Materials Included In Kit

Prelab Preparation

Cut thread into 30" pieces. Each lab group needs four 30" pieces.

Safety Precautions

This laboratory is considered nonhazardous. Always follow standard, safe laboratory procedures.

Disposal

All materials can be reused.

Teacher Tips

• The materials in this kit can be an excellent alternative to a written test. Have students manipulate and explain the processes of mitosis and meiosis as a form of evaluation.

• Have students make diagrams of each of the steps of mitosis and meiosis as they perform the simulation.
• Discuss the results of crossing-over. The genetic variation and “mixing” of maternal and paternal genes during gamete formation can produce variations not otherwise predicted.
• Keep a set of modeled chromosomes available throughout the genetics curriculum. Use the chromosome models to help model/explain other genetic concepts.
• Illustrate a deletion with a model chromosomes. Remove a portion of a chromatid. This loss of a portion of a chromatid simulates a deletion. The effect of a deletion can be very severe depending upon how much of the chromatid is lost.
• Use a washable marker to mark four beads on one chromatid along its length. Label them A, B, C and D. Form a loop along itself and exchange the ends of the chromatids where they touch each other. Straighten the chromatids again. What has happened to the order of A, B, C and D on the chromatid? The change of order of genes is called an inversion.

Teacher Handouts

10269_Teacher1.pdf

Introduction

If we get one set of genes from our mother and another set of genes from our father, why don’t we have four legs, four arms, four ears, and two noses? Chromosomes and genes are passed systematically from one generation to the next, assuring that these occurrences are rarities.

Concepts

• Cell cycle

• Mitosis and meiosis
• Chromosomes
• Crossing over

Background

The life of a eukaryotic cell consists of a continuous sequence of events known as the cell cycle (see Figure 1).

The cell cycle begins with the formation of a new cell and continues until that cell divides into two offspring cells. Each offspring cell then begins the cycle again. The dramatic events of cell division, M (the M from mitosis) phase, is only a brief segment in the total life cycle of the cell. Most of the life of a cell is spent performing normal metabolic activities, growing and preparing the cell for its next division. Although the main sequence of the phases of the life cycle of a cell is fixed, the amount of time spent in each phase varies among cells of different organisms and among different cells within an organism.

The G₁ (Growth 1) phase is the time of cell growth just following cell division. New cells are very metabolically active and are actively synthesizing RNA and new proteins. Some cells go into an extended G₁ phase and rarely ever divide again. This phase is often called G₀ phase. Nerve cells, for example, are very active and important cells but tend to remain in G₀ phase for extended periods of time.

From the G₁ phase, most cells go to the S (synthesis) phase. In the S phase, an exact copy of DNA is made in the nucleus of the cell. This exact copy or replication of the DNA ensures that a complete and identical set of genes is available for each new offspring cell following cell division.

G₂ (Growth 2) is the phase of growth and metabolism occurring after the S phase. The synthesis of protein, RNA and other macromolecules is small compared to G₁ growth. Because the DNA replicates in the S phase, a cell in G₂ has twice as much DNA in its nucleus as a cell in G₁. The duration of G₂ is usually short and then M phase starts.

The M phase occurs after G₂ and is easily identified because it is the only phase in which the chromosomes are visible with a light microscope. Mitosis is nuclear division in which the replicated chromosomes separate to form two nuclei from one. In most cells, mitosis (nuclear division) is followed quickly by cell division of the parent cell into two offspring cells completing the M phase. After the M phase, each offspring cell enters G₁ and the cycle continues.

Materials

Safety Precautions

This laboratory is considered nonhazardous. Always follow standard, safe laboratory procedures.

Procedure

I. Chromosome Assembly

1. Use four centromeres and sixty pop beads to assemble two chromosome models.
2. Snap a string of seven pop beads (all the same color) to one end of a centromere and a string of eight pop beads (same color) to the other end of the centromere.
3. Repeat this procedure with another centromere using the same color pop beads.
4. Make two more centromeres with different colored pop beads (see Figure 2).

II. Mitosis Simulation

The genetic material in a cell that is not dividing is contained within the nucleus. During the S phase of the cell’s life cycle, DNA contained within the chromosomes is replicated. Each chromosome is divided lengthwise into two sister chromatids, which are joined at a constriction called the centromere.

