Cell Growth and Division PDF

11 Cell Growth CHAPTER and Division 11.1 11.2 11.3 11.4 Cell Growth, The Process of Regulating the Cell...

11 Cell Growth CHAPTER and Division 11.1 11.2 11.3 11.4 Cell Growth, The Process of Regulating the Cell Division, and Cell Division Cell Cycle Differentiation Go Online to Reproduction access your ­digital course.  VIDEO  AUDIO  INTERACTIVITY  eTEXT  ANIMATION  VIRTUAL LAB  ASSESSMENT HS-ETS1-1, HS-LS1-4 336 Chapter 11 Cell Growth and Division CASE STUDY Will stem cells change the future of healing? More than 50 years ago, a young British scientist named John Gurdon carried out a revolutionary experiment. He transferred the nucleus from an adult frog cell into the cytoplasm of a frog egg cell. The result was that the cytoplasm “reprogrammed” the nucleus. The transformed cell developed first into an embryo and then into a fully functional tadpole. Most cells in an adult organism are special- Experts agree on the many potential ben- ized to carry out specific tasks, such as carry- efits of stem cell therapies. However, there ing oxygen, producing digestive enzymes, or also are dangers. Injected stem cells may not fighting disease. Gurdon’s work showed that develop into the cell types they are intended even adult cells could be changed back into to replace. They also might damage neigh- the unspecialized embryonic “stem cells” that boring tissues or produce tumors. In many have the potential to grow into nearly any cancerous tumors, the cells are undifferenti- cell type. ated and resemble stem cells in some ways. Today, many physicians are beginning to The Food and Drug Administration (FDA) and apply the biological principle that Gurdon other government agencies are considering demonstrated in order to replace cells dam- whether stem cell clinics should go through aged by injury or disease. In so doing, they an approval and regulatory process, such as have begun to establish a new field called that required for medicines and drugs. regenerative medicine. They hope that trans- Physicians, researchers, and patients plants of stem cells taken from embryos, from are faced with many questions about stem adult tissues like bone marrow, or from repro- cells. Are the benefits of stem cell therapies grammed adult cells will make it possible to worth the costs or the risks? How closely treat patients with failing eyesight, arthritic should the therapies be regulated? Who joints, or diseased livers. In the future, stem should make the important decisions? cells could also be used to treat heart attacks Throughout this chapter, look for and strokes. If stem cells could repair an connections to the CASE STUDY to help injured spinal cord, a paralyzed person might you answer these questions. walk again. Cell division in an animal cell. (False-color LM: 9000×) Unit 3 Cells 337  Cell Growth, Division, 11.1 LESSON and Reproduction KEY QUESTIONS What are some of the difficulties a cell faces as it increases in size? How do asexual and sexual reproduction compare? Eaglets will grow into adult eagles. HS-LS1-4: Use a model to illustrate the role of cellular division (mitosis) and differentiation in producing and maintaining complex organisms. When a living thing grows, what happens to its cells? Does an VOCABULARY organism get larger because each cell increases in size or because it cell division produces more cells? In most cases, living things grow by producing asexual reproduction sexual reproduction more cells. What is there about growth that requires cells to divide and produce more of themselves? READING TOOL Limits to Cell Size As you read, identify the Each of us begins life as a single cell. By the time we are adults, similarities and differences however, that single cell has grown and divided so many times that between sexual and asexual reproduction. the average human body contains nearly 40 trillion cells. To impress Include the advantages and your math teacher, you might want to calculate how many rounds disadvantages of each in of cell division would be required to go from one cell to 40 trillion the Venn diagram in your (40 × 1012). The answer might surprise you! Biology Foundations Cells can grow by increasing in size, but eventually, most cells Workbook. divide after growing to a certain point. There are two main reasons that cells divide rather than continuing to grow. The larger a cell becomes, the less efficient it is in moving nutrients and waste materials across its cell membrane. In addition, as a cell grows, it places increasing demands on its own DNA. A Problem of Size All cells are connected to the outside world through their cell membranes. To stay alive, a cell must allow food, oxygen, and water to enter through its cell membrane. Waste prod- ucts have to leave the cell in the same way. The rate at which this exchange takes place depends on the surface area of the cell, which is the total area of its cell membrane. The rate at which food and oxygen are used up and wastes are produced depends on a cell’s volume. As a cell gets larger, these rates increase, as does its surface area. However, volume and surface area do not increase at the same rate, and this is a key reason that cells must eventually divide rather than continue to grow without limit. 338 Chapter 11 Cell Growth and Division  Ratio of Surface Area to Volume in Cells 1 cm 2 cm 3 cm 1 cm 1 cm 2 cm 2 cm 3 cm 3 cm Surface Area 1 cm 3 1 cm 3 6 5 6 cm² 2 cm 3 2 cm 3 6 5 24 cm² 3 cm 3 3 cm 3 6 5 54 cm² (length 3 width) 3 6 sides Volume 1 cm 3 1 cm 3 1 cm 5 1 cm³ 2 cm 3 2 cm 3 2 cm 5 8 cm³ 3 cm 3 3 cm 3 3 cm 5 27 cm³ (length 3 width 3 height) Ratio of Surface Area to 6/156:1 24 / 8 5 3 : 1 54 / 27 5 2 : 1 Volume Figure 11-1 Ratio of Surface Area to Volume Imagine a cell that is shaped like a cube. The formula for area (l × w) is used to calculate the Ratio of Surface Area surface area. The formula for volume (l × w × h) is used to calculate to Volume the amount of space inside. By using a ratio of surface area to As the length of the sides of volume, you can see how the size of the cell’s surface area grows the cube increases, the cube’s compared to its volume. volume increases more than its Notice that for a cell with sides that measure 1 cm in length, the surface area. ratio of surface area to volume is 6/1 or 6 : 1. Increase the length of the cell’s sides to 3 cm, and the ratio becomes 54/27 or 2 : 1. As you can see from the calculations shown in Figure 11-1, surface area  INTERACTIVITY does not increase as fast as volume does. For a growing cell, this Investigate how and why decrease in the relative amount of cell membrane available cre- cells are limited in how large ates serious problems. The largest cells, such as the one shown in they can grow. Figure 11-2, use unusual shapes or structures to maintain the ratio.  READING CHECK Infer If a cell keeps growing, why must it eventually divide? Figure 11-2 A Long Cell Most single-celled organisms are too small to see without a microscope. However, the cell that makes up Caulerpa taxifolia, a type of algae, can grow up to 30 cm (12 in.). The cells of Caulerpa taxifolia are the largest known living cells of any organism! 11.1 Cell Growth, Division, and Reproduction 339  RY RA LIB Visual Analogy Traffic Problems Compare a growing cell to a small town with a two-lane main street running through it, such as the one shown Figure 11-3 in Figure 11-3. As the town grows, more and more traffic begins to Growing Pains clog the main street. Moving materials in and out of town becomes increasingly difficult. Businesses cannot get the goods they need and Lots of growth can mean lots trash piles up because garbage trucks are stuck in traffic jams. In the of trouble—both in a town and same way, if a cell were to get too large, it would be more difficult in a cell.  