Cell Cycle Textbook PDF

5.1 The Cell Cycle This frog began as a single cell that repeatedly divided to form this multicellular organism. CAN YOU EXPLAIN IT?...

5.1 The Cell Cycle This frog began as a single cell that repeatedly divided to form this multicellular organism. CAN YOU EXPLAIN IT? Explore Online FIGURE 1: A frog develops from a single cell. This cell divides into two cells, each of which divide again and again, trillions of times. As this pattern continues, many different cells develop. Gather Evidence All cells come from existing cells. This is easily observed in single-celled organisms, Image Credits: (t) ©Bill Byrne/Design Pics/Corbis; (bl) (bcl) (bcr) (br) ©Cytographics Pty Ltd./Image Bank Film/Getty Images As you explore the lesson, such as bacteria, some of which can reproduce in as little as 20 minutes. Multicellular gather evidence for how the cell organisms, such as a frog, begin as a single cell that repeatedly divides to form a complex multicellular organism. Some organisms reproduce asexually, making cycle is related to the growth and genetically identical clones of themselves. Other organisms, like humans, reproduce maintenance of organisms. sexually. A sexually reproducing organism begins as a fertilized egg. A sperm cell and an egg cell fuse to form a cell, called a zygote. The zygote divides trillions of times to produce a complex, multicellular organism. Predict Why aren’t all organisms made of just one cell? Why do cells divide, instead of simply growing larger? 222 Unit 5 Cells: Stability and Change  EXPLORATION 1 Overview of the Cell Cycle Living systems go through cycles of stable conditions and changing conditions. FIGURE 2: A dividing cell For example, when conditions in your outside environment change, homeostatic mechanisms in your body help restore internal stability. Cells also cycle between phases of stability and change. Some cells stay in a relatively steady state, without dividing, for long periods of time. Other cells are constantly dividing. Stages of the Cell Cycle The life cycle of organisms involves birth, growth and development, reproduction, and eventually death. A cell also has a life cycle, and cell division is only one part of that cycle. The cell cycle is the regular pattern of growth, DNA duplication, and cell division Predict Describe a that occurs in eukaryotic cells, or cells with nuclei. This pattern can be divided into specific situation in which stages that get their names from the earliest studies of cell division, when scientists’ an organism’s cells would need observations were limited by the microscopes of the time. to divide. Because these scientists were unable to observe activity in cells that were not actively dividing, they separated the cycle into two parts: a resting phase and a dividing phase. The resting stage was named interphase, and the division phase was named mitosis. Mitosis includes a final step for complete cell division called cytokinesis. FIGURE 3: The Cell Cycle INTERPHASE G1 Checkpoint Gap 1 (G1) Cells grow, carry out M Checkpoint CYT normal functions, and OKI replicate their organelles. NES IS Telophase Anaphase Mitosis (M) MITOSIS Cell division phase Meta Synthesis (S) se pha DNA is replicated. Pro Gap 2 (G2) Additional growth Image Credits: (t) ©Dr. Torsten Wittmann/Photo Researchers, Inc. G2 Checkpoint occurs. Over time, scientists developed techniques and tools that allowed them to detect the copying of DNA (DNA synthesis). As a result, the description of the cell cycle was revised to include the DNA synthesis stage. At the time, they were still unable to Analyze Why is it observe activity between the stages of synthesis and mitosis, so the periods between important that DNA is these two stages were labeled gap 1 (G1) and gap 2 (G2). Eventually scientists learned copied before the cell divides? that cells in interphase undergo critical growth and preparation for cell division while they carry out normal cellular functions. Lesson 1 The Cell Cycle 223  Checkpoints in the cell cycle keep cells from moving to the next stage before certain conditions are met. During G1, the cell must pass a critical checkpoint before it can proceed to the synthesis stage. This ensures that DNA is relatively undamaged and can be properly replicated. This checkpoint also allows other cells to signal the cell when more cell division is needed. G2 has its own critical checkpoint. Everything must be in order—adequate cell size, DNA correctly replicated—before the cell goes through mitosis and division. Collaborate If a cell has damaged DNA, what do you think happens during the G2 checkpoint? Rates of Cell Division FIGURE 4: Different cells divide All cells in your body undergo cell division, but the rate at which they divide is linked to at different rates. your body’s need for that type of cell. In human cells, the S, G2, and M stages together usually take about 12 hours. The length of the G1 stage differs the most from cell type Approximate Cell Type to cell type. The rate of cell division is greater in embryos and children than it is in Life Span adults. Children have a shorter cell cycle, and many of their organs are still developing. Skin cell 2–3 weeks But the rate of cell division also varies within different tissues of the adult body. For example, the internal lining of the digestive tract receives a lot of wear and tear. The Red blood cell 4 months cells of the lining also encounter toxins that enter the body through the digestive tract. As a result, cells that line the stomach and intestine are replaced every few days. In Liver cell 10–18 months contrast, cells that make up the rest of the intestine (mainly smooth muscle) and many of the internal organs, such as lungs, kidneys, and liver, divide only occasionally, in Intestine– response to cell injury or death. 