Chapter 10.3 - Regulating the Cell Cycle PDF

## 10-3 Regulating the Cell Cycle **Guide for Reading** **Key Concepts** - How is the cell cycle regulated? - How are cancer cells different from other cells? **Vocabulary** - cyclin - cancer **Reading Strategy: Summarizing** Summarizing helps you understand and remember what you read Write a ma...

## 10-3 Regulating the Cell Cycle **Guide for Reading** **Key Concepts** - How is the cell cycle regulated? - How are cancer cells different from other cells? **Vocabulary** - cyclin - cancer **Reading Strategy: Summarizing** Summarizing helps you understand and remember what you read Write a main idea statement for each blue head. When you have finished the section, compare your statements with those in the study guide. **Image:** Petri dish ### Controls on Cell Division One of the most striking aspects of cell behavior in a multcellular organism is how carefully cell growth and cell division are controlled. Not all cells move through the cell cycle at the same rate. In the human body, most muscle cells and nerve cells do not divide at all once they have developed. In contrast, the cells of the skin and digestive tract, and cells in the bone marrow that make blood cells, grow and divide rapidly throughout life. Such cells may pass through a complete cycle every few hours. This process provides new cells to replace those that wear out or break down. Scientists can observe the effects of controlled cell growth in the laboratory by placing some cells in a petri dish containing nutrient broth. The nutrient broth provides food for the cells. Most cells will grow until they form a thin layer covering the bottom of the dish, as shown in Figure 10-7. Then, the cells stop growing. When cells come into contact with other cells, they respond by not growing. If cells are removed from the center of the dish however, the cells bordering the open space will begin dividing until they have filled the empty space. These experiments show that the controls on cell growth and cell division can be turned on and off. Something similar happens within the body. When an injury such as a cut in the skin or a break in a bone occurs, cells at the edges of the injury are stimulated to divide rapidly. This action produces new cells, starting the process of healing. When the healing process nears completion, the rate of cell division slows down controls on growth are restored, and everything returns to normal. ### Cell Cycle Regulators For many years, biologists searched for a substance that might regulate the cell cycle - something that would “tell” cells when it was time to divide, duplicate their chromosomes, or enter another phase of the cycle. In the early 1980s, biologists found the substance. Several scientists, including Tim Hunt of Great Britain and Mark Kirschner of the United States, discovered that when injected into a nondividing cell, would cause a mitotic spindle to form. Such an experiment is shown in Figure 10-8. To their surprise, they discovered that the amount of this protein in the cell rose and fell in time with the cell cycle. They decided to call this protein **cyclin** because it seemed to regulate the cell cycle. Investigators have since discovered a family of closely related proteins, known as **cyclins**, that are involved in the cell cycle regulation.**Cyclins regulate the timing of the cell cycle in eukaryotic cells.** The discovery of cyclins was just the beginning. More recently, dozens of other proteins have been discovered that also help to regulate the cell cycle. There are two types of regulatory proteins: those that occur inside the cell and those that occur outside the cell. **Internal Regulators** Proteins that respond to events inside the cell are called internal regulators. Internal regulators allow the cell cycle to proceed only when certain processes have happened inside the cell. For example, several regulatory proteins make sure that a cell does not enter mitosis until all its chromosomes have been replicated. Another regulatory protein prevents a cell from entering anaphase until all its chromosomes are attached to the mitotic spindle. **External Regulators** Proteins that respond to events outside the cell are called external regulators. External regulators direct cells to speed up or slow down the cell cycle. Growth factors are among the most important external regulators. They stimulate the growth and division of cells. Growth regulators are especially important during embryonic development and wound healing. Molecules found on the surfaces of neighboring cells often have an opposite effect, causing cells to stop their cell cycles. These signals prevent excessive cell growth and keep the tissues of the body from disrupting one another. **What are cyclins?