Use Analogies How could cell growth create to get sufficient amounts of oxygen and nutrients in and waste a problem that is similar to a products out. traffic jam? Information Overload Many cells face another problem as they grow. Living cells store critical information in a molecule known as READING TOOL DNA. As a cell grows, that information is used to build the molecules needed for cell growth. But as a cell increases in size, its “library” Relate cause and effect of information in DNA remains the same. If a cell were to grow too to explain why cell size is large, an “information crisis” might occur. A growing town with an limited. overused library that no longer serves its needs might decide to build a new library. In the case of a cell, it might be time to make a duplicate copy of that DNA and divide it between two new cells. Cell Division The process by which a cell divides into two new daughter cells is called cell division. Before cell division can occur, its DNA must be copied, or replicated. DNA replication solves the problem of information overload because each daughter cell gets one complete copy of genetic information. Cell division also solves the problem of increasing size by reducing cell volume. This results in an increase in the ratio of surface area to volume for each daughter cell, allowing for a more efficient exchange of materials between the cell and its environment.  READING CHECK Summarize What potential problems of a cell are solved by cell division? 340 Chapter 11 Cell Growth and Division Cell Division and Reproduction  INTERACTIVITY All living things must be able to reproduce by forming new individu- als. For an organism composed of just one cell, cell division itself can Explore the reproductive strategies of algae. serve as a perfectly good form of reproduction. You don’t have to meet someone else, conduct a courtship, or deal with rivals. All you have to do is to divide, and presto—there are two of you! Asexual Reproduction For many single-celled organisms, such as the bacterium in Figure 11-4, cell division is their only form of reproduction. The process can be relatively simple, efficient, and effective, enabling populations to increase in number very quickly. BUILD VOCABULARY In most cases, the two cells produced by cell division are genetically Prefixes The prefix a- in identical to the cell that produced them. This kind of reproduction asexual means “without.” is called asexual reproduction. The production of genetically Asexual reproduction is repro- identical offspring from a single parent is known as asexual duction without the fusion of reproduction. reproductive cells. Asexual reproduction also occurs in many multicellular organisms— even in such organisms as plants that can reproduce sexually. The small bud growing off the hydra will eventually break off and become an independent organism, an example of asexual reproduction in an animal. Each of the small shoots or plantlets on the tip of the kalan- choe leaf may also grow into a new plant. Sexual Reproduction Unlike asexual reproduction, in which cells separate to form a new individual, sexual reproduction involves the fusion of two reproductive cells formed by each of two parents. Offspring produced by sexual reproduction inherit some of their genetic information from each parent. Most animals and Figure 11-4 plants reproduce sexually, as do many single-celled organisms. You will learn more about the special form of cell division that produces Asexual Reproduction these reproductive cells in Chapter 12. Cell division leads to reproduc- tion in single-celled organisms and some multicellular organisms. Bacterium Hydra Kalanchoe Prokaryotes undergo a form Hydras reproduce by budding. An Plants reproduce asexually of asexual reproduction known offspring starts off as a lump on its through vegetative propa- as binary fission, in which the parent. This bud grows, develops ten- gation. The plantlets at the cell splits in two after the chro- tacles, and eventually separates from edge of this kalanchoe leaf mosome replicates. its parent. (LM: 16×) will eventually drop off and (False-color TEM: 34,000×) grow on their own. 11.1 Cell Growth, Division, and Reproduction 341  Comparing Asexual and Sexual Reproduction Each  INTERACTIVITY type of reproduction has its own advantages and disadvantages Figure 11-5 in terms of survival strategies. For many single-celled organisms, asexual reproduction provides clear advantages. When conditions Asexual and Sexual are right, these organisms can reproduce quickly, enabling them Reproduction to compete successfully with other organisms. However, a lack The runners, or plantlets, growing of genetic diversity can become a disadvantage when conditions off this strawberry plant are the change in ways that do not fit the characteristics of an organism. result of asexual reproduction. Sexual reproduction involves finding a mate and then allowing The strawberries, which form for the growth and development of offspring. This may require more from the flowers, are the result of sexual reproduction. time than asexual reproduction. However, this can be an advantage for species that live in environments where seasonal changes affect weather conditions and food availability. Sexual reproduction pro- duces genetic diversity. If an environment changes, genetic diversity in a species may help to ensure that the population contains the right combination of characteristics needed to survive. Some organisms, such as the strawberry plant shown in Figure 11-5, reproduce both sexually and asexually. The different advantages of each type of reproduction may help to explain why the living world includes organisms that reproduce sexually, those that reproduce asexually, and many organisms that do both. HS-LS1-4  LESSON 11.1 Review KEY QUESTIONS 4. Use Models Compare a cell that has grown too large to be efficient with a wireless network that 1. What factors limit the size of a cell? has too many users. Explain how both have the 2. What are the advantages and disadvantages of same two problems noted for the city shown in both asexual and sexual reproduction? Figure 11.3. Illustrate how “division” helps in both cases. CRITICAL THINKING 5. Synthesize Information Aphids are a type of 3. Calculate The volume of a sphere increases with insect. They reproduce asexually in the spring the cube of its radius. If the radius of a sphere and summer. They reproduce sexually in the fall. increases from 2 cm to 6 cm, by what factor does How might this pattern improve the species’ its volume increase? chances of survival? 