Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Cut each chenille wires in half to obtain 30 six-inch wires.
2. Place two white six-inch chenille wires, two green six-inch chenille wires, 22 pink beads, 22 white beads and one permanent marker into each resealable bag.

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. Remind students to wash hands with soap and water before leaving the laboratory.

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. Dispose of isopropyl alcohol using Flinn Suggested Disposal Method #18a. Paper towels may be disposed of in regular trash, Flinn Biological Waste Disposal Type VI. All other materials can be reused.

Lab Hints

• This activity may be reasonably completed in one 50-minute lab period.
• Enough materials are provided in this kit for 6 students working in pairs or for 6 groups of students.
• 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 materials in this kit may also be used to simulate mitosis.
• The Mitosis, Meiosis and Cell Division microscope slide set (ML1409) is available from Flinn Scientific provides a microscopic survey of fission, budding, mitosis in plant and animal cells, oogenesis and spermatogenesis.
• Prometaphase has been incorporated in this activity as a separate 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 mentioned in your textbook.

Sample Data

Observations and Analysis

Answers to Questions

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

   Sister chromatids separate after the homologous chromosomes have separated. The sister chromatids stay attached at the centrioles during meiosis I when the homologous chromosomes are separated to opposite poles of the cell.

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

   Because sister chromatids are exact copies of each other, exchanging matching genes between them would not result in any new genetic combination of genes.

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

   No, each chenille wire contained beads from both the mother and the father. Each cell formed by meiosis contains a new combination of genes from both parents.

4. Structurally and functionally, how do the daughter cells 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. Haploid cells from two individuals combine during sexual fertilization to create a diploid offspring.

5. Explain the significance of the process of crossing over to living things in terms of biodiversity and the process of natural selection.
   

   Crossing over ensures that new variations of genes are present together in the next generation of a species. A particular combination of genes may lead to beneficial or harmful characteristics in the new organism. By having a variety of genetic combinations available in the next generation the species is more likely to withstand any environmental changes and continue the species through creation of the next generation.

References

Biology: Lab Manual; College Entrance Examination Board: 2001.

Introduction

All new cells come from previously existing cells. New cells are formed by the process of cell division, which involves both replication of the cell’s nucleus (karyokinesis) and division of the cytoplasm (cytokinesis) to form two genetically identical daughter cells. There are two types of nuclear division: mitosis and meiosis.

Concepts

• Cell cycle
• Crossing over
• Meiosis
• Centromere
• Cytokinesis
• Mitosis
• Chromatid
• Homologous chromosomes

Background

Mitosis typically results in new somatic (body) cells. Formation of an adult organism from a fertilized egg, asexual reproduction, regeneration and maintenance or repair of body parts are all accomplished through mitotic cell division. Meiosis, on the other hand, results in the formation of either gametes (in animals) or spores (in plants). These cells have half the number of the chromosomes in the parent cell.

Meiosis, the process of nuclear division that reduces the number of chromosomes in daughter cells by half, is required for sexual reproduction. In order for two individuals to produce offspring with the same number and types of chromosomes as themselves, they need to evenly reduce the number of their chromosomes by half in some cells. The resulting cells containing half the original number of chromosomes (referred to as haploid cells), combine with the haploid cells of a second individual to produce a new individual with the same number and types of chromosomes as the parents. Sexual reproduction allows for greater diversity in a population. When two individuals sexually reproduce, they bring forth a new individual with a unique mixture of genes. This variety of genes in a population allows for more diversity of characteristics and an overall stronger population. This strength is shown in a population’s ability to adapt to changes in the environment and also to evolve.

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, meiosis involves two cellular divisions, meiosis I and meiosis II. In meiosis, 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 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 sets of each chromosome, 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, which appear as a fan of lines in the diagram, control the migration of the chromosomes to the opposite ends of the cell during cell division. In prometaphase I, two homologous chromosomes—that is, 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. Consequently, interphase II is often called interkenesis. 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 separate and move 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.

Experiment Overview

In this laboratory, the events of meiosis will be simulated using beads to represent genes found on the chromosomes. Chromosomes and genes inherited from the mother are represented by white chenille wires and pink beads, respectively. Chromosomes and genes inherited from the father are represented by green chenille wires and white beads.

After completing this laboratory, you should be able to recognize the stages of meiosis in cells and relate chromosome activity to Mendel’s laws of segregation and independent assortment.

Materials

Safety Precautions

Isopropyl alcohol is a flammable liquid. Wear chemical splash goggles when working with isopropyl alcohol. Wash hands thoroughly with soap and water before leaving the laboratory. Follow all normal laboratory safety guidelines.

Procedure

Interphase I

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

1. Add 11 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, the mother.
2. Using a permanent marker, independently label the beads starting with the top bead as either “A” or “a” and ending with the lowest bead as “K” or “k.” 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: Any combination of upper case and lower case letters may be used.
3. Add 11 pink beads to the second white chenille wire.
4. Label the eleven 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 fifth and sixth beads. The twisted chenille wires represent one chromosome containing identical sister chromatids and the twist represents the centromere that binds the two sister chromatids together.
6. Add 11 white beads to one green chenille wire. The white beads on the green chenille wire represent the genes inherited by the organism from the second parent, the father.
7. Mark the eleven beads with the same sequence of letters as on the white wires, but vary the capitalization on some of the beads. (For example, if the pink bead “1” is “A,” then the white bead “1” may be “a” or “A.”)
8. Add 11 white beads to the second green chenille wire.
9. Label the eleven 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 fifth and sixth beads. The combination of upper case and lower case letters of one type represent the genotype that, when expressed by the organism, creates its visual appearance or phenotype.
11. Label Figure 1 on the Meiosis Worksheet with the sequence of upper case and lower case letter represented on the chenille wire models.

I. Meiosis I

1. Prophase I: Place the two chenille wire models (representing one chromosome from each parent) on the sheet of paper. The paper represents one cell.
2. Prometaphase I: Use the chenille wire models to simulate four events of crossing over by exchanging pink and white beads with the same letter on them. Label Figure 2 on the Meiosis Worksheet with the new sequence of upper case and lower case letters represented on the chenille wire models.
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.
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 each chromosome will be on its own section of the paper. Each chromosome is contained in its own cell.

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.
2. Prophase II: If the chromosome had unraveled during interphase II, it would condense again during this phase.
3. Prometaphase 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 the pole of its respective 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.
7. Use a small amount of isopropyl alcohol on a piece of paper towel to remove the letters off each bead. Return the materials to the zipper-lock bag. The paper may be recycled or disposed of in the regular trash.

Student Worksheet PDF

11715_Student1.pdf