In this laboratory, you will investigate the process of mitosis. You will use prepared slides of onion root tips to study plant mitosis.

Objectives:

• Before doing this laboratory you should understand the events of mitosis in animal and plant cells.
• After doing this laboratory you should be able to recognize the stages of mitosis in a plant cell.

Introduction: All new cells come from previously existing cells. New cell are formed by the process of cell division which involves both replication of the cell’s nucleus (karyokinesis) and division of the cytoplasm (cytokinesis). There are two types of nuclear division: mitosis and meiosis. 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 accomplished through mitotic cell division.

Where does one find cells undergoing mitosis? Plants and animals differ in this respect. In higher plants the process of forming new cells is restricted to special growth regions called meristems. These regions usually occur at the tips of stems or roots. In animals, cell division occurs anywhere new cells are formed or as new cells replace old ones. However, some tissues in both plants and animals rarely divide once the organism is mature.

To study the stages of mitosis, you need to look for tissues where there are many cells in the process of mitosis. This restricts your search to the tips of growing plants such as the onion root tip or, in the case of animals, to developing embryos such as the whitefish blastula.

Roots consist of different regions. The root cap functions in protection. The apical meristem is the region that contains the highest percentage of cells undergoing mitosis. The region of elongation is the area in which growth occurs. The region of maturation is where root hairs develop and where cells differentiate to become xylem, phloem, and other tissues.

Procedure: Examine prepared slides of onion root tips. Locate the meristematic region of the onion with the 10X objective, and then use the 40X objective to study individual cells. For convenience, biologists have described certain stages, or phases, of the continuous mitotic cell cycle, as outlined below. Identify one cell which clearly represents each phase.

Sketch and label the cell in the area provided (2 pts. each):

1. Interphase: The nondividing cell is in a stage called interphase. The nucleus may have one or more dark-stained nucleoli and is filled with a fine network of threads, the chromatin. During interphase, DNA replication occurs.

Cell in Interphase

2. Prophase: The first sign of division occurs in prophase. There is a thickening of the chromatin threads, which continues until it is evident that the chromatin has condensed into chromosomes. With somewhat higher magnification you may be able to see that each chromosome is composed of two chromatids joined together at a centromere. As prophase continues, the chromatids continue to shorten and thicken. In late prophase the nuclear envelope and nucleoli are no longer visible, and the chromosomes are free in the cytoplasm. Just before this time the first sign of a spindle appears in the cytoplasm; the spindle apparatus is made up of microtubules, and it is thought that these microtubules may pull the chromosomes toward the poles of the cell where the two daughter nuclei will eventually form.

Cell in Prophase

3. Metaphase: At metaphase, the chromosomes have moved to the center of the spindle. One particular portion of each chromosome, the centromere, attaches to the spindle. The centromeres of all the chromosomes lie at about the same level of the spindle, on a plane called the metaphase plate. At metaphase you should be able to observe the two chromatids of some of the chromosomes.

Cell in Metaphase

4. Anaphase: At the beginning of anaphase, the centromere regions of each pair of chromatids separate and are moved by the spindle fibers toward opposite poles of the spindle, dragging the rest of the chromatid behind them. Once the two chromatids separate, each is called a chromosome. These daughter chromosomes continue poleward movement until they form two compact clumps, one at each spindle pole.

Cell in Anaphase

5. Telophase: Telophase, the last stage of division, is marked by a pronounced condensation of the chromosomes, followed by the formation of a new nuclear envelope around each group of chromosomes. The chromosomes gradually uncoil to form the fine chromatin network seen in interphase, and the nucleoli and nuclear envelope reappear. Cytokinesis occurs simultaneously with the end of telophase or after telophase. This is the division of the cytoplasm into two cells. In plants, a new cell wall is laid down between the daughter cells. In animal cells, the old cell will pinch off in the middle along a cleavage furrow to form two new daughter cells.

Cell in Telophase