Magnetic Mitosis

Introduction

All cells are formed from pre-existing cells. New cells are formed by a process called mitosis. Use this handy magnetic demonstration to explain the process by which cell division occurs to form two genetically identical daughter cells. Demonstrate the activities of each stage of mitosis using magnetic models.

Concepts

• Cell cycle
• Mitosis
• Cytokinesis

Background

The process of growth and division in a typical eukaryotic cell is called the cell cycle and is composed of five stages—G1, S, G2, M and C (see Figure 1). The cell cycle begins with the formation of a new cell and continues until that cell divides into two offspring cells. In a mammal or higher order plant, a complete cell cycle lasts about 24 hours. Each offspring cell then begins the cycle again. The dramatic events of nuclear division take place during the karyokinesis or M (mitosis) stage, which represents only a brief segment—typically two to four hours—in the overall life cycle of the cell. Most of the life of a cell is spent performing normal metabolic activities, growing and preparing the cell for its next division. These stages are collectively termed interphase and include the G1, S and G2 stages. Interphase typically takes between twenty and twentytwo hours in a mammal or higher order plant cell. Although the main sequence of the phases of the life cycle of a cell is fixed, the amount of time spent in each phase varies among different organisms and among different cells within an organism. Interphase is divided into three stages. The G1 (Gap 1) stage is the cell’s primary growth stage and typically the longest time is spent in this stage. New cells, which are metabolically very active, are actively synthesizing RNA and new proteins. The G stage normally lasts about ten hours. Some cells go into an extended G1 stage and rarely ever divide again. This stage is called the G0 stage. Neurons, for example, are very active and important cells but tend to remain in the G0 stage. From the G1 stage, most cells proceed to the S (synthesis) stage. In the S stage, an exact copy of DNA is made in the nucleus of the cell. The S stage usually takes five to six hours to create an exact copy or replicate the DNA. During the G2 (Gap 2) stage, various organelles are replicated, the chromosomes start to condense, and microtubules are synthesized.

Because the DNA replicates in the S phase, a cell in G2 has twice as much DNA in its nucleus as a cell in G1. The duration of G2 is usually short, about three to four hours on average. After these three stages of the cell cycle, G1, S and G2, are complete, the nuclear division called karyokinesis or M (mitosis) stage can begin. The M stage is easily identified because it is the only phase in which the chromosomes are visible with a light microscope. In most cells, mitosis lasts only two hours of the entire twenty-fourhour cell cycle. Mitosis is quickly followed by cell division of the parent cell’s cytoplasm and organelles to produce two offspring cells in the C (cytokinesis) stage.

The length of the cell cycle is important because it determines how quickly an organism can multiply. For single-celled organisms, this rate determines how quickly the organism will reproduce new, independent organisms. For higher-order species the length of the cell cycle determines how long it takes to replace damaged cells. Certain simple multicellular organisms have cell cycles that last only 8 minutes. Some liver cells take up to a year to complete one cell cycle. Most of the differences in cell cycle duration among different species or different kinds of cells are found in the duration of specific cell cycle stages, generally the G1 and G2 stages. In complex organisms, early embryonic cells divide in 12 hours instead of the usual 24 hours, often omitting the G1 and G2 stages, and then quickly proceed through successive rounds of the S, M and C stages.

The M stage of the cell cycle is further subdivided into five phases as shown in Figure 2. During prophase, the nucleolus fades and chromatin condenses into chromosomes. Each replicated chromosome comprises two chromatids, both with the same genetic information. Microtubules of the cytoskeleton, which are responsible for cell shape, motility, and attachment to other cells during interphase, disassemble to be used to create the spindle fibers necessary for chromosome separation. In prometaphase, the nuclear envelope breaks down so there is no longer a recognizable nucleus. Some spindle fibers elongate and attach to the kinetochore protein bundles located on the chromosomes. Other spindle fibers elongate but instead of attaching to chromosomes, they overlap each other at the cell center. During metaphase, the chromosomes reach a position called the metaphase plate, which is midway between the poles. The chromosomes are at their most compact at this time. At the onset of anaphase, the kinetochore protein bundles separate and as a result the sister chromatids also separate, splitting the chromosome in half. The spindle fibers shorten and drag the attached chromatids to opposite poles of the cell. In telophase, the daughter chromosomes arrive at the poles and the spindle fibers that have pulled them apart disappear. A nuclear envelope reforms around each cluster of chromosomes and these chromosomes return to their more extended form while cytokinesis begins.