4–5 days internal lining Intestine– Analyze Why does a skin cell need to divide more frequently than a liver cell? muscle and 16 years other tissues Source: Spalding et al., Cell 122:1 G Zero (G0 ) Stage Not all cells need to divide regularly. Cells that divide rarely are thought to enter a gap phase called G0. These cells continue to carry out everyday functions, but they do not undergo any of the processes necessary to prepare for division. Some cells, such as neurons, may remain in G0 permanently. Other cells enter this stage temporarily until there is a need for them to divide. One such cell is a lymphocyte, which is a type of white blood cell that helps fight infections. Lymphocytes can remain dormant for years until they recognize an invading organism. Once the invading organism binds to a lymphocyte receptor, the lymphocyte goes through a series of rapid cell divisions to help fight infection. Explain Make a claim for how the cell cycle relates to the growth and maintenance of organisms. Discuss the stages of the cell cycle, mechanisms that regulate it, and how this cycle is related to the growth and maintenance of organisms. 224 Unit 5 Cells: Stability and Change EXPLORATION 2 Factors Affecting Cell Growth Many factors influence cell growth and division, including cell size. A typical animal cell only grows to a size of 10–20 micrometers. Cell size is often expressed as a comparison of two quantities: surface area and volume. A cell’s surface area-to-volume ratio is the relationship between the surface area of a cell’s membrane and the inner volume of a cell. Problem Solving Calculating Cell Size A ratio is a comparison of two numbers. For example, suppose there are 25 students in a class—10 boys and 15 girls. The ratio of boys to girls is 10 to 15. We can express this ratio in one of three ways: 10 __ 10 to 15 10:15 15 A ratio can be reduced, just like any other fraction. Ratios are reduced by determining the lowest common denominator. In the example above, the greatest common factor is 5. 10 __ 15 = __23 = 2:3 SAMPLE PROBLEM Study this sample problem for Cell A. FIGURE 5: Cells are measured by their surface area and volume. 1 2 3 Calculate the surface area-to-volume ratio for Cell A. 1. Surface area = length × width × number of sides = 1 × 1 × 6 = 6. 2. Volume = length × width × height = 1 × 1 × 1 = 1. 3. Surface area-to-volume ratio = 6:1. SOLVE Calculate the surface area-to-volume ratio for Cell B and Cell C. 1. Calculate the surface area of Cell B and Cell C. 2. Calculate the volume of Cell B and Cell C. 3. Calculate the surface area-to-volume ratio for Cell B and Cell C. Explain Describe the pattern you observe in the surface area-to-volume ratios as the cell gets larger. Lesson 1 The Cell Cycle 225  Cell Size Explore Online Recall that oxygen, nutrients, and wastes move across the cell membrane, or the Hands-On Lab surface of the cell. Some diffuse passively across the membrane, while others are transported actively via specialized proteins. No matter how materials move across the Modeling Cell Surface Area- membrane, they must be transported in adequate amounts and with adequate speed to-Volume Ratio Use model to maintain homeostasis. If there is not enough surface area for materials to cross cells to investigate how a cell’s into and out of the cell, the cell may not be able to absorb materials or expel wastes size affects its ability to transport effectively. To maintain a suitable cell size, growth and division must be coordinated. materials across the membrane and maintain homeostasis. Explain Make a claim for why cells must divide, rather than grow larger. Explain how surface area and volume, as well as transport across the cell membrane, are related to cell size and homeostasis. Regulating Cell Division Like other cellular processes, the cell cycle must be regulated. The cell cycle is regulated by both internal and external factors that work together to control when and how often a cell divides. Internal factors come from inside the cell and include several types of molecules found in the cytoplasm. External factors come from outside the cell, either from nearby cells or from another part of the organism’s body. An external factor that regulates the cell cycle can be either a physical signal or a chemical signal. One example of a physical signal—cell-to-cell contact—can be observed in a single layer culture of mammalian cells. Individual cells will divide in these cultures until they touch other cells. At this point, they stop dividing. Scientists are not yet sure what causes this to happen. One hypothesis is that receptors on the surfaces of neighboring cells bind to each other, causing the cell’s cytoskeletons to form structures that can block growth signals. Many cells also release chemical signals that can stimulate the growth of other cells. For example, growth factors are a broad group of proteins that stimulate cell division. When external factors bind to their receptors on a cell’s surface, they can trigger internal factors that affect