Stages of Mitosis in the Blastula of a Whitefish

Activity 1: Pre-Assessment

1. List three reasons why organisms need to produce new cells.
2. What cellular structures must be replicated to ensure that new cells are functional after cell division?
3. Why are karyokinesis and cytokinesis distinct steps in cell division?
4. Discuss the answers to the questions with a partner (think, pair, share) and then the class.

Activity 1: Observe the Stages of Mitosis in the Blastula of a Whitefish

A fundamental property of somatic (nonreproductive or body) cells of multicellular organisms is mitosis, which basically provides new cells for growth and regeneration or replacement of dying and dead cells of the living body. A simple example in humans is our continuous shedding of skin cells and their replacement by new skin cells. Mitosis is also vital for development. Many single-celled organisms depend on mitosis as their sole or primary way of asexual reproduction. This asexual reproduction is distinct from multicellular organisms which undergo meiosis to produce reproductive cells (sperms or eggs). Cell division involves the chromosomes and genes of the dividing cells, which are duplicated and passed on to the new cells or daughter cells and are the reason why, for example, all these new skin cells are genetically identical. Mitosis is a controlled process, and loss of control can lead to cancerous cells. Certain DNA sequences on the ends of the chromosomes called telomeres become shorter during every mitotic cycle of somatic cells, a regulatory mechanism that contributes to the number of mitotic cycles. If the telomeres fail to shorten, cells may become immortalized as they continue to divide indefinitely, typical of cancer cells.

As explained, all living organisms have the need to stay alive and reproduce. Mitosis, therefore, addresses the need for cell growth, maintenance, and repair. Mitosis is part of the cell cycle (see Figure 13.1). The cell cycle refers to the series of events that describe the metabolic processes of growth and replication of cells. The bulk of the cell cycle is spent in the “living phase,” known as interphase. Interphase is further broken down into 3 distinct phases: G1 (Gap 1), S (Synthesis), and G2 (Gap 2). G1 is the phase of growth when the cell is accumulating resources to live and grow. After attaining a certain size and having amassed enough raw materials, a checkpoint is reached where the cell uses biochemical markers to decide if the next phase should be entered. S phase is when metabolism is shifted towards the replication (or synthesis) of the genetic material. During S phase, the amount of DNA in the nucleus is doubled and copied exactly in preparation to divide. The chromosomes at the end of G1 consist of a single chromatid. At the end of S phase, each chromosome consists of two identical sister chromatids joined at the centromere. When the DNA synthesis is complete, the cell continues on to the second growth phase called G2. Another checkpoint takes place at the end of G2 to ensure the fidelity of the replicated DNA and to re-establish the success of the cell’s capacity to divide in the environment. If conditions are favorable, the cell continues on to mitosis. During interphase, the cell’s DNA is replicated so that there are two copies of each chromosome, called the sister chromatids. In eukaryotes, all chromosomes must be duplicated prior to mitosis and cytokinesis to ensure each new daughter cell has the full complement of genetic information. Other structures, such as organelles, are replicated during the G1 portion of the cell cycle (See Figure 13.1). The checkpoints are prone to DNA damage, which can cause a disease like cancer (see Figure 13.2).

During prophase, the chromosomes coil up, and sister chromatids become visible under a microscope. The nuclear membrane surrounding the chromosomes also disappears. In metaphase, the sister chromatids align in the center of the cell, attached to spindle fibers. During anaphase, the sister chromatids separate and move to opposite poles of the cell. In telophase, the chromosomes arrive at the poles and begin to decondense while the nucleus reforms. Figure 13.3 makes it look like the phases are very distinct. The phases, however, transition without stopping. Notice also other important structures, such as the spindle fibers and kinetochores on the centromeres of each chromosome. Each pair of sister chromatids has a protein structure, called a kinetochore, which becomes attached to spindle fibers. The spindle fibers pull the sister chromatids apart, toward opposite ends of the cell.

Cancer cells are not healthy cells. Healthy cells die through programmed cell death or apoptosis after a number of generations of cell divisions. Cells become mature or differentiate, which enables them to carry out their function in the body. Cancer is the uncontrolled growth and mitotic division of cells which can be caused by mutations in genes. Some chemotherapy drugs, including taxanes such as Taxol from the Yew tree and alkaloids from the Vinca plant, interfere with mitosis by binding to microtubules and preventing spindle fibers from separating sister chromatids, thus leading to cell death.

Defective Cell Cycle Checkpoints: A white light shone on a child’s eye should yield a clear view of the retina. In the above image, the right eye shows a white light reflecting and indicates a retinoblastoma (Figure 13.2). This cancer is caused by a defect in the Rb gene, a tumor suppressor gene. This defect permits the continuation of the cell cycle despite damage to DNA. Retinoblastoma is the most common primary childhood cancer which often stems from a genetic background.

Safety Precautions

• Be careful handling glass slides, as the edges may be sharp.
• Observe proper use of the microscope; avoid handling the electric cord with wet hands.
• Do not use the coarse adjustment knob of the microscope at higher magnifications.
• There is a separate marked disposal for sharp objects like broken glass. If you cannot locate it, inform your Instructor/Lab Technician immediately of any broken glassware, as it could cause serious injuries.

For this activity, you will need the following:

• Prepared slide of whitefish blastula cells, or use online images and resources

For this activity, you will work in pairs.

Structured Inquiry

Step 1: Hypothesize/Predict: Using Figure 13.1, predict the percent of time that a cell would spend in each phase. Based on this prediction, how much time do you think cells will spend in interphase as opposed to mitosis? Write your prediction in your notebook.

Step 2: Student-Led Planning: Look at Figure 13.3. In your notebook, make a table defining the characteristics of the stages of mitosis, as well as interphase, that you can use to identify each stage under the microscope. Note: There are four major phases of mitosis, plus prometaphase, which is a transition phase between prophase and metaphase.

Step 3: Using the prepared slide, record the number of cells in each phase of the cell cycle. Use the method shown in Figure 13.4 to count the cells. Record your count in a data table like that shown in Table 13.1. Share your data with the class to create a group total count.

Table 13.1: Results of Cell Stage Identification

Step 4: In your data table, calculate the percentage of cells in each phase.

Step 5: Critical Analysis: Are the predictions you made supported by your data (observations and calculations)? Do your results match the diagram as presented in Figure 13.1? Why or why not?

Assessments

1. Explain why interphase could be the longest phase and mitosis and cytokinesis are generally much shorter phases of the cell cycle.
2. Explain the importance of spindle fibers in mitosis and why antimitotic drugs that block spindle fiber formation are used to treat cancer.