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Regulating the Cell Cycle

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Presentation on theme: "Regulating the Cell Cycle"— Presentation transcript:

1 Regulating the Cell Cycle
Read the lesson title aloud to students or have a volunteer read the title aloud.

2 Learning Objectives Describe how the cell cycle is regulated.
Explain how cancer cells are different from other cells. Click to reveal each of the learning objectives. Give students the following scenario: Two athletes are brought before an investigating committee because both tested positive for growth hormones. One athlete, a record holder, denies ever knowingly taking the growth hormone. The other recently had knee surgery and was taking the drugs as prescribed by a doctor. As a class, discuss what the cell cycle has to do with this story. Encourage students to speculate how the growth hormones were being used by the athletes. Be sure students understand that, by the end of the presentation, they should be able to describe controls on cell division within the body and describe characteristics of cancer cells that differentiate them from normal cells.

3 Cell Division and Repair
Point out that a person recovering from a bone break or a wound relies on the cell cycle to make new cells to repair damaged tissues. Explain that in the human body, many cell types do not continue dividing. For example, most muscle and nerve cells do not continue dividing once they have developed. Other cell types, such as skin, bone marrow, and cells lining the digestive system, grow and divide rapidly and regularly. Ask students why certain cell types would need to grow and divide more quickly and more frequently than others. Emphasize that cell division is critical for repair to the body as well as general maintenance of body tissues. Ask students to consider how the body’s cells know when to divide and when to stop dividing. Tell students: For the girl in the photo, how do the cells in her broken leg bone know to divide more quickly than those in her healthy leg? Explain that scientists have answered this question by doing laboratory experiments.

4 Healing a Bone new bone cells Explain that when scientists grow cells in a laboratory, the cells will continue to grow and divide until they meet up with other cells. When neighboring cells are scraped away, the remaining cells start to grow and divide again. Ask: When you have a cut, what kind of healing pattern do you see? Where do the new cells form first? Answer: The cut heals from the edges inward. Explain that the image on the right of the screen is an illustration of how the break in the X-ray on the left would heal. Have a volunteer come to the board to point to where on the cutaway bone diagram the new cells are forming. Click to reveal the label and arrows indicating new bone cells. Ask: When will cells at the edge of the broken bone likely stop dividing rapidly? Answer: when they meet up with neighboring cells, meaning the bone is healed Click to reveal the slide text. Cells at the edge of an injury are stimulated to divide rapidly. As an injury heals, the rate of cell division slows.

5 The Discovery of Cyclins
Scientists found a protein in a cell undergoing mitosis. They injected the protein into a non-dividing cell. A mitotic spindle started to form. Cyclins: proteins that regulate the cell cycle Point out that scientists could describe what was happening as a wound or bone healed, but that they still needed to find out what signal actually regulates, or controls, the cell cycle. In other words, they needed to find what tells a cell when to divide, when to duplicate its chromosomes, or when to enter another stage of the cell cycle. Explain that in the 1980s scientists found a protein in a cell going through mitosis that was not present in a non-dividing cell. When they injected that protein into a non-dividing cell, a mitotic spindle formed. Click to reveal the first three bullet points. Ask: In mitosis, what is the mitotic spindle? What is its function? Answer: The mitotic spindle “grabs” onto duplicated chromosomes and pulls them apart so they can ultimately become part of separate nuclei. Click to reveal the fourth bullet point. Explain that the researchers called this protein “cyclin” because it seemed to regulate the cell cycle. Today scientists know of a whole family of proteins that they call cyclins that regulate the timing of the cell cycle in eukaryotic cells. Ask: What could go wrong if the cell cycle is not carefully regulated? Sample answer: DNA might not be divided evenly between the cells. Cells could divide when the organism does not need new cells, which could waste resources.

