Cell Growth and Reproduction

Transcription

1 Cell Growth and Reproduction Before You Read SC.F The student knows that cell behavior can be affected by molecules from other parts of the organism or even from other organisms. SC.F The student knows that every cell contains a blueprint coded in DNA molecules that specify how proteins are assembled to regulate cells. In this section you will learn about the way cells grow and divide. Have you ever watched someone trim a bush or a tree? What would have happened if the bush or tree had not been trimmed? Write down a few of your thoughts on the lines below. Read to Learn Cell Size Limitations As you have learned, the plasma membrane lets nutrients into the cell and allows wastes to leave. Inside the cell, nutrients and wastes move by diffusion. Because a cell s size can slow the rate of diffusion, cells have to have a way of limiting their growth. Fortunately, cells divide before they become too big and unable to function well. Cell division also has other purposes. Cell Reproduction When cells divide, new cells are produced from one cell. The two cells that are produced are identical to the original cell. Just before cells divide, several short, stringy structures appear in the nucleus. These structures are called chromosomes. What do chromosomes do? Chromosomes (KROH muh sohmz) contain DNA and are the carriers of the genetic material that is copied and passed from generation to generation. For most of a cell s lifetime, chromosomes exist as something called chromatin (KROH muh tihn). Chromatin is long, stringy strands of DNA. Without the proper amount of DNA, the cell cannot survive. Therefore the chromosomes must be accurately passed on to new cells. The Cell Cycle The cell cycle is the time of growth and division of a cell. A cell s life cycle is divided into two periods. There is a period of Identify the Main Idea As you read this section, stop after every paragraph and put what you read into your own words. Highlight the main idea in each paragraph. 1. What structures appear in the nucleus shortly before cell division? READING ESSENTIALS Chapter 8 83

2 Name Date Class Cell Growth and Reproduction, continued growth called interphase. There is also a period of nuclear division called mitosis. What is interphase? Analyze the Diagram As you read about mitosis, highlight each stage in the diagram below. What is mitosis? Mitosis (mi TOH sus) follows interphase. It is the process of nuclear division in which two daughter cells form. Each of these daughter cells contains a complete set Stages of Mitosis of chromosomes that are identical to those of the parent cell. A Prophase The chromatin coils to form chromosomes. Disappearing nuclear envelope Doubled chromosome Centromere B Metaphase The chromosomes move to the center of the cell. Sister chromatids C Anaphase Centromeres split and sister chromatids are pulled to the opposite sides of the cell. D Telophase Two new nuclei are formed and a double membrane begins to form between them. Nuclear envelope reappears Two daughter cells are formed 84 Chapter 8 What are the phases of mitosis? There are four phases of mitosis. Each phase merges into the next phase. The four phases are prophase, metaphase, anaphase, and telophase as shown in the illustration to the left. Prophase is the first and longest phase. During prophase the chromatin coils up to form chromosomes. Each duplicated chromosome is made up of two identical halves, called sister chromatids. Centromeres (SEN truh meers) hold the sister chromatids together. During metaphase, the second phase of mitosis, the doubled chromosomes are pulled to the center of the cell. Anaphase is the third phase of mitosis. During this phase, the centromeres of the sister chromatids split apart. This separates the sister chromatids from each other. In telophase, the last phase of mitosis, the chromatids move to opposite sides of the cell. Two nuclei are formed one on each side of the cell. Finally, a new double membrane begins to form between the two new nuclei. READING ESSENTIALS Spindle fibers During interphase a cell grows in size, carries on metabolism, duplicates chromosomes, and prepares for division. Interphase is the busiest phase of the cell cycle.

3 Cytokinesis Cell Growth and Reproduction, continued Following telophase, the cell s cytoplasm divides and separates into two new identical cells. This is called cytokinesis (si toh kih NEE sus). Results of Mitosis When mitosis is complete, one-celled organisms remain as single cells. The organism simply multiplied into two organisms. These daughter cells eventually will repeat the same cell cycle as the parent cell and will grow and divide. In larger organisms, cell growth and reproduction result in groups of cells that work together as tissue to perform a certain function. Tissues organize in combinations to form organs. Organs perform specific complex tasks within the organism. Multiple organs that work together form an organ system, such as the digestive system. The stomach is one organ in the digestive system. It functions to digest food. It is important to remember that no matter how complex the organ system or organism becomes, the cell is still the most basic unit of that organization. 2. What do tissues organize to form? After You Read Mini Glossary anaphase: the third phase of mitosis in which the centromeres split and the sister chromatids of each chromosome are pulled apart cell cycle: continuous sequence of growth (interphase) and division (mitosis) in a cell centromere (SEN truh meer): cell structure that joins two sister chromatids of a chromosome chromatin (KROH muh tihn): long strands of DNA found in the eukaryotic cell nucleus; coils up to form chromosomes chromosomes (KROH muh sohmz): cell structures that contain DNA and carry the genetic material that is copied and passed from generation to generation of cells cytokinesis (si toh kih NEE sus): cell process following mitosis in which the cell s cytoplasm divides and separates into new identical cells interphase: cell growth period where a cell increases in size, carries on metabolism, and duplicates chromosomes prior to division metaphase: short second phase of mitosis where doubled chromosomes move to the center of the cell mitosis (mi TOH sus): period of nuclear cell division in which two daughter cells are formed, each containing a complete set of chromosomes organ: group of two or more tissues organized to perform complex activities within an organism organ system: multiple organs that work together to perform a specific life function prophase: first and longest phase of mitosis where chromatin coils into visible chromosomes READING ESSENTIALS Chapter 8 85