342 Chapter 11 Cell Growth and Division  11.2 LESSON The Process of Cell Division KEY QUESTIONS What is the role of chromosomes in cell division? What are the main events of the cell cycle? Micrograph of onion cells undergoing mitosis (LM 750×) What happens during the phases of mitosis? How do daughter cells split apart after mitosis? What role does cell division play in your life? You know that small HS-LS1-4: Use a model to illustrate children grow bigger every year. This growth depends on the pro- the role of cellular division (mitosis) and differentiation in producing and duction of new cells through cell division. But what happens when maintaining complex organisms. you are finished growing? Does cell division simply stop? Now, think about what must be happening when your body heals a cut or a VOCABULARY broken bone. Where do the cells come from that heal a cut in your chromosome chromatin skin or seal together the fractured ends of a bone? Next, think about cell cycle interphase the daily wear and tear on your skin and on the cells of your diges- mitosis cytokinesis tive system. How about red blood cells that live for only about four prophase chromatid months in your circulatory system? Where do the cells that replace centromere centriole metaphase anaphase them come from? The more you think about it, the more you will telophase realize that cell division doesn’t stop when we stop growing. In fact, cell division takes place all the time, keeping us healthy by replacing READING TOOL worn cells and regenerating the tissue we lose to injury or disease. In the cell cycle diagram Chromosomes in your Biology Foundations Workbook, What would happen if a cell were simply to split in two, without any each section represents the advance preparation? The results might be disastrous, especially if time the cell spends in each some of the cell’s essential genetic information wound up in one of stage. Write the stages the daughter cells but not in the other. In order to make sure this of the cell cycle into the doesn’t happen, cells first make a complete copy of their genetic diagram. information before cell division begins. Even a small cell like the bacterium E. coli has a tremendous amount of genetic information in the form of DNA. In fact, the total length of this bacterium’s DNA molecule is 1.6 mm, roughly 1000 times longer than the cell itself. In terms of scale, imagine a 300-meter rope stuffed into a school backpack. Cells can handle such large molecules only by careful packaging. Genetic information is bundled into packages of DNA known as chromosomes. 11.2 The Process of Cell Division 343  Prokaryotic Chromosomes Prokaryotic cells lack membrane- bound nuclei and many of the organelles found in eukaryotes. The DNA molecules of prokaryotic cells are found in the cytoplasm, along with most of the other contents of the cell. Most prokaryotes contain a single circular DNA chromosome, as you can see in Figure 11-6. That Chromosome circular DNA contains all, or nearly all, of the cell’s genetic information. Figure 11-6 Prokaryotic Eukaryotic Chromosomes Eukaryotic cells generally have Chromosome much more DNA than prokaryotes have, and therefore they contain multiple chromosomes. Fruit flies, for example, have 8 chromosomes In most prokaryotes, a single per cell, carrot cells have 18, and human cells have 46. The chro- chromosome holds most of the mosomes in eukaryotic cells contain DNA tightly bound to proteins organism’s DNA. known as histones. This complex of DNA and protein is referred to as ­chromatin. DNA tightly coils around the histones, and together, the DNA and histone molecules form beadlike structures called nucleosomes. Nucleosomes pack together to form thick fibers, which condense even further during cell division. The X-like chromosome shape you often see drawn in textbooks is actually a duplicated chro- mosome with supercoiled chromatin, as shown in Figure 11-7. Why do cells go to such lengths to package their DNA into chro- Figure 11-7 mosomes? One of the principal reasons is to ensure equal division of Eukaryotic DNA when a cell divides. Chromosomes are precisely separated Chromosome into two daughter cells during cell division. As a eukaryotic cell prepares  READING CHECK Compare How are the chromosomes in for division, each chromosome eukaryotic cells different from those in prokaryotic cells? coils more and more tightly to form a compact structure. Duplicated chromosome Sister chromatids DNA double helix Centromere Coils Nucleosome Supercoils Histone proteins 344 Chapter 11 Cell Growth and Division The Cell Cycle READING TOOL Cells go through a series of events known as the cell cycle as they As you read, create a grow and divide. During the cell cycle, a cell grows, prepares timeline of the sequence for division, and then divides to form two daughter cells. Each of events of the cell cycle. daughter cell then moves into a new cell cycle of activity, growth, Include details for each and division. event. The Prokaryotic Cell Cycle The prokaryotic cell cycle is a regular pattern of growth, DNA replication, and cell division that can take place very rapidly under ideal conditions. Researchers are just beginning to understand how the cycle works in prokaryotes, and relatively little is known about its details. It is known that most pro- karyotic cells begin to replicate, or copy, their DNA chromosomes once they have grown to a certain size. When DNA replication is complete, or nearly complete, the cell begins to divide. The process of cell division in prokaryotes is a form of asexual reproduction known as binary fission. Once the chromosome has been replicated, the two DNA molecules attach to different regions of the cell membrane. A network of fibers forms between them, stretching from one side of the cell to the other. These fibers con-  INTERACTIVITY strict and the cell is pinched inward, dividing the cytoplasm and Learn about the various chromosomes between two newly formed cells. Binary fission results stages of the cell cycle. in the production of two genetically identical cells. The Eukaryotic Cell Cycle In contrast to prokaryotes, much Figure 11-8 more is known about the eukaryotic cell cycle. As you can see in The Cell Cycle Figure 11-8, the eukaryotic cell cycle consists of four stages: G1, S, G2, and M. The length of each stage of the cell cycle—and the During the cell cycle, a cell grows, length of the entire cell cycle—varies depending on the type of cell. prepares for division, and divides to form two daughter cells. The At one time, biologists described the life of a cell as one cell divi- cell cycle includes four phases— sion after another separated by an “in-between” period of growth G1, S, G2, and M. called interphase. We now appreciate that a great deal happens in the time between cell divisions, so interphase is now divided into three phases: G1, S, and G2. on si G1: Cell Growth Cells do most of their growing dur- e ivi Cy G1 phase as ll d ph ing the G1 phase. In this phase, cells increase in size and to (Cell growth) Ce ki M Mi n synthesize new proteins and organelles. The G in G1 tos esi is s and G2 stands for “gap,” but the G1 and G2 phases are actually periods of intense growth and activity. G2 phase S phase S: DNA Replication The G1 phase is followed by (Preparation (DNA replication) the S phase. The S stands for “synthesis.” During the for mitosis) S phase, new DNA is synthesized as the chromosomes are replicated. By the end of the S phase, the cell contains twice as much DNA as it did at the beginning of the phase. Inte rp h as e 11.2 The Process of Cell Division 345  G2: Preparing for Cell Division When DNA replication is com-  INTERACTIVITY pleted, the cell enters the G2 phase. G2 is usually the shortest of the Observe how cells duplicate three phases. During the G2 phase, many of the organelles and mol- themselves through mitosis. ecules required for cell division are produced. When the events of the G2 phase are completed, the cell is ready to enter the M phase and begin the process of cell division. M Phase: Cell Division The M phase of the cell cycle, which follows interphase, produces two daughter cells. The M phase takes its name from the process of mitosis. During the normal cell cycle, interphase can be quite long. In contrast, the process of cell division usually takes place quickly. In eukaryotes, cell division occurs in two main stages. The first stage of the process, the division of the cell nucleus, is called mitosis. BUILD VOCABULARY The second stage, the division of the cytoplasm, is called cytokinesis. Root Words The word root In many cells, the two stages may overlap, so that cytokinesis begins kinesis is a noun that means while mitosis is still taking place. “movement” or “motion.”  