Mitosis

Materials

1. Slides of mitosis in onion (Allium) root tip

2. Two fresh onions with growing roots

3. One dropper bottle of toulidine blue stain

4. Models and/or charts of cells undergoing mitosis          

 

Some Suggested Learning Goals

1. Understand and know what takes place in each of the phases of mitosis.

2. Understand the nature, structure, or function of spindle fibers, poles, equator, chromatids, cell plate, and phragmoplast.

Introduction

In meristematic tissues (plant tissues in which cells multi­ply), cells go through an orderly sequence of events known as the cell cycle. This cycle is usually divided into inter­phase and mitosis; mitosis is usually accompanied by cy­tokinesis (cell division). A clear understanding of the process of mitosis is fundamental to an understanding of growth and reproduction, and helps in the comprehension of the important process of meiosis to be studied later. All living organisms undergo mitosis in at least some stages of their life cycles. Plant and animal cells differ in some de­tails of the process, but not in the overall scheme.

Mitosis in Plant Cells

Examine a slide labeled "Allium root tip." This is a slide with stained, thin longitudinal sections (usually three sec­tions) of an onion root tip that was actively growing until it was harvested to make slides. The end of the root is pro­tected by a root cap consisting of cells that are irregular in shape. Immediately behind the root cap is a meristematic region (apical meristem) where cells actively divide. Above the meristematic region, in the region of elonga­tion, the cells increase in length without dividing. To find cells in the various stages of mitosis, you will have to confine your examination of the slide to the meristematic region.

Interphase Look for cells with distinct nuclei, each containing one, two, or more nucleoli (which appear as more or less spherical miniature nuclei within a nucleus). Most of the cells will be in interphase. The fine, wispy, lightly stained material is called chromatin.

Prophase In prophase, the nuclear ern'elope ("casing" of the nucleus) and nucleoli disappear, the chromosome strands coil tightly, becoming shorter and thicker, and both strands of each chromosome become distinguishable.

Metaphase-In metaphase, the centronreres of the chromosomes are aligned at the equator (an invisible plate in the center) of the cell. A top-shaped spindle, consisting of numerous spindle fibers, becomes fully developed. The spindle fibers extend in arcs between two invisible poles located toward opposite ends of the cell. Each chromosome has a single spindle fiber attached to its centromere, but most of the spindle fibers arc directly between the poles.

Anaphase In anaphase, the two strands of each chromosome (chromatids) separate longitudinally at their centronreres. One strand of each pair migrates to a pole, and the other strand of each pair simultaneously migrates to the opposite pole.

Telophase--In telophase, the tight coiling of the chromatids now called chromosomes again relaxes, and the strands become longer and thinner ; a new nuclear envelope develops around each group; and nucleoli reappear. The spindle gradually disappears, and a set of shorter fibers, comprising the plrraginoplast, develops at right angles to the spindle between the two new nuclei. While this is taking place, dictyosornes in the cytoplasm produce vesicles containing raw materials for cell walls and membranes. Some of these vesicles are channeled to the center of the spindle (equator) by the remaining fibers. As the vesicles accumulate, they fuse together, forming a double membrane called the cell plate. The cell plate grows outward from the center until it contacts and unites with the plasma membrane. Cellulose is then deposited on the membranes, resulting in new cell walls. At the same time, pectin is added between the membranes, creating a middle lamella that is shared at the end of telophase by what has become two new daughter cells.

Drawings to Be Submitted

Label the four phases of mitosis (prophase, metaphase, anaphase, telophase) from cells in the meristematic region of an onion root tip. Include the following: CHROMO­SOMES, CELL WALL, SPINDLE FIBERS, CEN­TROMERE, CYTOPLASM, CELL PLATE, NUCLEUS, NUCLEOLUS, NUCLEAR ENVELOPE, and PHRAG­MOPLAST. You do not need to label each item more than once.

