Mitosis and Meiosis Lab Instructions:
Observing Mitosis and Cytokinesis in Plant Cells

Materials

• prepared slide of onion root tip
• compound microscope

Introduction

The behavior of chromosomes during the cell cycle is similar in animal and plant cells. However, differences in cell division do exist. Plant cells have no centrioles, yet they have bundles of microtubules that converge toward the poles at the ends of a spindle. Cell walls in plant cells dictate differences in cytokinesis. In this exercise, you will observe dividing cells in the zone of cell division of a root tip.

Procedure

1. Examine a prepared slide of a longitudinal section through an onion root tip using low power on the compound microscope.

    

2. Locate the region most likely to have dividing cells, just behind the root cap.

   At the tip of the root is a root cap that protects the tender root tip as it grows through the soil. Just behind the root cap is the zone of cell division. Notice that rows of cells extend upward from this zone. As cells divide in the zone of cell division, the root tip is pushed farther into the soil. Cells produced by division begin to mature, elongating and differentiating into specialized cells, such as those that conduct water and nutrients throughout the plant.

3. Focus on the zone of cell division. Then switch to the intermediate power, focus, and switch to high power.

    

4. Survey the zone of cell division and locate stages of the cell cycle: interphase, prophase, prometaphase, metaphase, anaphase, telophase, and cytokinesis.

    

5. As you find a dividing cell, speculate about its stage of division, read the following descriptions given of each stage to verify that your guess is correct, and, if necessary, confirm your conclusion with the instructor.

    

6. Locate the nucleus, nucleolus, chromosome, chromatin, mitotic spindle, and cell plate when appropriate.

Interphase (G1, S, G2)

Nuclear material is surrounded by a nuclear envelope. Dark-staining bodies, nucleoli, are visible. Chromosomes appear only as dark granules within the nucleus. Collectively, the chromosome mass is called chromatin. The chromosomes are not individually distinguishable because they are uncoiled into long, thin strands. Chromosomes are replicated during this phase.

Prophase

Chromosomes begin to coil and become distinguishable thin, threadlike structures, widely dispersed in the nucleus during prophase. Although there are no centrioles in plant cells, a spindle begins to form. Nucleoli begin to disappear. The nuclear envelope is still intact.

Prometaphase

By prometaphase, the chromosomes are thick and short. Each chromosome is double-stranded, consisting of two chromatids held together by a centromere. The nuclear membrane breaks down in prometaphase. Chromosomes move toward the equator.

Metaphase

Metaphase begins when the centromeres of the chromosomes lie on the equator of the cell. The arms of the chromatids extend randomly in all directions. A spindle may be apparent. Spindle fibers are attached to centromeres and extend to the poles of the cell. As metaphase ends and anaphase begins, the centromeres split.

Anaphase

The splitting of centromeres marks the beginning of anaphase. Each former chromatid is now a chromosome. Single-stranded chromosomes are drawn apart toward opposite poles of the cell. Anaphase ends when the migrating chromosomes reach their respective poles.

Telophase and Cytokinesis

Chromosomes have now reached the poles. The nuclear envelope reforms around each compact mass of chromosomes. Nucleoli reappear. Chromosomes begin to uncoil and become indistinct. Cytokinesis is accomplished by the formation of a cell plate that begins in the center of the equatorial plane and grows outward to the cell wall.

William R. Morgan wmorgan@acs.wooster.edu