6 Regulatory Proteins Internal regulators: External regulators:
respond to events inside the cell let cell cycle proceed only when certain steps have already happened External regulators: respond to events outside the cell growth factors: wound healing and embryonic development others cause cells to stop or slow their cell cycles. Explain that scientists have since found many other proteins that help regulate the cell cycle. The cell cycle is controlled by regulatory proteins both inside and outside the cell. Explain that internal regulatory proteins respond to events inside the cell and let the cell cycle proceed only when certain steps have already been completed. For example, some regulatory proteins function to keep mitosis from starting until the chromosomes have been duplicated. Another regulatory protein keeps the cell from proceeding to anaphase until the spindle fibers have attached to chromosomes. Click to reveal each of the bullet points for internal regulators. Ask: What do you think the role of external regulators is? Answer: to respond to events that happen outside the cell Click to reveal each of the bullet points for external regulators. Explain that external regulatory proteins direct the cell cycle to speed up or slow down. Growth factors are an important group of external regulatory proteins. Growth factors stimulate the growth and division of cells. Other external regulators have the opposite effect, causing cells to stop or slow their cell cycles. Stopping or slowing a cell cycle can be just as critical as stimulating growth. Stopping the cell cycle can prevent excessive cell growth and keep body tissues from disrupting one another. Ask students to think back to the opening scenario of the two athletes that you discussed. Ask: What kind of regulators were most likely present in the medications given to the athlete who had knee surgery? Answer: external regulators

7 Apoptosis A process of programmed cell death
Important role in structuring tissues during growth and development Cell undergoes a series of controlled steps for self- destruction. Point out that while many new cells are forming almost constantly in a multicelluar organism, cells are also dying. Some die due to normal wear and tear. Other cells are programmed to die. This process is called apoptosis. Explain that apoptosis plays an important role in structuring the body during growth and development. Ask: If old skin cells did not die and shed, what would happen to a person’s skin? Answer: The skin would become too thick and lose some of its properties. Click to reveal the mouse image. Explain that the magnified image shows the foot of an embryonic mouse. Ask a volunteer to describe how the embryonic foot and the adult mouse foot differ. Ask a volunteer to come to the board to point out the regions of the embryonic foot that are likely programmed for cell death. Click to reveal the circle indicating the area of webbing between the toes. Ask: What is one way the pattern of apoptosis would differ in foot development for a duck? Answer: Not all of the cells between a duck’s toes would undergo apoptosis. Some of the cells would remain, causing the duck to have webbed feet. Make sure students understand that apoptosis is a normal cell process and is beneficial to many organisms. In fact, apoptosis is a necessary process for multicellular organisms. Apoptosis allows organisms to more fully control which cells continue to grow and divide and which do not. If a cell is old or damaged, apoptosis allows this cell to be destroyed rather than continuing to grow, divide, and use the resources that other healthy cells need to survive.

8 Cancer: Uncontrolled Cell Growth
Cancer cells don’t respond to normal regulatory signals. Cell cycle is disrupted. Cells grow and divide uncontrollably. tumor Ask students to share what they already know about what cancer is. Encourage them to relate what they already know to what they have been learning about the cell cycle. Explain that cancer is a disease of the cell cycle. Ask a volunteer to point to the area in the diagram that shows cancer. Click to reveal the label for the tumor. Tell students: Cancer cells form a mass called a tumor. Ask: How do you think a tumor like this formed? Answer: One cell started dividing too fast; there was no control over the cell cycle. Click to reveal the bullet points, reading each one in turn. Ask students why they think a rapidly growing mass of cells can be harmful to healthy cells. Guide students to realize that cells need resources to grow and divide. When cancer cells grow uncontrollably, they not only displace healthy cells and tissues by taking up space but they also use up resources that normal cells need. blood vessel

