Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Cut each chenille wire in half to obtain 30 six-inch wires.
2. Place two white, six-inch chenille wires and two green six-inch chenille wires into each resealable bag. Add 16 pink beads, 16 white beads and one permanent marker to each bag also.

Safety Precautions

Isopropyl alcohol is a moderate fire risk; flammable liquid; slightly toxic by ingestion and inhalation. Please wear eye protection when handling isopropyl alcohol. Dispose of isopropyl alcohol using Flinn Suggested Disposal Method #18a. Remind students to wash hands with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Paper towels may be disposed of in regular trash, Flinn Biological Waste Disposal Type VI. All other materials can be reused.

Lab Hints

• Ten extra beads of each color are included in case a few become lost.
• This lab may be simplified by having students label the pink beads with all capital letters and the white beads with all lowercase letters.
• The lab may be extended to simulate reproduction by combining haploid cells from two groups. If a phenotype key for each letter has been created, students can draw the offspring.
• The same materials can be used to simulate mitosis.

Teacher Tips

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students.

• Prometaphase is included as a phase in meiosis I and meiosis II since it is included in literature published by the National Institute of Health. Include prometaphase events with metaphase events if prometaphase is not included in your textbook.

Answers to Prelab Questions

1. Is a homologous chromosome the same as a sister chromatid? What similarities or differences can you identify?

No. Sister chromatids are identical to each other and joined at a centromere whereas a homologous chromosome is composed of pairs of sister chromatids.

2. In Figure 13, given the chromosome marked “1” below, could “2” be a homologue? If not, circle the reason on the diagram marked “2.” If true, indicate under what conditions it would be possible by drawing in an appropriate homologue to “1.” (K circled on “2.”)

No.

3. Could “3” be a homologue? If not, circle the reason on the diagram marked “3.” If true, indicate under what conditions it would be possible by drawing in an appropriate homologue to “1.”

Yes.

4. Label “4” and “5” to create two chromosomes that have different alleles from one another but are both homologous to “1.”

Answers to Questions

Interphase I

7. What does the potential for differences in capitalization on the alleles indicate?

Student answers may include different alleles of the same gene, the parent’s chromosomes have different alleles, heterozygous form.

Alleles may be capital or lower-case at each position but the two halves must match.

11. What does bead color indicate? How are the beads of the same color but on different chenille wires related to one another?

Bead color indicates which parent supplied that allele. Beads of the same color but on different chenille wires originated on the same chromosome but separated during crossing over.

What does chenille wire color indicate?

Wire color indicates which parent supplied the DNA.

I. Meiosis I

2. Prometaphase I

When two homologous chromosomes lineup next to each other they form a structure called a tetrad.

The two chromosomes should have three changes from Figure 14.

3. Metaphase I

How is this different from metaphase in mitosis?

In mitosis each chromosome is split into two halves so that each daughter cell contains one complete copy of DNA from each parent.

5. Telophase I

How much genetic material does each new cell have as compared to the original cell (before any genetic material was copied)? Explain.

Each cell has two copies of one parent’s DNA rather than one copy of each parent’s DNA

II. Meiosis II

1. Interphase II

In interphase II, no replication occurs. Why not?

The purpose of meiosis is to reduce to a haploid number so that two haploid cells can unite to form a new diploid offspring.

Identical to Figure 15, each chromatid is in its own box.

6. Telophase II

Are the four resulting cells identical or different from one another and why?

The four cells are different from each other because the alleles on each chromatid may differ from each other. They do each have one copy of each chromatid.

Post-Lab Questions

1. Do sister chromatids separate before or after homologous chromosomes separate? Explain.

After. Homologous chromosomes separate during anaphase I while sister chromatids separate during anaphase II.

2. Functionally, why would it be ineffective for sister chromatids to engage in crossing over?

Sister chromatids are identical to each other.

3. If you were to compare mitosis and meiosis, which would you say was a more complex process and why?

Meiosis is more complex because it involves genetic recombination and also twice as many steps to completion.

4. At the end of the process, did you end up with beads of all one color on the chenille wires? Explain what this means to a real cell.

No. Cells created for reproduction undergo crossing over to ensure genetic variation in the next generation.

5. Structurally and functionally, how do the offspring produced from meiosis differ from those produced by mitosis?

The daughter cells of mitosis are diploid with two copies of each gene whereas the daughter cells of meiosis are haploid with one copy of each gene. The haploid cell will combine with another haploid cell during sexual fertilization to create a diploid offspring.