READING CHECK Define What events occur during interphase? Mitosis Biologists divide the events of mitosis into four phases: prophase, metaphase, anaphase, and telophase. Depending on the type of cell, mitosis may last anywhere from a few minutes to several days. Prophase The first phase of mitosis, prophase, is usually the l­ongest and may take up to half of the total time required to com- plete mitosis. During prophase, the genetic material inside the nucleus condenses and the duplicated chromosomes become ­visible. Outside the nucleus, a spindle starts to form. During prophase, each duplicated chromosome condenses to appear as two thick strands known Spindle Centrioles as sister chromatids (kroh muh tids), attached at forming a point called the centromere. When the process of mitosis is complete, the sister chromatids will have separated, one to each of the daughter cells. Also during prophase, the cell starts to build a spindle, a fanlike system of microtubules Prophase that will help to separate the duplicated chromo- somes. Spindle fibers extend from a region called the centrosome, where tiny paired structures called centrioles are located. Early in prophase, the centrioles move toward opposite ends, or Nuclear poles, of the cell. Plant cells lack centrioles, and envelope Centromere organize spindles directly from their centrosome Chromosomes regions. 346 Chapter 11 Cell Growth and Division  HS-LS1-4 Quick Lab  Open-Ended Inquiry Make a Model of Mitosis ANALYZE AND CONCLUDE 1. With your partner, discuss a plan for modeling the 1. Use Models How many chromosomes did stages of mitosis. Choose available materials, such you include in your model? as yarn, chenille stems, or candy pieces, to represent 2. Evaluate Models How accurately does your the chromosomes. Then describe how to use the model show an original cell and the two materials to demonstrate each stage. daughter cells that are produced after mito- 2. Carry out your plan. Make sketches or take photo- sis? Compare your model with other represen- graphs of each stage. tations of mitosis, such as those shown in the lesson. How could you improve your model? 3. Organize the sketches or photos to show all the stages of mitosis in the proper order. Add labels or 3. Use Models Use your model to explain the captions to create a flip book, slide show, or video function of mitosis to your classmates. presentation. Metaphase The second phase of mitosis, metaphase, is gen- erally the shortest. During metaphase, the centromeres of the duplicated chromosomes line up across the center of the cell. Spindle fibers connect the centromere of each chromosome to the two poles of the spindle. The cell is now ready to separate those sister chromatids. Anaphase The third phase of mitosis, anaphase, begins when sister chromatids suddenly separate and begin to move apart. Spindle Once anaphase begins, each sister chromatid is now considered Metaphase an individual chromosome. During anaphase, the chromo- somes separate and move along spindle fibers to opposite ends of the cell. Anaphase movement requires the rapid disas- sembly of microtubules as chromosomes move toward the poles of the mitotic spindle. Anaphase comes to an end when this movement stops and the chromosomes are completely separated into two groups. The microtubules that once made up the mitotic spindle have almost completely disassembled by the end of anaphase. Individual chromosomes Telophase Following anaphase is telophase, the final phase Anaphase of mitosis. During telophase, the chromosomes, which were distinct and condensed, begin to spread out into a tangle of chromatin. A nuclear envelope re-forms around each cluster of chromosomes, and gradually a nucleolus becomes visible in each daughter nucleus. Mitosis is complete, but the process of cell ­division has one more step.  READING CHECK Review What structures are responsible for the movement of chromosomes to the center of the cell in Nuclear metaphase and their separation in anaphase? envelopes re-forming Telophase 11.2 The Process of Cell Division 347  Cytokinesis As a result of mitosis, two nuclei—each with a duplicate set of ­chromosomes—are formed. All that remains to complete the M phase of the cycle is cytokinesis, the division of the cytoplasm to form two separate cells. Cytokinesis usually occurs at the same time as telophase. Cytokinesis completes the process of cell division by dividing one cell into two. The process of cytokinesis differs in animal and plant cells, as shown in Figure 11-9. Cytokinesis in Animal Cells During cytokinesis in most animal cells, the cell membrane is drawn inward until the cytoplasm is pinched into two nearly equal parts. Each part contains its own nucleus and cytoplasmic organelles. Cytokinesis in Plant Cells Cytokinesis in plant cells proceeds differently. The cell membrane is not flexible enough to draw inward because of the rigid cell wall that surrounds it. Instead, a structure known as the cell plate forms halfway between the divided nuclei. The cell plate gradually develops into cell membranes that separate the two daughter cells. A cell wall then forms in between the two new membranes, completing the process. The membrane A cell plate forms. draws inward Figure 11-9 Cytokinesis Animal Cell Plant Cell The division of the cytoplasm TEM 1200x LM 800x occurs differently in animal and plant cells. HS-LS1-4  LESSON 11.2 Review KEY QUESTIONS CRITICAL THINKING 1. What are chromosomes? How are they different 5. Construct an Explanation Explain how mitosis between prokaryotes and eukaryotes? maintains the chromosome number of the original cells when forming new cells. 2. What is the cell cycle? 6. Construct an Explanation Describe the role of 3. What happens during each of the four phases microtubules in mitosis. Why must microtubules of mitosis? Write one or two sentences for both assemble and disassemble for mitosis to each phase. occur properly? 4. What happens during cytokinesis? 348 Chapter 11 Cell Growth and Division  ANIMATION Visual Summary Figure 11-10 Mitosis The phases of mitosis shown here are typical of animal cells. These light micrographs are from a developing whitefish embryo (LM 415×). Interphase The cell grows and replicates its DNA and centrioles. Cytokinesis Prophase The cytoplasm pinches in The chromatin condenses half. Each daughter cell into chromosomes. The has an identical set of centrioles separate, and a duplicate chromosomes. spindle begins to form. The nuclear envelope breaks down. Metaphase Telophase The chromosomes line up The chromosomes gather at across the center of the cell. opposite ends of the cell and lose Each chromosome their distinct shapes. Two new is connected to nuclear envelopes will form. spindle fibers at its centromere. Anaphase The sister chromatids separate into individual chromosomes and are moved apart. 