the cell cycle. Two well-studied kinds of internal factors are kinases and cyclins. A kinase is an enzyme that, when activated, transfers a phosphate group from ATP to a specific target molecule. This action typically increases the energy of the target molecule, changes its shape, or both. Your cells have many types of kinases, and they are almost always present in the cell. Those kinases that help control the cell cycle are activated by cyclins. Cyclins are a group of proteins that are rapidly made and destroyed at certain points in the cell cycle. These two factors help a cell advance to different stages of the cell cycle when they bind to each other. This cyclin- kinase interaction plays an important role in cell cycle checkpoints, ensuring that cells start and stop dividing at appropriate times. Model Create a graphic organizer to describe the different factors that influence cell division. Include information related to the cell cycle, rates of cell division, cell size, and internal and external factors. 226 Unit 5 Cells: Stability and Change  Apoptosis FIGURE 6: In early stages of Some cells are programmed to die at a predetermined time in their life cycle or after development, human embryos a certain number of cell divisions. Programmed cell death is known as apoptosis, and have webbing between their it occurs when internal or external signals activate genes that help produce self- fingers and toes. destructive enzymes. Apoptosis may occur in cells with damaged DNA or in cells that are harmful to, or simply no longer needed by, the body. Normally immune system cells ignore other cells in the body, but some immune cells are specialized to recognize apoptotic cells. These cells very tidily gobble up the apoptotic cell and recycle its chemical parts for use in building other molecules. Apoptosis is also an important process in normal embryological development in animals, including humans. Collaborate Human embryos have webbed digits (fingers and toes) early in their development. The cells between the digits undergo apoptosis during later stages of development. With a partner, draw a model to show how apoptosis leads to changes in the structure of digits during later stages of development. Cancer Cancer is the common name for a class of diseases characterized by uncontrolled cell division. It arises when regulation of the cell cycle is disrupted. Because they do not respond to factors regulating growth, cancer cells divide more often than healthy cells. This results in the formation of disorganized clumps of cells called tumors. Some tumors can be removed successfully if they remain localized. However, some cells break away and are carried to other places in the body where they create new tumors in a process called metastasis. Cancer cells are hazardous because they do not perform normal cell functions. For example, in the lungs, cancer cells do not develop into healthy lung tissue and do not properly carry out gas exchange. Cells become cancerous when mutations occur in sections of DNA that code for regulatory factors. Some mutations are caused by radiation or chemical exposure while others are inherited. Substances that promote or produce cancerous growth are called carcinogens. These include tobacco smoke and certain air pollutants. Some cancers are inherited when the abnormal gene that causes the cancer is passed on from generation to generation. Image Credits: (t) ©Anatomical Travelogue/Photo Researchers, Inc.; (br) ©Girand/Science Source FIGURE 7: Normal animal cells respond to external factors and stop dividing when they touch each other. Cancer cells fail to respond to these factors. The cancerous growth shown here is a form of skin cancer called melanoma. formation of a tumor Explain Describe the cancer cells normal cells differences in the normal dividing cells and the cancerous cells shown in Figure 7. Analyze A sensory neuron serving the toe of a giraffe has an average length of nearly 4.6 meters. Use what you have learned about cell surface area and volume to explain how this cell can function properly. Lesson 1 The Cell Cycle 227  EXPLORATION 3 A Brief History of Cell Theory In order to learn more about cells and how they function, scientists first depended on simple instruments. Over time, advancements in science and technology resulted in microscopes that allowed us to not only see cells, but to observe processes occurring within them. Before the 1600s, people had no idea that cells existed, and so had other explanations for the basis of life. That all began to change after the English scientist Robert Hooke first viewed cork under a microscope. He observed that cork is made of tiny, hollow compartments. The compartments reminded Hooke of small rooms found in a monastery, so he gave them the same name: cells. However, it took nearly 200 years before scientists made the connection between biological cells and life. Image Credits: (tl) ©Photo by SSPL/Getty Images; (tr) ©Omikron/Photo Researchers, Inc.; (bl) ©Ted Kinsman/Science Source; (br) ©UNIVERSITY OF DUNDEE/DR PAUL ANDREWS/Science Source FIGURE 8: The cells viewed under Hooke’s microscope are from cork, dead plant tissue. The cell viewed under the modern microscope is in the process of dividing. Predict Advances in which fields most likely influenced changes in microscope technology? Cell Theory Almost all cells are too small to see without a microscope. Magnifying lenses had