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Q What does it mean for a cell to be immortal? This photograph shows a lung cancer cell undergoing cell division. Unlike healthy cells, cancer cells can divide without limit—they are what scientists call immortal. This property is useful to scientists who culture cancer cells for research purposes. However, cancer cells are very dangerous in the body, where they may form tumors and invade tissues.

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R E a d IN G T ool bo x This reading tool can help you learn the material in the following pages.

USING LANGUAGE Your Turn Cause and Effect In biological processes, one step Identify the cause and effect in the following sentences. leads to another step. When reading, you can often 1. People often shiver as a result of being cold. recognize these cause-and-effect relationships by words 2. The light got brighter, so the pupil of the eye got that indicate a result, such as so, consequently, if-then, smaller.

and as a result. 3. If the cell passes the G2 checkpoint, then the cell may begin to divide.

Chapter 5: Cell Growth and Division 129 DO NOT EDIT--Changes must be made through “File info” CorrectionKey=B

5.1 The Cell Cycle

5a Key Concept Cells have distinct phases of growth, reproduction, and normal functions. VOCABULARY MAIN IDEAS cell cycle The cell cycle has four main stages. mitosis Cells divide at different rates. cytokinesis Cell size is limited.

5A describe the stages of the cell cycle, including Connect to Your World deoxyribonucleic acid (DNA) Many of life’s little chores can be quietly satisfying and rather fun. Washing dishes by replication and mitosis, and the importance of the cell cycle to the hand, however, is not so fun, which is why some clever person made the dishwasher. growth of organisms This handy invention soaks, washes, and rinses your dishes to a spot-free, sanitary sparkle. You unload the dishes, and the machine is ready to start the cycle all over again. A cell goes through a cycle, too. This cycle of growth, DNA synthesis, and division is essential for an organism to grow and heal. If it goes out of control, abnormal cell growth may occur, resulting in cancer cells like those shown on the previous page.

Main Idea 5a The cell cycle has four main stages. Just as all species have life cycles, from tiny chihuahuas to massive beluga whales, cells also have a life cycle. The cell cycle is the regular pattern of Figure 1.1 Cells grow and copy growth, DNA duplication, and cell division that occurs in eukaryotic cells. their DNA during interphase. Figure 1.1 shows its four main stages: gap 1, synthesis, gap 2, and mitosis. They also carry out cell-specific Gap 1, synthesis, and gap 2 together make up what is called interphase. functions in G1 and G2. During M stage, both the nucleus The stages of the cell cycle get their names from early studies of cell (in mitosis) and cytoplasm division. Scientists’ observations were limited by the micro (in cytokinesis) are divided. INTERPHASE scopes of the time. When a cell was not actively divid ing, they could not see activity in it. Thus, they originally divided the cell cycle into two parts: interphase, when the cell appeared to be at rest,

Gap 1 (G1) and mitosis, when the cell was dividing. Cells grow, carry out normal functions, CY Improved techniques and tools later allowed TOKIN and replicate their organelles. ESIS scientists to detect the copying of DNA T elophase (DNA synthesis), and they changed their phase Mitosis (M) description of the cell cycle to include the Ana Cell division e synthesis stage. Since they still could not see

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Gap 1 (G1) The first stage of the cell cycle is gap 1 (G1). During G1, a cell carries out its READING TOOLBox normal functions. If it is a skeletal muscle cell, it contracts to move joints. If it is an adrenal cell, it secretes hormones such as adrenaline. If it is an intestinal TAKING NOTES Construct your own cycle cell, it absorbs nutrients. During G1, cells also increase in size, and organelles diagram to take notes about increase in number. A cell spends most of its time in the G1 stage, although processes such as the cell cycle. the length of this stage varies by cell type. G1 S During G1, the cell must pass a critical checkpoint before it can proceed to Growth Synthesis the synthesis stage. Just as it would be dangerous for you to run a marathon if you had not slept or eaten for several days, it would also be dangerous for your cells to continue dividing if certain conditions were not met. For instance, most animal cells need enough nutrition, adequate size, and relatively un damaged DNA to divide successfully. They also need specific signals from other cells, telling them whether more cell division is needed.