11.2 The Process of Cell Division 349  11.3 Regulating the Cell Cycle LESSON KEY QUESTIONS How is the cell cycle regulated? How do cancer cells differ from other cells? This cancer cell is dividing abnormally, with four spindle poles instead of the usual two. (LM 700×) VOCABULARY growth factor cyclin apoptosis How do cells know when it is time to divide? One striking fact about cancer cells in multicellular organisms is how carefully cell growth and cell tumor division are controlled. Think of what might happen, for example, if the cells in one of your internal organs were to suddenly start grow- READING TOOL ing while the other parts of the body did not. That’s why careful As you read, fill in the control of the cell cycle is essential for orderly growth and develop- graphic organizer in your ment. If something goes wrong with that control, serious diseases Biology Foundations such as cancer sometimes result. In the human body, for example, Workbook with the key most muscle cells and nerve cells do not divide at all once they have words from this lesson. developed. In contrast, blood-producing cells in the bone marrow, as well as cells of the skin and digestive tract, grow and divide regularly throughout life. In this way they produce new cells to replace those that wear out or break down. Controls on Cell Division When scientists grow cells in the laboratory, most cells will divide until they come into contact with one another. Once they do, they usually stop dividing and growing. What happens if those neighbor- ing cells are suddenly scraped off the culture dish? The remaining cells will begin dividing again until they once again make contact with other cells. This simple experiment shows that controls on cell growth and division can be turned on and off. Something similar happens inside the body. Look at Figure 11-11.  INTERACTIVITY When an injury such as a cut in the skin or a break in a bone occurs, Explore how cell growth is cells at the edges of the injury are stimulated to divide rapidly. New regulated. cells form, starting the process of healing. When the healing process nears completion, the rate of cell division slows, controls on growth are restored, and normal activities return. 350 Chapter 11 Cell Growth and Division Regulatory Proteins For many years, biologists searched for BUILD VOCABULARY a signal that might regulate the cell cycle—something that would Academic Words The verb “tell” the cell when it was time to divide, duplicate its chromosomes, regulate means “to control or or enter another phase of the cell cycle. They found out that there direct.” Therefore, a substance is not just one signal, but many. Scientists have identified dozens of that regulates the cell cycle proteins that help to regulate the cell cycle. The cell cycle is con- tells the cell whether to divide and when. trolled by regulatory proteins both inside and outside the cell. Internal Regulators One group of internal regulatory proteins responds to events inside the cell. These proteins act as checkpoints, allowing the cell cycle to proceed only when certain events have taken place. For example, one set of checkpoint proteins makes sure a cell does not enter mitosis until its chromosomes have replicated. Another checkpoint prevents a cell from entering anaphase until spindle fibers have attached to each of the chromosomes. External Regulators Proteins that respond to events outside the cell are called external regulatory proteins. External regulatory pro- teins direct cells to speed up or slow down the cell cycle. One impor- Up Close tant group of external regulatory proteins is the group made up of Figure 11-11 growth factors. Growth factors stimulate the growth and division of cells. These proteins are especially important during embryonic Cell Growth development and wound healing. Other external regulatory proteins and Healing on the surface of neighboring cells often have the opposite effect. When a person breaks a bone, These regulatory proteins cause cells to slow down or stop their cell cells at the edges of the injury cycles. This prevents excessive cell growth and keeps body tissues are stimulated to divide rap- from disrupting one another. idly. The new cells that form begin to heal the break. As  READING CHECK Summarize Why must multicellular the bone heals, the cells stop organisms tightly control the cell cycle? dividing and growing. New bone cells 11.3 Regulating the Cell Cycle 351  Cyclins Biologists had been searching for years for the signal that regulates the cell cycle because they realized that it could help them treat diseases. Learning that there is not just one signal but many has made that job more complicated. Scientists discovered the first regulatory protein in the early 1980s. When they injected this protein into a nondividing cell, a mitotic spindle would form. They named this protein cyclin because it seemed to regulate the cell cycle. Investigators have since discov- ered a family of proteins known as cyclins that regulate the timing of the cell cycle in eukaryotic cells. Cyclins rise and fall in a pattern, as Figure 11-12 shown in Figure 11-12. Cyclin Levels Cyclin Levels in Fertilized Clam Eggs Cyclin binds with an enzyme MPF Enzyme to produce mitosis-promoting Concentration factor (MPF). The levels of MPF n Cyclin rise and fall to control the cli Cy cell cycle. PF M Mitosis Factor P G1 S G2 M phase G1 S G2 M phase G1 S G2 M phase G1 S Cyclin Enzyme 60 70 80 90 100 110 120 130 140 Minutes After Fertilization P Apoptosis Just as new cells are produced every day in a multicel- lular organism, many other cells die. A cell may die by accident due to damage or injury, or a cell may actually be “programmed” to die. Apoptosis (ayp up toh sis) is a process of programmed cell death. Once apoptosis is triggered, a cell undergoes a series of controlled steps leading to its self-destruction. First, the cell and its chromatin READING TOOL shrink, and then parts of the cell’s membranes break off. Neighboring cells then quickly clean up the cell’s remains. Review the timeline you created for Lesson 2, which Apoptosis plays a key role in growth and development by shap- showed the events of the ing the structure of tissues and organs. When apoptosis does not cell cycle. Create a similar occur as it should in humans, a number of diseases can result. For timeline for apoptosis. example, the cell loss seen in AIDS and Parkinson’s disease can result if too much apoptosis occurs. Analyzing Data The Rise and Fall of Cyclins ANALYZE AND CONCLUDE Scientists measured cyclin levels in clam embry- 1. Analyze Graphs How long does cyclin production onic cells as the cells went through their first last during a typical cell cycle in embryonic clam cells? mitotic divisions after fertilization. The data are 2. Apply Scientific Reasoning During which part of the shown in the graph in Figure 11-12. cell cycle does cyclin production begin? How quickly Cyclins are continually produced and destroyed is cyclin destroyed? within cells. Cyclin production signals cells to 3. Form a Hypothesis Suppose that the regulators that enter mitosis, whereas cyclin destruction signals control cyclin production are no longer produced. cells to stop dividing and to enter interphase. Hypothesize two possible outcomes. 