9 Cancer Formation: A Closer Look
A cell begins to divide abnormally. Cells produce a tumor and start to displace normal cells and tissues. Cancer cells move to other parts of the body. Ask for student volunteers to “narrate” what the images are showing about the development of cancer. Click to reveal each step in turn as volunteers correctly describe the step. Elaborate on step 3 to explain that cancer is so dangerous because these abnormal cells can break free from their original site and move to other parts of the body through the bloodstream or lymphatic system, where the cells can continue to grow and divide out of control. The formation of secondary tumors is called metastasis. Ask: What makes cancer cells different from healthy cells? Answer: Cancer cells do not respond to the signals that regulate cell growth and division. They continue to divide when healthy cells would stop. Ask: When researchers develop drugs to fight cancer, what characteristics of cancer cells do you think they target? Sample answer: They might target rapidly developing cells. Misconception alert: Explain that while we use the term “tumor” to refer to a clump of cancer cells, not all tumors are truly cancerous. A benign, or noncancerous, tumor does not spread to surrounding healthy tissue. A malignant, or cancerous, tumor invades and destroys surrounding healthy tissue.

10 What Causes Cancer? In all cancers, control over has broken down.
Cancer results from a defect in genes that control cell growth and division. the cell cycle Ask a volunteer to come to the board and write in the missing terms or to verbally identify what correctly fills in the blank. Click to reveal the correct answer. Ask students to share some of the things they have heard in the news about what causes cancer. Answers may vary, but many students will offer ideas such as smoking, tanning, and exposure to X-rays. Click to reveal the text. Point out that there are, in fact, many triggers for cancer, but that all of these ultimately disrupt the genes that regulate cell growth and division. The result of the gene defects is that some cancer cells no longer respond to external regulators and others no longer produce internal regulators. Defects in these genes may be caused by tobacco use, radiation exposure, other defective genes, and even viral infections.

11 Cancer Incidence Point out that cancer can affect almost every organ of the body. Ask: What information does this graph provide? Answer: the number of incidences of several cancer types for males and females over a five-year span Ask: Does this graph indicate the total number of people who had each kind of cancer during the time period? Answer: No, it shows the number of people with each cancer type for every 100,000 individuals. Click to reveal the circle around the y-axis label. Ask: Based on this data, how many cases of melanoma in men would you expect in a population of 1 million (1,000,000) individuals? Answer: around 250 cases Ask: What was the most common form of cancer during the time period? Answer: prostate cancer Ask: How many cases of breast cancer were reported compared to prostate cancer for the time period shown? Answer: Per 100,000 individuals, there were about 125 cases of breast cancer reported, compared to about 165 cases of prostate cancer.

12 Treatments for Cancer Surgery to remove localized tumor
Radiation to destroy cancer cell DNA Chemotherapy to kill cancer cells or slow their growth Explain that for a tumor that has not spread, surgery can often be sufficient to remove the cancer. This treatment is common for skin cancer. Radiation is used to destroy the DNA of cancer cells, which must divide quickly and therefore must duplicate their DNA rapidly. In chemotherapy, chemicals targets cells as they are dividing. Ask: What unintended effects might radiation and chemotherapy have? Answer: They could also disrupt healthy cells that are dividing. Point out that unintended consequences of cancer treatments are motivating researchers to learn even more about the proteins that control the cell cycle so they can find more specific ways to target cancer cells without harming healthy cells. Distribute the lesson worksheet to the class. Allow students to work in pairs or small groups to develop their concept maps to summarize what they learned from the presentation. To help students get started, you may wish to suggest a number of terms that they should be sure to include in their concept maps: cell cycle, divide, injury, growth, development, growth factors, cyclins, regulate, external regulatory proteins, internal regulatory proteins, cancer, tumor, gene, radiation, tobacco use, and defective genes.

13 Student Worksheet Answers
Allow students to work together in pairs or small groups to develop their concept maps. If time permits, ask several volunteer pairs to sketch out their concept maps on the board. Review the concepts maps as a class, using the opportunity to identify and discuss any points of confusion or misconceptions. Worksheet Answers: Student concept maps will vary widely, but should include reference to the following: cell cycle, divide, injury, growth, development, growth factors, cyclins, regulate, external regulatory proteins, internal regulatory proteins, cancer, tumor, gene, radiation, tobacco use, and defective genes. Accept all logically formatted and arranged concept maps in which all the indicated terms have been used and in which arrows, lines, and intervening text describe accurate relationships between terms.

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