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Laboratory: Meiosis

INTRODUCTION
It is important to note that organisms that are the product of sexual reproduction have two complete sets of chromosomes. One set was donated by the sperm from the father and the other set was present in the egg. If a special cell division responsible for reducing chromosome number by half did not precede sexual reproduction then each subsequent generation would have twice as many chromosomes as the previous one.

Meiosis involves two successive nuclear divisions that produce four haploid (one of each chromosome - half the diploid number) cells. Melosis I is the reduction division. It is this first division that reduces the chromosome number from diploid (two oif each) to haploid (one of each) and separates the homologous pairs. Meiosis II, the second division, separates the sister chromatids. The result is four haploid gametes.

Mitotic cell division produces new cells genetically identical to the parent cell and meiosis does not. Meiosis increases genetic variation in the population. In addition, chromatids of homologous chromosomes may exchange parts in a process called crossing over. Croossing over generally does not produce new characteristics what it does do is establish new combinations of traits that may have never existed before.

Simulation of Meiosis

In this exercise you will study the process of meiosis using chromosome simulation kits. Your kit should contain two strands of beads of one color and two strands of another color. A homologous pair of chromosomes is represented by one strand of each color, with one of each pair coming from each parent. The second strands of each color are to be used as chromatics for each of these chromosomes.
Interphase. Place one strand of each color near the center of your work area. (Recall that chromosomes at this stage would exist as diffuse chromatin and not as visible structures.) DNA synthesis occurs during interphase and each chromosome, originally composed of one strand, is now made up of two strands, or chromatids, joined together at a region of the chromosome called the centromere. When viewed under a microscope the centromere is easily seen as a narrowest part of the chromosome. We will uses magnets to help simluate the centromere of a chromosomes. Simulate DNA replication by bringing the magnetic centromere region of one strand in contact with the centromere region of the other of the same color. Do the same with its homolog (Figure 3.4).

Interphase: Place one strand of each color near the center of your work area. In interphase recal that the DNA is relaxed and in the form of threadlike Chromatin. (We will be unable to model the chromatin phase but can simulate what happens to the total number of DNA molecules) DNA synthesis occurs during Interphase (S-stage). At the conclusion of the S stage each strand of DNA has bee copied (Replication) and when chromosomes are formed in Prophase they will be actually double stranded (Two identical DNA chains). Each strand is called a Chromatid and they are held together by a mass of protein called the Centromere. Simulate DNA replication by bringing the magnetic centromere region of one strand in contact with the centromere region of the other of the same color. Do the same with its homolog.

Summary: DNA Replication

Prophase I: Homologous chromosomes come together and synapse along their entire length. Synapsis does not occur in mitosis and therefore this is the first big difference between Mitosis & Meiosis.. (In mitosis homologous chromosomes line up independently of one another) A Tetrad consisting of four chromatids is formed. Entwine the two chromosomes to simulate synapsis and the process of crossing over. Crossing Over can be simulated by popping some beads apart on one chromatid at the fifth bead or "Gene" and doing the same with the other chromatid. Reconnect the beads to those of the other color. Proceed through Prophase I of meiosis and note how crossing over results in recombination of genetic information.

Summary: Synapsis & Crossing Over

 

Metaphase I

The Crossed Over Tetrads line up in the center of the cells. Position the chromosomes near the middle of the cell.

Summary: Tetrads Align On The Equator

 

Anaphase I

During Anaphase I, the hmologous chromosomes are separated and pulled to opposite sides of the cell. This represents a second significant difference between the events of mitosis and meiosis.

Summary:Tetrads Separate Chromosome number reduced

Telophase I: Place each chromosome at opposite sides of the cell. Centriole duplication is completed in Telophase in preparation for the next division. Formation of a nuclear envelope and division of the cytoplasm (cytokinesis) often occur at the same time to produce two cells, but this is always not the case. Some cells are large and depend upon many nuclei to control metabolism (cardiac muscle cells). In their case Mitosis can occur that is not accompanied by cytokinesis. Notice in your simulated cell that each chromosome within the two daughter cells still consists of two chromatids.

Summary:Two Haploid Cells Formed, each chromosome composed of two chromatids

Meiosis II

A second meiotic division is necessary to separate the chromatids of the chromosomes in the two daughter cells formed by the first division. This will reduce the amount of DNA to one strand (chromatid) per chromosome. This second division is called Meiosis II. It resembles mitosis except that it begins with only one homolog from each homologous pair of chromosomes.

The following simulation procedures apply to haploid nuclei produced from Meiosis I.

Interphase II (Interkinesis): The amount of time spent "at rest" following telophase I depends upon the type of organism which will impact the formation of new nuclear envelopes, and the degree of chromosomal uncoiling. Because Interphase II does not necessarily resemble Interpahse I, it is often given a different name - interkinesis. DNA replication does not occur during Interkinesis. This represents the Third difference between mitosis & meiosis.

Prophase II: No DNA replication occurs. replicated centrioles (not shown) move to opposite sides of the cell. Double stranded chromosomes formed during Meiosis I become attached to the developing spindle fibers.

Summary: nuclear envelope disappears, chromatids condense to chromosomes, centrioles move to opposite sides, spindle attaches to centromere of chromosomes.

Metaphase II:Chromosomes are moved so that are centered in the middle of each daughter cell

Summary: Chromosomes are moved to the center of the cell (cells typicaly have many chromosomes , humans have 23 homologs, at this stage of meiosis the chromosomes move independently of one another just like in Mitosis)

Anaphase II: The centromere regios of the chromatids split. separate the chromatids of the chromosomes and pull the daughter chromosomes toward the opposite sides of each daughter cell. Now that each chromatid has its own visibly separate centromere region it can be called a chromosome.

Summary : Chromatids Separate

Telophase II: After the single chromatids have reached the opposite sides of the cell a nuclear envelope is reformed, spindle fibers break down, chromosomal DNA uncoils and forms chromatin, the cytoplasm divides and cleaves the cell into two separate cells each having one of each type of chromosome.

Check your understanding of Meiosis by using the web site shown below.
http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/problems.html

Before moving on to the analysis questions use the pop it beads to construct four single stranded chromosomes. Use one homologous pair to represent a cell going through mitosis use the other homologous pair to represent a cell going through meiosis. Move the beaded chromosome models to simulate mitosis in the one cell and meiosis in the other. Compare the two and look for major differences.

Questions:

1. List three major differences between the events of Mitosis & Meiosis.

2. Compare mitosis and meiosis with respect to each of the following.

 
Mitosis
Meiosis
Chromosome number of
parent cell
Number of DNA
Replications
Number of Divisions
Number of Daughter cells
produced
Chromosome number in
daughter cells
Number of genetically
unique daughter cells
   
Purpose of division

3. How are Meiosis I and Meiosis II different?

4. Oogenesis (formation of the egg) and spermatogenesis (formation of the sperm) have some differences. Refer to your text to identify what these differences are and explain their significance?

5. Where in an organism's body do meiotic cell divisions occur?

6.Why is meiosis important for sexual reproduction?

7. What is meant by SYNAPSIS?

8. Why doesn't crossing over occur in Mitotic cell divisions?

9. How does crossing over impact the evolution of a species?