352 Chapter 11 Cell Growth and Division Cancer: Uncontrolled  ANIMATION Cell Growth Why is cell growth regulated so carefully? The princi- Figure 11-13 pal reason may be that the consequences of uncon- Growth of Cancer Cells trolled cell growth in a multicellular organism are very severe. Cancer, a disorder in which body cells lose Normal cells grow and divide in a carefully the ability to control growth, is one such example. controlled fashion. Cells that are cancerous lose this control and continue to grow and divide, Cancer cells do not respond to the signals producing tumors. that regulate the growth of most cells. As a result, the cell cycle is disrupted, and cells grow and divide uncontrollably. Cancer cells form a mass of cells called a tumor. However, not all tumors are can- cerous. Some are benign, or noncancerous. A benign tumor does not spread to surrounding healthy tissue or to other parts of the body. Cancerous tumors, such as the one shown in Figure 11-13, are malignant. Malignant tumors invade and destroy surrounding healthy tissue. As the cancer cells spread to surrounding healthy tissue, they absorb the nutrients needed by other cells, 1 A cell begins to divide abnormally. block nerve connections, and prevent the organs they invade from functioning properly. Soon, the delicate balances that exist in the body are disrupted, and life- threatening illness results. What Causes Cancer? Cancers are caused by defects in the genes that regulate cell growth and division. There are several sources of such defects, including smoking or chewing tobacco, radiation exposure, and even viral infection. All cancers, how- ever, have one thing in common: The control over the cell cycle has broken down. Some cancer cells will no 2 The cancer cells produce a tumor, which longer respond to external growth regulators, while begins to displace normal cells and tissues. others fail to produce the internal regulators that ensure orderly growth. An astonishing number of cancer cells have a defect in a gene for a checkpoint protein known as p53, which normally halts the cell cycle until all chro- mosomes have been properly replicated. Damaged or defective p53 genes cause cells to lose the infor- mation needed to respond to signals that normally control their growth.  READING CHECK Make Generalizations Why 3 Cancer cells are particularly dangerous because is the presence of cancer cells so harmful to the body? of their tendency to spread once they enter the bloodstream or lymph vessels. The cancer then moves into other parts of the body and forms secondary tumors, a process called metastasis. 11.3 Regulating the Cell Cycle 353  Treatments for Cancer When a cancerous tumor is local-  INTERACTIVITY ized, it can often be removed by surgery. Skin cancer, one of the Investigate how the cell most common forms of the disease, can usually be treated this way. cycle is regulated and what Melanomas, the most serious form of skin cancer, can be removed happens when things go surgically, but only if spotted very early. wrong. As shown in Figure 11-14, cancerous tumors tend to grow more rapidly than normal cells. For this reason, they need to copy their DNA relatively quickly. This makes them especially vulnerable to damage from high-energy radiation. As a result, many tumors can be Figure 11-14 effectively treated with carefully targeted beams of radiation. Cancer Research Medical researchers have worked for years to develop chemical Thanks to the work of researchers compounds that would kill cancer cells, or at least slow their growth. around the world, cancer can be The use of such compounds against cancer is known as chemo- treated more effectively than it therapy. Great advances in chemotherapy have taken place in recent was in the past. The false-colored years and have even made it possible to cure some forms of cancer. micrograph shows a small cancer- However, because most chemotherapy compounds target rapidly ous tumor (blue) in the air sacs of dividing cells, they also interfere with cell division in normal, healthy the lungs. cells. This produces serious side effects in many patients, which is why researchers are trying to find highly specific ways in which can- cer cells can be targeted for destruction while leaving healthy cells unaffected. 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 an even deeper understanding of the processes that control cell division. (SEM: 240×)  LESSON 11.3 Review KEY QUESTIONS CRITICAL THINKING 1. Name the two types of proteins that regulate the 3. Translate Scientific Information How did experi- cell cycle. How do these proteins work? mental results show the effect of cyclins on the cell cycle? 2. Why is cancer considered a disease of the cell cycle? 4. Construct an Explanation How might a drug that alters events in mitosis or the cell cycle be useful for treating cancer? 354 Chapter 11 Cell Growth and Division  11.4 LESSON Cell Differentiation KEY QUESTIONS How do cells become specialized for different functions? What are stem cells? What are some possible benefits and issues associated with Cell division in sea stem cell research? urchin eggs (lm: 42x). HS-LS1-4: Use a model to illustrate the role of cellular division (mitosis) and differentiation in producing and maintaining complex organisms. Each of us started life as just one cell. So, for that matter, did your pet dog, a sea urchin, and the petunia on a windowsill. Cell division VOCABULARY explains how one cell could produce millions or even billions of cells embryo in each of these organisms, but it leaves one critical question unan- differentiation swered: Why do some cells turn into muscle cells, some into nerve totipotent cells, and others into bone and skin cells? Plainly stated, how do cells blastocyst become so different from one another? pluripotent stem cell From One Cell to Many multipotent Animals and higher plants pass through a developmental stage READING TOOL called an embryo, from which the adult organism is gradually pro- duced. During the developmental process, cells become more and In the chart in your more different from one another and specialized for particular Biology Foundations functions. Figure 11-15 shows some of the specialized Workbook, fill in the details that support the main ideas cells found in the parts of a plant. from this lesson. Figure 11-15 Specialized Plant Cells The first cell of a plant forms inside a reproductive structure, such as a flower. After many cell divisions, the new plant develops the specialized tissues it needs to survive as a multicellular organism.  VIDEO Learn how meat can be grown in a lab using stem cells. Leaf tissue Root tissues Cross-section of (lm: 255x) (lm: 46x) a stem (lm: 27x) 11.4 Cell Differentiation 355  Defining Differentiation The process by which cells become specialized is known as differentiation (dif ur en shee ay shun). During the development of an organism, cells differentiate into many distinct cell types. A differentiated cell has become, quite literally, different from the embryonic cell that produced it and specialized to perform certain tasks, such as contraction, photosynthesis, or protection. Our bodies, and the bodies of all multicellular organisms, contain highly differentiated cells that carry out the jobs we need to perform to stay alive. Mapping Differentiation The process of differentiation deter- mines a cell’s ultimate identity, such as whether it will spend its life as a nerve cell or a muscle cell. In some organisms, a cell’s role is rigidly determined at a specific point in the course of development. In the  INTERACTIVITY microscopic worm Caenorhabditis elegans, for example, biologists Figure 11-16 have mapped the outcome of each and every cell division from fertil- ized egg to adult. Differentiation in C. elegans The process of cell differentiation in C. elegans begins with the very first division and continues throughout embryonic develop- A fertilized egg develops into ment. Figure 11-16 shows when some of the cells found in the adult an adult worm after many cell begin to differentiate during development. Every time a new worm divisions. Daughter cells from develops, the process is the same, resulting in exactly 959 cells with each cell division follow a precisely determined functions. specific path toward a role as a particular kind of cell. Cuticle By the 8th cell division, some of the cells that secrete the worm’s Nervous System By the 5th cell outer covering begin to differentiate. division, cells in the nervous system begin to differentiate. Pharynx After 9 to 11 cell divisions, cells in the feeding organ differentiate. Eggs Muscle Gonad Intestine Differentiation in Mammals Other organisms, including mammals, go through a more flexible process in which cell differen- tiation is controlled by a number of interacting factors in the embryo, many of which are still not well understood. What is known, however, is that adult cells generally do reach a point at which their differentia- tion is complete—when they can no longer turn into other types of cells.  