Synthesis (S) CONNECT TO The second stage of the cell cycle is the synthesis (S) stage. Synthesis means DNA replication “the combining of parts to make a whole.” During the S stage, the cell makes a As you will learn in the chapter copy of its nuclear DNA. In eukaryotes, DNA is located in the nucleus. From DNA to Proteins, DNA During interphase, it is loosely organized and appears grainy in photographs. synthesis is also called DNA By the end of the S stage, the cell nucleus contains two complete sets of DNA. replication. During this process, the DNA molecule unzips and each strand is used as a pattern Gap 2 (G2) for a new DNA strand. Gap 2 (G2) is the third stage of the cell cycle. During G2, cells continue to carry

out their normal functions, and additional growth occurs. Like G1, this stage includes a critical checkpoint. Everything must be in order—adequate cell size, undamaged DNA—before the cell goes through mitosis and division. Mitosis (M) Mitosis (M), the fourth stage of the VISUAL VOCAB cell cycle, includes two processes: Mitosis is the division of the cell mitosis and cytokinesis. Mitosis nucleus and its contents. (my-TOH-sihs) is the division of the cell nucleus and its contents. During parent cell mitosis, the nuclear membrane mitosis dissolves, the duplicated DNA con denses around proteins and separates, cytokinesis and two new nuclei form. Lastly, cytokinesis (sy-toh-kuh-NEE-sihs) daughter cells is the process that divides the cell Cytokinesis divides the cell cytoplasm. cytoplasm. The result is two daughter cells that are genetically identical to the original cell. The stages of the cell cycle and the proteins that control it are similar in all eukaryotes. For example, scientists have demonstrated that some of the molecules that regulate checkpoints in the yeast cell cycle can work in human cells, too. Such similarities suggest that eukaryotes share a common ancestry.

Predict What might happen if the G2 checkpoint stopped working in cells?

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body’s for need cells. In those human S,G cells, the divide much faster cells. than do eukaryotic found ineukaryotes. Thus, prokaryotic cells typically have membrane the thatthat prokaryotes of cells. Recall eukaryotic do not The prokaryotic is cycle cell similar but not identical to muscle) and many of your internal organs, such as lungs, that make up rest the of your intestine (mainly smooth and intestine are few days. replaced In every contrast, cells lot of wear and tear. As aresult, cells that line your stomach body. The internal lining of your digestive receives tract a division varies within also different tissues of adult the of organs their are developing. still But rate the of cell it is inadults. Children have ashorter cycle, cell and many rate of division cell is greater inembryos and children than Rates of division cell widely, vary as shown in Cells dividerates. at different Therefore, increase insizecould result afurther in a

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FIGURE 1.3 Ratio of Surface Area to Volume in Cells As a cell grows, its volume increases more rapidly than its surface area. When the surface area–to-volume ratio is too small, the cell cannot move materials into and out of the cell at a sufficient rate or in sufficient quantities.

Relative size

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Compare Which cell has the largest surface area? Which cell has the largest surface area–to-volume ratio?

Some cells, however, must be large. A neuron running down a giraffe’s neck to its legs may be several meters long, for instance. But it is not shaped like a cube or a sphere. Instead, it is extremely long and thin. This structure gives the neuron a large surface area with a relatively small increase in volume. To maintain a suitable cell size, growth and division must be coordinated. If a cell more than doubled its size before dividing, the daughter cells would be larger than the original cell. If this happened with each generation, cells would quickly become too large to live. Similarly, if a cell did not double its size before dividing, the daughter cells would be smaller than the original cell. If this happened with each generation, cells would become too small to live. Connect Which has the larger ratio of surface area to volume, a tennis ball or a soccer ball? Explain your reasoning.

Self-check Online HMDScience.com 5.1 Formative Assessment GO ONLINE Reviewing Main Ideas Critical thinking CONNECT TO 1. During which stage of the cell cycle is 4. Infer Suppose you were to draw a Scientific Process the DNA copied? 5a diagram representing the cell cycle of 6. Predict how the rate of cell 2. Which stages of the cell cycle a neuron. Explain where and how you division would differ 5a generally require about the same would represent G0. between single-celled algae amount of time in all human cells? 5. Predict Suppose you treat cells with living in a sunny, nutrient-rich 5a chemicals that block cytokinesis. pond versus algae living in a shady, nutrient-poor pond. 3. What limits the maximum size of a Describe what you think the cells How could you test your cell? would look like. prediction?