already been around for hundreds of years before Robert Hooke developed his microscope, but their quality was limited by the lens-grinding technology of the times. Therefore, even though Robert Hooke had designed a state-of-the-art microscope for his time, he would most likely not have seen anything inside the cork cells when he studied them, even if they had been alive. So how did scientists come to learn so much about cells, and how long did it take? 228 Unit 5 Cells: Stability and Change  FIGURE 9: A timeline of the study of cells 1595 Zacharias Janssen 1674 Antonie van 1855 Rudolf Virchow Dutch eyeglass maker Leeuwenhoek German scientist who who invented the Dutch tradesman who stated that all cells compound microscope developed a more powerful come from other cells. by placing two lenses in microscope. He observed He also described the a tube. numerous single-celled microscopic structure organisms swimming in a of cells such as nerve drop of pond water, which cells. he called “animalcules.” 1665 Robert Hooke 1838 Matthias Schleiden English scientist who used a three-lens German botanist who used compound compound microscope to examine microscopes to study plant tissue and thin slices of cork from an oak tree Image Credits: (tl) ©Photo by Universal History Archive/Getty Images; (b) ©Science & Society Picture Library/Getty Images; (tr) ©Universal History Archive/UIG via Getty Images proposed that plants are made of cells. (Figure 8). He called the tiny, hollow compartments he saw “cells.” 1839 Theodor Schwann German animal physiologist who noticed structural similarities between plant cells and the animal cells he had been studying. He concluded that all living things are made of cells and cell products. Analyze Using the development of cell theory as an example, make a claim for how science influences technology and technology influences science. The cell theory is one of the first unifying concepts developed in biology. Theodor Schwann, influenced by the work of Matthias Schleiden and other scientists, published the first statement of the cell theory. Schwann’s theory helped lay the groundwork for all biological research that followed. However, Schwann stated in his publication that cells form spontaneously by free-cell formation. As later scientists studied the process of cell division, they realized that this part of Schwann’s idea was wrong. The cell theory is an example of a theory that changed over time as new discoveries were made. The major principles of the cell theory are: All organisms are made of cells. All existing cells are produced by other living cells. The cell is the most basic unit of life. Explain Before the cell theory was developed, many people claimed that spontaneous generation was possible. In other words, that cells arose from nonliving matter, such as dust or rotting meat. Respond to that claim using the principles of cell theory. Lesson 1 The Cell Cycle 229  CONTINUE YOUR EXPLORATION Hands-On Lab Modeling Cell Surface Area-to-Volume Ratio Cells must transport materials across their membranes in order to maintain homeostasis. In this lab, you will use model cells to investigate the relationship between cell size and homeostasis. Your model cells will consist of agar cubes of different sizes. Agar is a gel-like material used as a growth medium for bacteria. The agar has been soaked in an indicator that turns pink when exposed to a basic solution. The indicator will allow you to measure how quickly materials diffuse across the model cell’s membrane. SAFETY Sodium hydroxide is corrosive. Wear goggles and gloves, and dispose of chemicals as instructed by your teacher. PROBLEM How does a cell’s size affect its ability to maintain homeostasis? PREDICT Make a prediction for how the diffusion of materials into the cell will change as the model cell gets larger. Explain your reasoning. MATERIALS PROCEDURE beaker, 250 mL 1. Make three model cells by using the knife to cut three cubes from the graduated cylinder, 100 mL phenolphthalein agar. Cell A should be 3 cm on each side, cell B should be 2 cm knife, plastic on each side, and cell C should be 1 cm on each side. Use the ruler to make exact measurements. metric ruler FIGURE 10: The cube-shaped cells model what happens to the volume as surface area paper towel increases. phenolphthalein agar sodium hydroxide solution (1.0 M HCl), 100 mL spoon, plastic 3 cm timer 2 cm 1 cm 3 cm 2 cm 1 cm 2 cm 1 cm 3 cm Cell A Cell B Cell C 230 Unit 5 Cells: Stability and Change 2. Calculate the total surface area of each cell. Record your data in a data table. surface area of a cube = length × width × number of sides 3. Calculate the volume of each cell. Record your data. volume of a cube = length × width × height 4. Calculate the surface area-to-volume ratio for each cell. For example, if the surface 2 3 area was 27 cm and the volume was 9 cm , the surface area-to-volume ratio would be 3:1. Record your data. 5. Put the model cells in the beaker. Carefully cover them with sodium hydroxide solution, which turns the agar pink. 6. Soak the cells in solution for four minutes. Use the spoon to turn the cells repeatedly throughout that time. 7. Remove the cells from solution and gently dry them on the paper towel. 8. Use the knife to cut each cube in half. Measure the distance (in cm) from the edge of the cell to the inner edge of the pink line. This shows how far the sodium hydroxide diffused. Record your data. ANALYZE 1. How does the surface area-to-volume ratio change as cell size increases? 