READING CHECK Identify How does a single fertilized egg cell develop into so many different types of specialized cells? 356 Chapter 11 Cell Growth and Division  Stem Cells and Development BUILD VOCABULARY One of the most important questions in biology is how all of the Prefixes The prefix toti- in ­specialized, differentiated cell types in the body are formed from just totipotent means “entirely.” The prefix pluri- in pluripotent a single cell, the fertilized egg, called a zygote. Biologists say that means “several.” Totipotent the zygote is totipotent (toh tip uh tunt)—literally, able to do every- cells can develop into any type thing, to develop into any type of cell in the body (including the cells of cell, whereas pluripotent that make up the extra-embryonic membranes and placenta). Only cells can develop into many the fertilized egg and the cells produced by the first few cell divisions different types of cells. of embryonic development are truly totipotent. If there is a “secret” by which cells start the process of differentiation, these are the cells that know that secret. Human Development After about four days of development, a human embryo forms into a blastocyst, a hollow ball of cells with a clus-  INTERACTIVITY ter of cells inside known as the inner cell mass. Even at this early stage, Explore stem cells and the cells of the blastocyst have begun to specialize. The outer cells ­differentiated cells. form tissues that will attach the embryo to its mother, while the inner cell mass becomes the embryo itself. The cells of the inner cell mass are said to be pluripotent (plu ri poh tunt). Pluripotent cells can develop into any of the body’s cell types, although they generally cannot form the tissues surrounding the embryo. Stem Cells As the name implies, stem cells sit at the base of a branching “stem” of development from which different cell types form. Stem cells are the unspecialized cells from which ­differentiated cells develop. Stem cells are found in the early embryo, of course, but they are also found in many places in the adult body. Adult Stem Cells Cells in some tissues, like blood and skin, have a limited life span and must be constantly replaced. Pools of adult stem cells, found in various locations throughout the body, produce the new cells needed for these tissues. New blood cells differenti- ate from stem cells found in the bone marrow, and many skin stem cells are found in hair follicles. Small clusters of adult stem cells are even found in the brain, in the heart, and in skeletal muscle. These adult cells are referred to as multipotent (muhl tip uh tunt) stem cells, because the types of differentiated cells they can form are usually limited to replacing cells in the tissues where they are found.  Exploration Lab Open-ended Inquiry Regeneration in Planaria Problem Are planarian cells multipotent or totipotent? In this lab, you will design an experiment to determine whether planarian stem cells are multipotent or totipotent. Then you will share your results with the rest of the class. From these combined results, you will infer where multipotent or totipotent stem cells are found in a planarian’s body. You can find this lab in your digital course. 11.4 Cell Differentiation 357  Embryonic Stem Cells Pluripotent embryonic stem cells are more versatile than adult stem cells, since they are capable of ­producing every cell type in the body, as shown in Figure 11-17. In laboratory experiments, ­scientists have managed to coax embry- onic stem cells to differentiate into nerve cells, muscle cells, and even sperm and egg cells. In 1998, researchers at the University of Wisconsin found a way to grow human embryonic stem cells in culture, making it possible to explore the potential of these remark- able cells.  READING CHECK Compare and Contrast How are pluripo- tent and multipotent cells similar? How are they different? Figure 11-17 Embryonic Stem Cells After fertilization, the human embryo develops into a hollow ball of cells known as a blastocyst. The actual body of the embryo develops from the inner cell mass, a cluster of cells inside the blastocyst. Because of their ability to differentiate into each of the body’s many cell types, the cells are known as embryonic stem cells. Blastocyst Inner cell mass SEM 2610x Neuron Embryonic stem cells in culture SEM 261x SEM 1925x Fat cells Macrophage SEM 871x Epithelial cells (red) 358 Chapter 11 Cell Growth and Division  3 The environment of the heart stimulates injected stem cells to differentiate into new heart muscle cells. 1 Stem cells are 2 The stem cells are filtered from bone injected into the heart’s marrow removed from damaged area. a patient’s hip. Frontiers in Stem Cell Research CASE STUDY Basic research on stem cells takes on a special urgency in light of the Figure 11-18 importance it might have for human health. Heart attacks destroy Future Treatment for cells in the heart muscle, strokes injure brain cells, and spinal cord Heart Disease? injuries cause paralysis by breaking connections between nerve cells. Not surprisingly, the prospect of using stem cells to repair such Stem cell research may lead ­cellular damage has excited medical researchers. to new ways to reverse the damage caused by a severe Figure 11-18 shows how stem cells might be used in the future to heart attack. The diagram repair the damage caused by a heart attack. During a heart attack, shows one method currently the blood supply to part of the heart muscle is cut off, causing the being investigated. cells to die. This damages the heart and prevents it from functioning properly. Stem cells harvested from the bone marrow might be cul- tured and then injected into the damaged portion of the heart. Once in place, the stem cells would ”learn” what kind of cells they needed to be from the surviving cells around them. The stem cells would dif- ferentiate to become heart muscle cells. Ethical Issues Because adult stem cells can be harvested from READING TOOL a willing donor, research with these cells has raised few ethical questions. This is not the case with embryonic stem cells, which are Make a two-column chart to list the benefits and generally obtained in ways that cause the destruction of an embryo. issues related to stem cell For this reason, individuals who seek to protect human embryonic research. Fill in the chart as life oppose such research as unethical. Other groups support such you read. research as essential for saving human lives and argue that it would be unethical to restrict research. Human embryonic stem cell research is controversial because the arguments for it and against it both involve ethical issues of life and death. However, new developments in research may help to address these concerns. 