Chapter 5: Cell Growth and Division 133 CorrectionKey=A DO NOT EDIT--Changes mustbemadethrough “File info” Unit 134 hsht base phosphate acids. of thelong chainscalled nucleic bond, they form thebackbones the sugars andphosphate groups molecule called abase. When and anitrogen-containing asugar,parts: aphosphate group, nucleotide ismade ofthree From DNA to Proteins, a As you learn will inthechapter B telophase anaphase metaphase prophase telomere centromere chromatid chromatin histone chromosome VOCABULARY data and5 inferences, andpredict trends from growth oforganisms importance ofthecell cycle to the replication andmitosis, andthe deoxyribonucleic acid (DNA) the cell cycle, including ioc CO sugar

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T t O ry 2G, 5a ages of Mitosis andCytokinesis between two nuclei. Mitosis isanamazingprocess that efficiently sortstwo sets ofDNAanddividesthem properly. Dividing DNAisacomplicated task because theDNA isso long andstringy. a similar way, your cells mustreceive set afull ofDNA—nomore, noless—to work mannered child (not you, ofcourse) mighthave raised afuss ifanitem was In missing. exactly numberoferasers, thesame candies, andstickers. Otherwise, some ill- were Whoever handedout. stuffed thebags had to make sure that each bag had When you were achild, perhaps you attended abirthday where party goody bags Your to World Connect Ke Chromosomes condense at the start ofmitosis. Chromosomes at start condense the y

the association between DNA and proteins. a linear strand of DNA to its highly condensed form. The key to this process is nucleotides. A DNA is adouble-stranded made molecule of four different subunits called a different one. gled. Perhaps a cell would get two copies of one chromosome and no copies of If chromosomes remained stringy during mitosis, they could become entan- must condense to be divided between two nuclei. During mitosis, the duplicated chromosomes proteins or to copy the entire DNA sequence. access specific genes for a cell to make specific functions. During all of interphase, proteins must organization allow a cell to carry out its necessary How it does fit inside nucleus the of amicroscopic cell? DNAthe injust one of your cells would about be 3meters (10feet) long. cells have 46chromosomes each. If stretched out straight and laid end to end, consists of numerous genes along with regulatory information. Your body shown in your chromosomes are tightly condensed, as looks a bit like spaghetti. During mitosis, however, is not dividing, DNA is loosely organized—it and condense it. During interphase, or when a cell M MAI C Mitosis Chromosomes ai o DNA n I n n N c ID ept d wraps around proteins that help organize

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chromatid FIGURE 2.2 Chromosome Structure telomere DNA condenses tightly during the early stages of mitosis.

centromere histone

telomere DNA double helix DNA and histones Chromatin Supercoiled DNA Condensed, duplicated chromosome Each continuous, DNA wraps at regular Interactions between The chromatin The condensed, duplicated chromo- double-stranded intervals around pro- parts of the histones coils more and more somes can be aligned and separated DNA molecule makes teins called histones, further compact the tightly around orga- during mitosis. one chromosome. forming chromatin. DNA. nizing proteins. Infer Overall, DNA has a negative charge. Look at the histone proteins in the figure. What type of overall charge do you think they have? Explain.

At almost all times during the cell cycle, each of your chromosomes is R E ADI N G TO OLB ox associated with a group of proteins called histones. DNA wraps around histones at regular intervals, similar to beads on a string. The complex of TAKING NOTES Use a main idea web to help protein and DNA that makes up the chromosome is called chromatin. Parts you study the makeup and of the histones interact with each other, further compacting the DNA. At the organization of chromosomes. “spaghetti” stage, the combination of DNA and proteins is loose. The word “loose” describes how much the DNA strand folds back on itself; it does not chromosomes mean the DNA is loosely wrapped around the histones. As a cell progresses into mitosis, chromatin further condenses. It continues to coil more and more tightly around organizing proteins, finally forming small, thick rods. Recall that each chromosome has already been copied during the previous S stage. Thus, the chromosome looks similar to an “X” in which the left and right halves are two identical DNA double helixes. One half of a duplicated chromosome is called a chromatid (KROH-muh-tihd). Together, the two identical chromatids are called sister chromatids. Sister chromatids are held together at the centromere (SEHN-truh-meer), a region of the condensed chromosome that looks pinched. In addition, the ends of DNA molecules form structures called telomeres (TEHL-uh-meers), which are made of repeating nucleotides that do not form genes. They prevent the ends of chromosomes from accidentally attaching to each other, and they help prevent the loss of genes. A short section of nucleotides is lost from a new DNA molecule each time it is copied. It is important that these nucleotides are lost from telomeres, not from the genes themselves. Apply What is the relationship between a molecule of DNA and a chromosome?