In animal cells, cytokinesis results when the membrane is pulled inward by the cytoskeleton at a point called the cleavage furrow. The pulling in of this cleavage furrow continues until the deepest parts on opposite sides meet in the center of the cell. At that point, when membrane hits membrane, the cell membrane fuses together, separating the two daughter cells. In plant cells, the rigid wall requires that a cell plate be synthesized between the two daughter cells. To do this plant cells send vesicles filled with cell wall material to their equator. When the vesicles reach the equator, they bump into other vesicles and fuse together, forming the cell plate. As more vesicles go to the equator, the cell plate expands until it bumps into the cell membrane. When the cell plate reaches the cell membrane, it fuses with it to form the complete cell wall.

Materials

Safety Precautions

Exercise caution when cutting the magnets with scissors. Follow all laboratory safety guidelines.

Disposal

All materials may be stored for future demonstrations.

Prelab Preparation

Using scissors, cut out magnetic chromosomes.

Draw a cell on the board using chalk or a dry erase marker. This cell should include centrosomes and a nuclear envelope. It should be modified throughout the demonstration to reflect the changes in each stage of meiosis. Refer to the Background section frequently for explanation of each step.

Procedure

Prophase

1. Pair the two identical sister chromatids of each chromosome, joining them together at the centromere (see Figure 3).
2. Surround the chromosomes with a nuclear envelope which has started to break down. Draw the centrosomes separating and forming the early mitotic spindle.

3. Draw the microtubules invading the nuclear area so that they interact with the chromosomes. The centrosomes are at opposite poles (see Figure 4).
4. Place each chromosome so that it is attached to a microtubule at its centromere.

5. Arrange the chromosomes so that they are attached to the metaphase plate at the centromere (see Figure 5).

6. Each chromosome separates at the centromere. One sister chromatid moves towards one pole and its identical sister chromatid moves toward the other pole (see Figure 6).
7. At the end of anaphase each pole has an equivalent and complete set of chromosomes.

8. The cells begin to separate at the cleavage furrow (see Figure 7).
9. Using chalk, draw the nuclear envelope beginning to form.
10. Explain that cytokinesis is occurring and the cytoplasm is divided between the two daughter cells.

Student Worksheet PDF

11015_Student1.pdf

Teacher Tips

• Use the mitosis cell overhead before, during or after the demonstration and point out how the phases appear in actual cells.

• This demonstration may be performed an infinite number of times.
• Although Interphase is not technically part of the mitotic (M) cycle students may find it beneficial to see what the cell looks like during this time (see Figure 8).
  {11015_Tips_Figure_8_Interphase}

• A complementary model kit for teaching meiosis, Magnetic Meiosis Models, Catalog No. FB1992, is also available from Flinn Scientific.
• Explain to students that in this demonstration only 3 pairs of sister chromatids are used to model the process of mitosis. In human cells this occurs with 22 pairs of autosomes and one set of sex chromosomes.

Answers to Questions

1. Referring to the drawings above, describe what is happening during each stage of mitosis.

   Prophase—the nucleolus fades and chromatin condenses into chromosomes. Microtubules disassemble and are used to create the spindle fibers necessary for chromosome separation.

   Prometaphase—the nuclear envelope breaks down so there is no longer a recognizable nucleus. Some spindle fibers elongate and attach to the kinetochore protein bundles located on the chromosomes. Other spindle fibers elongate but instead of attaching to chromosomes, they overlap each other at the cell center.

   Metaphase—the chromosomes reach a position midway between the poles, called the metaphase plate.

   Anaphase—the kinetochore protein bundles separate and as a result the sister chromatids separate, splitting the chromosome in half. The spindle fibers shorten and drag the attached chromatid to opposite poles of the cell.

   Telophase—the daughter chromosomes arrive at the poles and the spindle fibers that have pulled them apart disappear. A nuclear envelope reforms around each cluster of chromosomes and these chromosomes return to their more extended form.

2. 1. Approximately what percentage of the cell’s life cycle is spent in the mitotic phase?

      Approximately 10% of the cell’s life cycle consists of the mitotic M phase.

   2. Imagine looking at a microscope slide that contains 130 cells. Estimate how many cells on the slide will be undergoing the process of mitosis. Explain your reasoning.

      At any given time approximately 10% of cells are going through mitosis. Therefore if 130 cells are in a sample around 13 of those cells should be in the mitotic phase.

3. Why is it important that the microtubules have the ability to stretch? It is important because one centrosome goes to one daughter cell and the second goes to the other daughter cell. The mitotic spindles must stretch the entire length of the cell so that the chromosomes can attach and divide evenly.
4. During prometaphase the microtubule attaches to the chromosomes at a location called the kinetochore. Explain why this chromosome component is necessary for successful mitosis.

   The kinetochore proteins act as the attachment point on the chromosome for the microtubule formed by the mitotic spindle. The kinetochore proteins separate and allow the sister chromatids to be divided evenly as they are pulled to opposite poles of the cell for cell division.

References

Campbell, N. Biology; 6th edition, Benjamin Cummings: San Francisco, CA; 2002; pp 218–219.