11.4 Cell Differentiation 359  Induced Pluripotent Stem Cells A fundamental break- through took place in 2007 when Shinya Yamanaka of Kyoto University in Japan was able to convert human fibroblasts into cells that closely resembled embryonic stem cells. His work is summarized in Figure 11-19. These induced pluripotent stem cells (iPS cells), as they are known, are now widely used in research. Under certain con- ditions, iPS cells may be able to replace embryonic stem cells. In a sense, what Yamanaka and his lab achieved was to take the work of John Gurdon in cloning frogs to its logical conclusion. Gurdon had shown that the nucleus of an adult cell could be repro- grammed to develop into an embryo by unknown factors in the cytoplasm of an egg cell. To produce iPS cells, Yamanaka found a set of precise conditions that could reprogram an entire cell to put it back into an embryonic state. For these two discoveries, more than 50 years apart, Gurdon and Yamanaka shared the Nobel Prize in Physiology or Medicine in 2012. Today, it seems clear that this work has the potential to solve the ethical problems that can make embry- onic stem cell research highly controversial. CASE STUDY Induced Pluripotent Stem Cells Figure 11-19 Shinya Yamanaka 1 Genes are added to adult cells Dr. Shinya Yamanaka’s break- Dr. Yamanaka introduced four transcription factor genes into mouse skin fibroblasts. through research on induced pluripotent stem cells has made a huge impact on the field of regenerative medicine. Study the diagram to see how specialized cells can become stem cells. 2 They display properties similar to embryonic stem cells. red blood cell 3 They now have the capacity to develop into a number of specialized cell types. When injected into an embryo, the new stem cells (called induced pluripotent stem cells) can differentiate into any type of cell smooth found in an adult organism. nerve cell muscle cell 360 Chapter 11 Cell Growth and Division Regenerative Medicine Work on many types of stem cells, including adult stem cells and iPS cells, has now opened up an entirely new field of medicine. Regenerative medicine makes use of stem cells to repair or replace damaged cells and tissues. By study- ing what happens when stem cells differentiate, researchers have now developed laboratory “recipes” that can remake cells into certain other cell types. These differentiated cells may then be used to repair or replace damaged or diseased cells, tissues, and even whole organs. One promising treatment, now in clinical trials, uses stem cells to treat a form of macular degeneration. This condition affects the most sensitive part of the retina, the light-sensing layer of cells within the eye. When these cells break down, the result is a loss of vision. Experimental treatments have taken cells from a patient’s own body and converted them into iPS cells. These cells were then stimulated to differentiate into cells that could be transplanted directly into the eye. In at least a few cases, this treatment seems to have reversed the process of macular degeneration. Such research is not without risk, of course, since the transplanted cells may behave in unpredictable ways. They could differentiate into unwanted cell types, spread beyond the site of the transplant, or even grow uncontrollably into a tumor. This means that there are good rea- sons to proceed cautiously before putting these techniques into wide  VIDEO use. However, given its potential to alleviate human pain and suffering, Discover how animals it seems that the age of regenerative medicine is now upon us. regenerate lost body parts. As researchers also know, some organisms do an excellent job of regenerating lost body parts. For example, if a sea star loses one or more of its arms, the central part of its body is capable of grow- ing back the lost parts. Scientists continue to study the steps of this process. The research may lead to a method of replicating the steps in the human body. HS-LS1-4  LESSON 11.4 Review KEY QUESTIONS CRITICAL THINKING 1. What happens during differentiation? 4. Construct an Explanation Why is cell ­differentiation essential for every complex 2. What are stem cells? How are embryonic stem ­multicellular organism? cells different from adult stem cells? 5. Communicate Information Use what you 3. What do the arguments for and against the use learned in this lesson to discuss how cells of stem cells in medical research share? become specialized for different functions. Include an explanation of how the potential for specialization varies with cell type and how it ­varies over the life span of an organism. 11.4 Cell Differentiation 361 CASE STUDY WRAP-UP Will stem cells change the future of healing? Scientists are able to reprogram certain differentiated cells to make them act like stem cells. However, stem cell technology involves both benefits and risks. HS-ETS1-1 Make Your Case Different people may analyze new technologies in different ways. Even when observers understand the technology and agree on the evidence, they may draw different conclusions about whether the benefits outweigh the drawbacks. As you research stem cell thera- pies, you will likely find conflicting opinions and judgments about them. Be sure to evaluate the reliability of your sources as you form your opinion. Construct an Argument 1. Conduct Research Research stem cell technology. Find out about both sides of the issue, including scientific and ethical considerations. The United States Food and Drug Administration (FDA) is one useful source. 2. Engage in Argument From Evidence Based on your research, pick one side of the debate. Construct a useful set of guidelines for regulat- ing stem cell therapies. Your guidelines should account for constraints such as costs, reliability, and safety, as well as social impacts. 362 Chapter 11 Cell Growth and Division  Technology on the Case Here Come the Clones A clone is an exact genetic duplicate of a cell or an organism. John Gurdon created the first cloned frog when he transferred a nucleus from an adult frog cell into an egg cell. The result was a new frog, or clone, that had the same genes as the adult. False-colored fibroblast cell. Cloning technology has advanced greatly (TEM ×25,800) since Gurdon’s original experiments. In the 1990s, researchers in Scotland welcomed Dolly the sheep, the first cloned mammal. Today, more than 20 different kinds of animals have Careers on the Case been cloned, including cattle, horses, and pigs. The technique is generally the same as Gurdon Work Toward a Solution pioneered: An adult cell nucleus is transferred Researching and writing about stem cells and into the cytoplasm of an egg cell. other scientific findings includes the coordina­ tion of many different types of careers. Cloning provides many benefits. Ranchers are expanding their herds by cloning their Science Journalist strongest, healthiest animals. Then the clones Science journalists can be used as breeding stock. Government are required to have a agencies, including the Federal Drug Adminis­ knowledge of science tration (FDA), have confirmed the safety of as well as communica­ milk and meat products from animal clones tion skills. Newspapers, and their offspring. television networks, Could cloning be used for human repro­ and online media all duction? It might be possible, but the practice employ journalists to raises many serious ethical issues. Many coun­ report on developments in ­science, technology, tries now prohibit human cloning. In other coun­ and ­engineering. tries, including the United States, bans have been proposed but not yet enacted.  VIDEO Watch t