each member of the group does this process individually, explaining it to the other members as they proceed.

a.Replicate DNA: place the second of each kind of chromosome next to the one you placed on the table (Step 4 above) joining the centromeres. You now have a model of replicated chromosomes.

b.Meiosis proceeds without crossing over:

  1. Pairs of replicated homologous chromosomes line up together across the center of the cell. This pairing arrangement is called a tetrad.
  2. Continue to simulate the remaining phases of meiosis until you have chromosome models of gametes, displaying haploid nuclei. Refer to your text if you need assistance with the sequence of meiosis. Fill in diagrams of each phase in the chart provided labeled “Diagram 1” using colored pencils to draw the chromosomes.

c.The effects of independent assortment:

  1. Simulate meiosis again, only this time, show an outcome different from the first simulation due to the effects of independent assortment. Draw the stages for this simulation in the chart labeled “Diagram 2”.

d.Meiosis proceeds with crossing over:

  1. In Prophase I, when the chromosomes are in the tetrad arrangement, segments of nonsister chromatids overlap. Overlapping segments are often exchanged between nonsister chromatids of the two homologous pairs. This is called recombination. To simulate crossing over and recombination, break four beads from the long arm of each color of one sister chromatid (2 breaks total). Reattach each four bead segment to the other color chromatid. Repeat this procedure for the small chromosome, but only removing 2 beads.
  2. Continue to simulate the remaining phases of meiosis until you have models of haploid nuclei. Are these nuclei different than the ones generated without crossing over? Draw the stages for this simulation in the chart labeled “Diagram 3”.

7. When you are finished, use acetone and a paper towel to wipe the black letters off of the pop beads before putting them away.

Diagram 1. Meiosis without crossing over. Use colored pens, pencils, etc. to show the series of steps as a cell proceeds through meiosis.

Meiosis I

Prophase 1 / Metaphase I / Anaphase I / Telophase I
Meiosis II
Prophase II / Metaphase II / Anaphase II / Telophase II

notes:

Diagram 2. Meiosis without crossing over, but showing the effects of independent assortment. Use colored pens, pencils, etc. to show the series of steps as a cell proceeds through meiosis. Show a different arrangement of the chromosomes in Metaphase I than you did in the previous diagram to illustrate the effects of independent assortment.

Meiosis I

Prophase I / Metaphase I / Anaphase I / Telophase I
Meiosis II
Prophase II / Metaphase II / Anaphase II / Telophase II

notes:

Diagram 3. Meiosis with crossing over. Use colored pens, pencils, etc. to show the series of steps as a cell proceeds through meiosis. Include a crossover and recombination in Prophase I. Be sure the gametes produced show the effects of the crossover.

Meiosis I

Prophase I / Metaphase I / Anaphase I / Telophase I
Meiosis II
Prophase II / Metaphase II / Anaphase II / Telophase II

notes:

D. Quiz Questions

  1. Fundamentally, how does meiosis differ from mitosis?
  1. If a cell’s nucleus has 46 chromosomes before mitosis, how many chromosomes are in each nucleus after mitosis? If this same cell went through meiosis, how many chromosomes would be in each nucleus in the end?
  1. What are gametes? What are the names for the human male and female gametes? In what organs do these gametes develop?
  1. Define diploid and haploid. Are gametes diploid or haploid? Explain the rationale for your response and the consequences for the incorrect answer.
  1. What is meant by the terms meiosis I and meiosis II?
  1. Summarize the events of meiosis I and meiosis II (your text is an excellent reference).
  1. What is independent assortment? How does it contribute to genetic variation in gametes?
  2. Explain the process and consequences of crossing over. Your response should include the following terms: prophase I, sister chromatids, homologous chromosomes, genetic recombination, variability, nonsister chromatids, and alleles.