Interphase

1. Place the two chromosomes (each consisting of two chromatids) in the plastic bag (nuclear membrane).
2. Place the nucleus with its chromosomes in the center of the work area.
3. Place two centrioles near the nuclear membrane.

Prophase

1. Separate the two centrioles and tape them down about 40 cm to each side of the nucleus. Point the open end of each centriole toward the nucleus.
2. Remove the chromosomes from the nucleus and set the nuclear membrane (bag) aside.
3. Form a loop at one end of each string. Draw one loop tightly around each centromere of a chromatid.
4. Thread the other end of the string through opposite centrioles. The setup should look like Figure 2.

Metaphase

1. Adjust the strings and chromosomes to bring them to the midpoint between the two centrioles (see Figure 2).

Anaphase

1. Continue pulling the strings until the centromeres are separated (see Figure 3). When the centromeres separate, the chromosomes become daughter chromosomes.
2. Continue pulling the chromosomes toward the centrioles.

Telophase

1. Continue pulling until the chromosomes are near the centriole.
2. Remove the strings.
3. Move the chromosomes near each centriole (see Figure 4).

The nuclear division is now complete. This would usually be followed by a division of the cytoplasm and the completion of cell division resulting in two identical cells.

1. Place the two chromosomes (each consisting of two chromatids) in the plastic bag (nuclear membrane). Place the bag near the center of your work area.
2. Place two centrioles near the nuclear membrane.

First Division: Reduction Division

Prophase I

1. Remove the chromosomes from the nucleus and set the nuclear membrane aside (even though the events of Prophase I do occur within the nuclear membrane).
2. Separate the two centrioles and tape them down about 40 cm to each side of the nucleus. Point the open end of each centriole toward the nucleus.
3. Use the string to form a loop tightly around the double centromere of each chromosome as shown in Figure 5. Be sure the loop is around both centromeres.

Metaphase I

1. Position the chromosomes near the midpoint between the centrioles with the homologous pair (one maternal and one paternal chromosome) lined up at right angles to the strings.

Anaphase I

1. Gently pull on the strings and separate the homologous pair of chromosomes.
2. Pull them toward the opposite centrioles (see Figure 6).

Telophase I

1. Remove the strings. Pile each chromosome near its centriole.
2. Place an additional centriole near each centriole.
3. Formation of a nuclear membrane and division of the cytoplasm often occurs at this point to produce two cells.

Second Division: Mitotic Division

Perform each of the following procedures to each of the two cells produced from the first reduction division.

Prophase II

1. Separate the two new centrioles and tape them down about 40 cm to each side of the nucleus. (Do this to all four centrioles.)
2. Form a loop at one end of each of four pieces of string. Tighten a loop around each centromere of the chromatids as shown in Figure 7.

Metaphase II

1. Gently pull on the strings to bring the chromosomes to the center (midpoint) between the centrioles (see Figure 8).

Anaphase II

1. Continue pulling the strings until the centromeres are separated and the daughter chromosomes are pulled toward the centrioles.

Telophase II

1. Remove the strings.
2. Place each chromosome by its centriole.
3. There should be four centrioles and four chromosomes, with one chromosome in each of the potential cells.
4. Formation of a nuclear membrane and division of the cytoplasm would occur at this time producing four gamete cells each with one-half of the number of original chromosomes.

IV. Crossing-Over Simulation

Work with a homologous pair of chromosomes like those used in the meiosis simulation. Consider each pop bead to be a gene.

1. Line up a homologous pair of chromosomes as shown in Figure 5. Entwine the chromatids as shown in Figure 9.

2. Detach the pop beads at places where the chromosomes of homologous pairs are entwined.
3. Exchange the chromosome pieces at these entwined areas. Note that the exchanged sections must be of equal length.
4. This exchange of the genetic material between homologous chromosomes is called crossing-over. What will be the consequences to the gametes resulting after crossing-over?
5. Run these chromosomes through the rest of the steps in meiosis and compare the resulting chromosomes with those secured earlier in the meiosis simulation.