4 Cell Growth and Reproduction, continued sister chromatid: identical half of duplicated parent chromosome formed during the prophase stage of mitosis; the halves are held together by a centromere telophase: final phase of mitosis during which new cells prepare for their own independent existence tissue: groups of cells that work together to perform a specific function 1. Circle the terms from the Mini Glossary that identify the phases of mitosis. Then, in the space provided, list them in the order they occur Use the diagram below to help you review the cell cycle by providing two facts for each period. The Cell Cycle Interphase Mitosis Fill in the blanks with the following terms: tissues, sister chromatids, cell cycle, mitosis, and chromosomes. a. Two identical halves of a duplicated parent chromosome are called. b. The process of cell division is called. c. are the carriers of genetic material that is copied and passed from generation to generation d. The is the period of growth and division of a cell. e. are groups of cells that work together to perform specific functions. Visit the Glencoe Science Web site at science.glencoe.com to find your biology book and learn more about cell growth and reproduction. 86 Chapter 8 READING ESSENTIALS

5 8.3 Control of the Cell Cycle Before You Read SC.F The student knows that complex interactions among the different kinds of molecules in the cell cause distinct cycles of activity governed by proteins. SC.F The student knows that cell behavior can be affected by molecules from other parts of the organism or even from other organisms. Also covers SC.F This section is about cancer and what happens when cells grow out of control. Experts agree that a healthful diet can reduce the risk of cancer. On the lines below list foods that you think belong in a healthful diet. Read to Learn Normal Control of the Cell Cycle The cell cycle is controlled by conditions both inside the cell and in the cell s environment. When something goes wrong with normal cell conditions, cells lose control of the cell cycle. Cancer is a growth that occurs when uncontrolled cell division takes place. This loss of control may be caused by changes in enzyme production inside the cell or by some outside factor, such as air or water pollution. An enzyme, you will remember, is a type of protein found in all living things that changes the rate of chemical reactions. Cancer: A Mistake in the Cell Cycle Scientists think that cancer is caused by changes to one or more of the genes that produce the substances that help to control the cell cycle. A gene is a part of DNA that controls the production of a protein. Cancer occurs when something causes the damaged genes to go into action. When that happens, cancerous cells form masses of tissue called tumors. Tumors keep normal cells from getting needed nutrients. This causes damage to organs. Cancer cells can spread throughout the body to other organs as well. What are the causes of cancer? Cancer is the second leading cause of death in the United States. Only heart disease leads to more deaths each year. But the causes of cancer are hard to pinpoint. This is because both environmental and genetic factors play a part. We do know that many environmental factors damage the genes that control the Summarize As you read this section, highlight the main point in each paragraph. Then, write a short paragraph summarizing what you have learned. 1. What occurs when uncontrolled cell division takes place? READING ESSENTIALS Chapter 8 87

6 8.3 Control of the Cell Cycle, continued cell cycle. Environmental factors include cigarette smoke, air and water pollution, and exposure to ultraviolet radiation from the sun. Cancer may also be caused by genetic factors such as viral infections that damage the genes. 2. Infer Which snack is more healthful? (Circle your choice.) a. an apple b. a chocolate candy bar How can cancer be prevented? A healthful lifestyle reduces the risk of cancer. Experts agree that diets low in fat and high in fiber content can reduce the risk for many kinds of cancer. Fruits, vegetables, and grain products are good food choices because they are low in fat and high in fiber. Other healthful choices such as daily exercise and not using tobacco also are known to lower the risk of cancer. After You Read Mini Glossary cancer: uncontrolled cell division that may be caused by environmental factors and/or changes in enzyme production in the cell cycle gene: segment of DNA that controls protein production and the cell cycle 1. Write a sentence using both of the terms in the Mini Glossary above. 2. Place the following events in the appropriate box based on the order in which they occur: Uncontrolled cell division occurs. Cancerous cells form tumors that deprive normal cells of nutrients. Cancer occurs. Environmental factors or viral infections damage genes Visit the Glencoe Science Web site at science.glencoe.com to find your biology book and learn more about control of the cell cycle. 88 Chapter 8 READING ESSENTIALS