** **Image:** Explains when cytoplasm from a cell in mitosis is injected into another cell, the second cell enters mitosis, and it triggers cell division because of a protein called *cyclin*. ### Uncontrolled Cell Growth **Image:** A microscopic image of cancer cells. Why is cell growth regulated so carefully? The principal reason may be that the consequences of uncontrolled cell growth in a multicellular organism are very severe. **Cancer**, a disorder in which some of the body’s own cells lose the ability to control growth, is one such example. **Cancer cells do not respond to the signals that regulate the growth of most cells.** As a result, they divide uncontrollably and form masses of cells called tumors that can damage the surrounding tissues. Cancer cells may break loose from tumors and spread throughout the body, disrupting normal activities and causing serious medical problems or even death. Figure 10-9 shows typical cancer cells. What causes the loss of growth control that characterizes cancer? The various forms of cancer have many causes, including smoking tobacco, radiation exposure, and even viral infection. All cancers, however, have one thing in common: The control over the cell cycle has broken down. Some cancer cells will no longer respond to external growth regulators, while others fail to produce the internal regulators that ensure orderly growth. An astonishing number of cancer cells have a defect in a gene called p53, which normally halts the cell cycle until all chromosomes have been properly replicated. Damaged or defective p53 genes cause the cells to lose the information needed to respond to signals that would normally control their growth. Cancer is a serious disease. Understanding and combating cancer remains a major scientific challenge, but scientists at least know where to start. Cancer is a disease of the cell cycle, and conquering cancer will require a much deeper understanding of the processes that control cell division. --- ### 10-3 Section Assessment 1. **Key Concept** What chemicals regulate the cell cycle? How do they work? 2. **Key Concept** What happens when cells do not respond to the signals that normally regulate their growth? 3. How do cells respond to contact with other cells? 4. Why can cancer be considered a disease of the cell cycle? 5. **Critical Thinking: Formulating Hypotheses** Write a hypothesis about what you think would happen if cyclin were injected into a cell that was in mitosis. ### Sharpen Your Skills Imagine that you are developing a drug that will inhibit the growth of cancer cells. Use your knowledge of the cell cycle to describe how the drug would target and prevent the multiplication of cancer cells. Use the Internet to compare your anticancer drug with those currently in use --- ### TECHNOLOGY SOCIETY **Stem Cells: Promises and Problems** **Image:** Shows how Liver cells, Red blood cells, Brain cells, and Muscle cells are all stem cells Where do the different cells and tissues in your body come from? Incredible as it seems, every cell was produced by mitosis from a small number of cells called stem cells. Stem cells are unspecialized cells that have the potential to differentiate - to become specialized in structure and function - into a wide variety of cell types. In embryonic development, stem cells produce every tissue in the body. Evidence indicates that stem cells also are found in adults. Stem cells in bone marrow, for example, produce more than eight different types of blood cells, replacing those lost due to normal wear and tear. ### Stem Cells in Medicine Although your body produces billions of new cells every day, it is not always able to produce the specific kind of cell to replace those damaged by injury or disease. For example, the body is not able to produce new neurons to repair serious spinal cord injuries, such as those that cause paralysis. Because of this, at present, there is no way for doctors to restore movement and feeling to people who are paralyzed. Stem cells may be the perfect solution to this problem. Recently, researchers have found that transplants of stem cells can reverse the effects of spinal injuries in mice. There is hope that the same will hold true for humans and that stem cells might be used to reverse brain and spinal cord injuries. It also may be possible to use stem cells to grow new liver tissue, to replace heart valves, and to reverse the effects of diabetes. ### Sources of Stem Cells Human embryonic stem cells were first isolated in 1998 by scientists in Wisconsin. Many scientists are now experimenting with ways to produce such cells by transferring adult cell nuclei into the cytoplasms of egg cells. However, since these techniques use or produce early human embryos, these techniques raise serious moral and ethical questions. Because of such issues, embryonic stem cell research is highly controversial. Researchers have also found that nerve, muscle, and liver cells sometimes can be grown from adult stem cells isolated from the bone marrow and other tissues in the body. Experiments such as these, although still in the early stages of development, may usher in a new era of therapy in which replacement tissue is grown from a person's own stem cells. ### Research and Decide Use library or Internet resources to learn more about stem-cell research. Then, write a brief report on how this technology will impact the future of medicine.