 

Review questions 3

1. What changes in a cell's cytoplasm and nucleus take place during interphase?

2. In which region of a root tip does mitosis occur? 3. What is a spindle?

4. What are the main events of metaphase?

5. In which phase of mitosis do chromosomes first become distinguishable from one another? 6. 6. What happens to chromosomes during anaphase?

7. Of what substance is a middle lamella composed?

8. What becomes of the cell plate at the conclusion of mitosis?

 

1. Which plant is used for the study of mitosis in this exercise?

2. When do single-stranded chromosomes become two-stranded?____

3. In which specific regions of a plant does mitosis commonly take place?_______

4. In which phase of mitosis do nucleoli disappear?____

When do nucleoli reappear?

5. What takes place in metaphase?___

6. Which of the following is visible in at least one stage of mitosis: poles, equator, spindle?

7. What occurs in anaphase?__

8. From what does a new cell wall develop?

9. Which cell organelle produces the materials for a new middle lamella?_____

10. What is the function of a centromere?

Optional Exercise

 

Mitosis Lab

Objectives

1. Prepare a slide of stained chromosomes using the squash technique.

 

2. Identify the stages of mitosis from prepared stained slides of onion root tips seen under the compound microscope.

 

3. Identify the structures visible under the compound microscope of cells in various stages of cell division.

PROCEDURE

Each human originates with the fertilization of an egg. At this stage of development the individual consists of a single cell about the size of the period at the end of this sentence. Division of this cell and many divisions of sequential cells over a period of several years, may result in the formation of an adult human, consisting of about 1018 cells. Among these are cells of many kinds, each having a different structure, function, and life span. This process of mitosis, cell division, continues as long as the human or parts of his body are alive.

 

In this investigation you will see the cell in the various stages of mitosis. I. Staining Chromosomes

Procedures:

1. With forceps, place an onion root tip in the center of a clean slide. Using a scalpel or razor blade, cut off the terminal cm of the tip of the root. Dispose of the rest of the root.

2. Cover immediately with 3 or 4 drops of the 1N HC1. The acid is to act on the root tip for 5 minutes. During this period the slide is to be heated gently. This is done by placing the slide on the diffused light source for 5 to 7 minutes. The onion root tip must be covered with the HCl solution during this warming period.

 

3. Blot off the acid without touching the root tip and cover the root tip with 2 or 3 drops of the toluidine blue solution. The stain is to set for about 5 minutes and is to be warmed in the same manner as with the acid solution

 

4. Blot off the stain solution carefully. Add 2 drops of fresh stain solution carefully and add a cover glass.

 

5. With the slide flat on the table, place your thumb on the cover glass exactly above the specimen. Press straight down hard.

 

6. Blot the back of the slide and examine on low power. Look for embryonic cells. They appear square. When you find mitotic figures, switch to high power.

III. Draw and label cells in interphase ,prophase, metaphase, anaphase, and telophase stages of mitosis from slides you prepared.

STAGES OF MITOSIS By definition:

Prophase            begins when chromosomes are visible with the light microscope and contin­ues until the nuclear membrane is no longer visible.

Metaphase begins when nuclear membrane is no longer visible. Chromosomes are in mass in the center of the cell.

Anaphase            begins when the identical chromosomes start moving toward opposite ends of the cell.

Telophase            begins with the invagination of cell membrane in animal tissue and with the first visible evidence of a forming cell plate in plant cells.

Interphase chromosomes not visible with light microscope (not a stage of mitosis).

 

 

 

1. Describe the prominent cell morphology as seen through the compound microscope of animal tissue in each of the following phases:

Interphase

 

 

 

 

 

 

anaphase

Prophase

 

 

 

 

 

 

telophase

Metaphase

 

 

 

 

 

 

 

 

2. Normal cell functions such as protein synthesis and respiration occur during which phase?

3. The duplication of DNA and chromatids occurs during which phase?

4. If a parent cell has 32 chromosomes, then after mitosis daughter cells each have how many chromosomes?

5. Contrast the telophase in animal and plant cells.