Laboratory 10: Mitosis and Meiosis

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Objectives

After completing this lab, you should be able to:

• Describe the stages of mitosis and meiosis.

• Demonstrate the processes of mitosis and meiosis using models.

• Identify stages of mitosis in stained onion root tip and fish blastula slides.

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Key Terms

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Introduction

Every multicellular organism begins as a single cell called a zygote, formed when an egg is fertilized by a sperm. Through continuous cell divisions, this zygote develops into a complete organism composed of trillions of cells.

Cell division occurs through two main processes:

• Mitosis – for growth and repair (produces identical diploid cells).

• Meiosis – for reproduction (produces genetically unique haploid gametes).

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Mitosis

Mitosis produces two identical diploid (2n) daughter cells from one diploid parent cell. Each daughter cell has the same genetic information as the parent. The process is divided into four major stages:

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase, followed by Cytokinesis

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Meiosis

Meiosis occurs in reproductive organs to produce gametes (sperm and eggs).
It reduces the chromosome number by half — from diploid (2n) to haploid (n) — ensuring genetic stability across generations.

During Meiosis I, homologous chromosomes separate, and during Meiosis II, sister chromatids separate, resulting in four haploid cells.

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Materials

• Purple yarn

• White string (spindle fibers)

• Two rod-shaped beads (centrioles)

• Two strands of red beads (paternal chromosomes)

• Two strands of yellow beads (maternal chromosomes)

• Magnets (centromeres)

• Onion root tip mitosis slide

• Fish blastula mitosis slide

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Procedures

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Experiment 1 – Modeling Mitosis

1. Model Components:

   • Purple yarn → Nuclear envelope

   • Red beads → Paternal chromosome

   • Yellow beads → Maternal chromosome

   • Magnets → Centromeres

   • Rod-shaped beads → Centrioles

   • White string → Spindle fibers

2. Interphase:

   • DNA replication occurs, forming sister chromatids joined at centromeres.

   • Centrioles duplicate.

   • Protein synthesis and metabolic activities continue.

   Note: This phase cannot be fully modeled but is essential before mitosis begins.

   Question:
   If you begin mitosis with one diploid cell, how many cells result at the end? Are they haploid or diploid?

3. Prophase:

   • Chromosomes condense and become visible.

   • Nuclear envelope breaks down.

   • Spindle fibers form and attach to kinetochores.

   Modeling Steps:

   • Use purple yarn as the nucleus; place paired red and yellow chromosomes inside.

   • Move the centrioles to opposite poles.

   • Remove the yarn (nuclear envelope).

   • Attach spindle fibers (white string) to centromeres.

4. Metaphase:

   • Chromosomes align along the equatorial plane.

   • Spindle fibers from opposite poles attach to sister chromatids.

   Model: Align chromosomes at midline using the white string to represent tension.

5. Anaphase:

   • Sister chromatids separate and move to opposite poles.

   • The kinetochore leads, chromatids trail.

   Model: Gently pull on the spindle fibers to simulate chromatid separation.

6. Telophase and Cytokinesis:

   • Chromosomes decondense.

   • Nuclear envelopes reform.

   • In animal cells, a cleavage furrow forms.

   • In plant cells, a cell plate develops between daughter cells.

   Model:

   • Remove spindle fibers.

   • Use yarn to form two new nuclei.

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Experiment 2 – Observing Mitosis in Onion Root Tips and Fish Blastula

1. Examine the slides under low power (40×) to locate active regions of cell division.

2. Focus on the root tip (plant) or blastula (animal) for high mitotic activity.

3. Increase magnification to 400× to view distinct stages.

4. Identify and record the following:

5. Draw and label the stages observed for both the onion and fish cells.

6. Record descriptions in Table 1 below.

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Table 1. Stages of the Cell Cycle in Onion and Fish Cells

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Experiment 3 – Modeling Meiosis

1. Model Components:
   Same as in Experiment 1 (yarn, beads, string, etc.).

2. Interphase:

   • DNA replicates, producing sister chromatids joined at centromeres.

3. Prophase I:

   • Homologous chromosomes pair (synapsis) to form tetrads.

   • Crossing over occurs at chiasmata, exchanging genetic material.
     Model: Pair red and yellow chromatids and swap one bead between homologs.

4. Metaphase I:

   • Tetrads align along the equator.

   • Spindle fibers attach to homologous chromosomes, not sister chromatids.

5. Anaphase I:

   • Homologous chromosomes separate and move to opposite poles.

6. Telophase I and Cytokinesis:

   • Two haploid daughter cells form (each still has duplicated chromosomes).

7. Prophase II:

   • No DNA replication occurs.

   • New spindle apparatus forms in each haploid cell.

8. Metaphase II:

   • Chromosomes align individually along the cell midline.

9. Anaphase II:

   • Sister chromatids separate and move to opposite poles.

10. Telophase II and Cytokinesis:

• Nuclear membranes reform; chromosomes decondense.

• Four haploid gametes are produced.

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Review Questions

1. Explain how mitosis differs between plant and animal cells.

2. Compare and contrast mitosis and meiosis in terms of purpose and outcome.

3. Why is it biologically important to produce haploid gametes?

4. Compare and contrast Meiosis I and Meiosis II with respect to chromosome behavior and outcome.