References

Special thanks to Romena Holbert, Xenia High School, Xenia, OH, for providing the idea and the instructions for this activity to Flinn Scientific.

Introduction

The sequence of events occurring during meiosis is explained in this simple kinesthetic activity.

Concepts

• Meiosis

• Crossing over
• Homologous chromosomes
• Sister chromatid

Background

Two processes must be accomplished when cells, called gametes, are created for reproduction. First, the amount of genetic information must be cut in half so that chromosome numbers do not double in the next generation. Second, genetic variation must be added to the next generation of organisms. Events that occur during meiosis address both of these issues.

Meiosis is the type of cell division that occurs in reproductive tissues. In contrast to mitosis in which only one division occurs, two cellular divisions occur during meiosis, meiosis I and meiosis II, during which the cells reduce their normal diploid (di = two in Greek) chromosome number by half to create four haploid (hap = one in Greek) cells.

Interphase occurs just before meiosis I begins. In this stage, the chromosomes are in the chromatin or thread-like form. This loose form is needed so that the DNA can replicate itself in preparation for cell division. In humans this means that the two versions of a gene, one from the mother and one from the father, are both replicated, creating two identical copies of each version. The result is that there are a grand total of four copies of each gene.

Meiosis I begins with prophase I when the duplicated threads of chromatin condense to form two identical, or sister, chromatids. These sister chromatids attach to each other at a special point called the centromere. This whole structure is called a chromosome. There are two chromosomes, one with two copies of the mother’s genes and one with two copies of the father’s genes. Also during prophase, the centrioles are copied. Centrioles control the migration of the chromosomes to the opposite ends of the cell during cell division.

In prometaphase I, two homologous chromosomes—that is, the chromosomes that contain the same genes—move adjacent to each other to form a structure called a tetrad (Tetra = four in Greek). While these two homologous chromosomes or homologues are aligned as a tetrad, they may exchange sections of similar genetic code with each other. This process is called crossing over because the homologues appear to “cross” each other as DNA strands are exchanged. It is this exchange of genetic information that creates new genetic variation in living organisms. Crossing over does not occur between sister chromatids because they are identical and no genetic change would occur if identical pieces of DNA switched places. Also during prometaphase I, the spindle fibers start to attach to the centromeres and the nuclear membrane breaks apart.

In metaphase I, the nuclear membrane completely disappears and the genetically altered and attached chromosomes align in the middle of the cell. The orientation is random, with the homologue from either parent on a side. This means that there is a 50–50 chance for the daughter cells to receive either the mother’s or the father’s genetically altered chromosome.

During anaphase I, the altered homologous chromosomes separate and migrate to opposite ends of the cell. The chromosomes migrate when they are pulled toward opposite centrioles by spindle fibers that are attached between the centrioles and the centromere on each homologue.

Telophase I occurs when the chromosomes reach the centrioles on the opposite sides of the cell. Cytokinesis occurs at the same time. In animal cell, the cell membrane pinches in to divide the cytoplasm and organelles into two cells. In plant cells, new cell walls form along the center of the cell creating two cells.

Interphase II is the period between meiosis I and meiosis II. Interphase II is very different from interphase I because no DNA replication occurs. The nuclear membrane reforms in many organisms. In some organisms the chromatids separate and unravel while in others they do not separate or unravel.

Meiosis II begins with prophase II. During prophase II, the chromatin condenses (if it unraveled during interphase II), the centrioles are duplicated and spindle fibers begin to reform.

In prometaphase II, the nuclear membrane begins to break apart and the new spindle fibers attach to the centromeres of the chromosomes. One spindle fiber from each centriole attaches to the centromere on each chromosome. Recall that there is just one copy of each chromosome in each cell but that each chromosome is composed of two sister chromatids.

Metaphase II is characterized by the alignment of the chromosomes along the center of the cell in preparation for the separation of the sister chromatids. Keep in mind, the sister chromatids are no longer identical because of the crossover events that occurred in prometaphase I.

In anaphase II, the centromere is split in half as the sister chromatids are separated, moving to opposite sides of the cell.

In telophase II each sister chromatid moves toward a centriole located on the opposite side of the cell. At the same time cytokinesis occurs, splitting the cell in half again. A total of four new haploid cells have been produced from the original cell. Each haploid cell contains one sister chromatid, which includes a single complete set of genes.