2. Identify which cell turned pink in the greatest proportion, and explain how this relates to cell size and diffusion. EXPLAIN Write an explanation that addresses each of the points below. Claim How is a cell’s size related to its ability to maintain homeostasis by transporting materials across the membrane? Was your prediction correct? Evidence What evidence from your data supports your claim? Reasoning Explain how the evidence you cited supports your claim. REFINE Explain whether the model used in this investigation provided an adequate level of accuracy and how you would change the model for future investigations. Precision and accuracy Did the model provide the level of precision needed to make a valid conclusion? Propose changes What changes would you make to this model if you were to carry out this investigation again? Why would you make these changes? Go online to choose one of MEASURING CELL DIVISION CELL BIOLOGIST: CANCER RESEARCH these other paths. Lesson 1 The Cell Cycle 231  EVALUATE Lesson Self-Check CAN YOU EXPLAIN IT? Explore Online FIGURE 11: All plants and animals begin with a single cell. One cell divides into two, each of which will then divide. This pattern continues until an organism is formed. Cells have a life cycle made up of periods of rest, growth, and division. When a multicellular organism develops, a single cell divides over and over to produce the trillions of cells that make up the organism. Throughout the organism’s lifetime, internal and external signals regulate cell growth and cell division. These factors include physical and chemical signals, as well as limits on cell size. Explain Refer to the notes in your evidence notebook to construct an explanation for why cells divide instead of simply growing larger. In your explanation, address the following questions. 1. How are the cell cycle and cell division related to the growth, development, and maintenance of the organism? 2. How do different factors influence cell growth and cell division? 3. How has technology influenced our understanding of cells and cell division? Image Credits: (l) (cl) (cr) (r) ©Cytographics Pty Ltd./Image Bank Film/Getty Images 232 Unit 5 Cells: Stability and Change  CHECKPOINTS Check Your Understanding 6. Imagine a cell has six sides, each measuring 4 micrometers (μm) in length. Use this information to 1. Which of these is not a principle of the cell theory? answer the following questions. a. The cell is the basic unit of life. a. What is the surface area of the cell? b. All living things are made of cells. b. What is the volume of the cell? c. All organisms are made up of many cells. c. What is the surface-area-to-volume ratio for the cell? d. All cells come from other cells. d. If this cell grew larger in size, how would the transport of materials across the cell membrane be affected? 2. Which term describes the resting phase of the cell cycle? How does this relate to the cell’s ability to maintain a. mitosis homeostasis? b. interphase c. prophase 7. Complete this statement using these terms: d. telophase growth factors, cyclins, volume, surface area Different factors regulate cell growth and division. Cells 3. Which of these best explains how advancements in are limited in size because they need a large _____ as technology influenced the development of the cell compared to their _____. This ensures that materials can theory? move into and out of the cell at adequate rates. The cell a. Communication between scientists improved. cycle is also regulated by external factors such as _____ b. Microscopes enabled scientists to see cells. and internal factors such as _____. These factors work together to make sure the cell enters the appropriate c. Increased knowledge allowed scientists to make phase of the cell cycle at the correct time. predictions. d. Printing increased the number of books about the cell. 8. Use an example to explain how apoptosis contributes to the growth and maintenance of an organism. 4. In which of these situations would cells most likely receive signals instructing them to enter the M phase of the cell cycle? Select all correct answers. a. A tissue needs repairing. MAKE YOUR OWN STUDY GUIDE b. Cells need to grow larger. c. More cells are needed to defend the body. In your Evidence Notebook, design a study guide that d. Cells need to decrease in number during supports the main idea from this lesson: development. The cell cycle is a sequence of events in which cells grow and 5. Place these events in the correct order to illustrate the divide. Internal and external factors regulate the cell cycle to sequence of events in the cell cycle. ensure that cells grow and divide at appropriate times. a. Mitosis occurs, and one cell divides into two. Remember to include the following information in your b. DNA is replicated to make two copies. study guide: c. Organelles are copied, and the cell grows. Use examples that model main ideas. d. Additional growth occurs before the cell divides. Record explanations for the phenomena you investigated. Use evidence to support your explanations. Your support can include drawings, data, graphs, laboratory conclusions, and other evidence recorded throughout the lesson. Consider how models help scientists learn more about cells, the cell cycle, and how cells maintain homeostasis. Lesson 1 The Cell Cycle 233