Materials

Prelab Questions

1. Is a homologous chromosome the same as a sister chromatid? What similarities or differences can you identify?
2. In Figure 13, given the chromosome marked “1” below, can “2” be a homologue? If not, circle the reason on the diagram marked “2.” If true, indicate under what conditions it would be possible by drawing in an appropriate homologue to “1.”
3. Could “3” be a homologue to “1”? If not, circle the reason on the diagram marked “3.” If true, indicate under what conditions it would be possible by drawing in an appropriate homologue to “1.”
4. Label “4” and “5” to create two chromosomes that have different alleles from one another but are both homologous to “1.”

Safety Precautions

Isopropyl alcohol is a moderate fire risk; flammable liquid; slightly toxic by ingestion and inhalation. Please wear eye protection and avoid sources of ignition when handling isopropyl alcohol. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Interphase I
Use the supplies provided in the resealable bag to create two homologous chromosomes.

1. Add eight pink beads to one white chenille wire. The chenille wire represents one chromatid and the beads represent genes inherited by the organism from one parent.
2. Independently label the beads starting with the top bead as either “A” or “a” and ending with the lowest bead as “H” or “h.” The letters written on each bead represent alleles of each gene and together they are the genotype. By convention, an upper case letter represents the dominant form of a gene while the lower case letter represents the recessive form of the same gene. Note: A combination of capital and lower case letters may be used.
3. Add eight pink beads to the second white chenille wire.
4. Label the eight pink beads to create an exact match to the original white chenille wire. This second chenille wire represents a duplicated copy of the DNA.
5. Twist the two white chenille wires around one another between the fourth and fifth beads. The twisted chenille wires represent one chromosome and the twist represents the centromere that binds the two sister chromatids together.
6. Add eight white beads to one green chenille wire. The green chenille wire and white beads represent the chromosome inherited by the organism from the second parent.
7. Mark the eight beads with the same sequence of letters as on the white wires, but vary the capitalization on some of the beads. (For example, if pink beads “1” are “A” white beads “1” may be “a” or “A.”)

What does the potential for differences in capitalization on the alleles indicate?

8. Add eight white beads to the second green chenille wire.
9. Label the eight white beads to create an exact match to the original green chenille wire.
10. Twist the two green chenille wires around one another between the fourth and fifth beads.
11. Create a color–coded diagram of the two wire chromosome structures in Figure 14. Indicate each letter (allele) on each arrow.

What does bead color indicate? How are the beads of the same color but on different chenille wires related to one another?
What does chenille wire color indicate?

I. Meiosis I

1. Prophase I

Place the models on the sheet of paper. The paper represents one cell.

2. Prometaphase I

Use the chenille wire models to simulate three events of crossing over by exchanging pink and white beads with the same letter on them.
When two homologous chromosomes lineup next to each other they form a structure called a _____________________.
Create a color-coded diagram of the new bead arrangement on Figure 15. Indicate the letter (allele) on each arrow.

3. Metaphase I

Line up the chenille wire models in the center of the paper with the green chromosome on the left-hand side of the paper and the white chromosome on the right-hand side of the paper.
How is this step different from the metaphase that occurs in mitosis?

4. Anaphase I

Separate the chromosome models by color, moving the green chromosome to the left and the white chromosome to the right.

5. Telophase I

Tear the paper “cell” in half so that each chromosome will be on its own section of the paper. Each chromosome is contained in its own cell.
How much genetic material does each new cell have as compared to the original cell (before any genetic material was copied)? Explain.

II. Meiosis II

1. Interphase II

Remember, no DNA is replicated for the next cell division. The cell prepares for the next cell division by increasing the amount of intercellular organelles and cytoplasm.
In interphase II, no replication occurs. Why not?

2. Prophase II

If the chromosome had unraveled during interphase II, it would condense again during this phase.

3. Promotaphase II

New spindle fibers attach to the centromeres.

4. Metaphase II

Line up the chromosome models in the center of each “cell” again, but 90° from their positions in metaphase I.

5. Anaphase II

Untwist the chenille wires. This represents the splitting of the centromere.
Separate the two chenille wires from each other and move each wire to poles of its cell.

6. Telophase II

Tear the paper in half again so that each chenille wire is on its own quarter sheet of paper. Each chromatid is now contained in its own haploid cell.
Do not destroy the models yet!
Create a color-coded diagram of the chromatid models on Figure 16. Indicate each letter (allele) on each arrow.

Are the four resulting cells identical or different from one another? Explain.

Student Worksheet PDF

10754_Student1.pdf