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Main Idea 5a Mitosis and cytokinesis produce two genetically identical daughter cells. The combined processes of mitosis and cytokinesis produce two genetically identical daughter cells. Follow along in Figure 2.4 as you read about the process in more detail below.

Figure 2.3 The nucleus and Interphase INTERPHASE chromosomes go through dramatic Interphase plays an important role in preparing the cell to changes in a dividing cell. divide. It provides critical time for the duplication of organ- Parent cell elles and for DNA replication. By the end of interphase, an centrioles individual cell has two full sets of DNA, or chromosomes, and is large enough to divide.

spindle Mitosis MITOSIS fibers Mitosis divides a cell’s nucleus into two genetically identical nucleus nuclei, each with its own single, full set of DNA. This process with DNA centrosome occurs in all of your body cells—except those that form eggs or sperm—and prepares them for cytokinesis. Although mitosis and cytokinesis are continuous processes, scientists have divided them into phases to make them easier to understand and discuss. The four main phases of mitosis are prophase, metaphase, anaphase, and telophase. Cytokinesis begins during late anaphase or telophase. CONNECT TO 1 During prophase, chromatin condenses into tightly coiled chromosomes. Cells Each consists of two identical sister chromatids. The nuclear envelope As you will learn in the chapter breaks down, the nucleolus disappears, and the centrosomes and centrioles Meiosis and Mendel, your body begin to migrate to opposite sides of the cell. Organized microtubules has two major cell types. Germ called spindle fibers grow from the centrioles and radiate toward the cells develop into eggs or sperm. Somatic cells make up the rest of center of the cell. your body. 2 In metaphase, the spindle fibers attach to a protein structure on the centromere of each chromosome and align the chromosomes along the cell equator, around the middle of the cell. 3 During anaphase, sister chromatids separate from each other. The spindle fibers begin to shorten, pulling the sister chromatids away Biology HMDScience.com from each other and toward opposite sides of the cell. GO ONLINE 4 In telophase, a complete set of identical chromosomes is positioned Mitosis Stage Matching at each pole of the cell. The nuclear membranes start to form, the chromosomes begin to uncoil, and the spindle fibers fall apart.

Cytokinesis CYTOKINESIS Cytokinesis divides the cytoplasm into two cells and completes a full stage of the cell cycle. Cytokinesis differs in animal and plant cells. In animal cells, the membrane forms a furrow, or trench, that is pulled inward by tiny filaments, like a drawstring. Gradually, the membrane pinches closed, forming a separate cell around each nucleus.

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FIGURE 2.4 the Cell Cycle in Detail

Following interphase, mitosis divides duplicated chromosomes between two nuclei. Cytokinesis divides the cytoplasm. In this diagram, the mitosis stage is greatly expanded to highlight its four major phases. (micrographs; magnification about 1003)

INTERPHASE MITOSIS The cell copies its DNA Mitosis divides a cell’s nucleus into two and grows in preparation nuclei, each with an identical set of DNA. for division. The DNA is loosely organized during interphase.

1 Prophase DNA and proteins condense into tightly coiled chromosomes. The nuclear envelope breaks down, centrioles begin to move to opposite poles, and spindle fibers form.

CYTokinesis Cytokinesis divides cytoplasm between two daughter cells, each with a genetically identical 2 Metaphase Spindle fibers nucleus. The cells enter attach to each chromosome. They align the chromosomes interphase and begin along the cell equator. the cycle again.

3 Anaphase Chromatids separate 4 Telophase Nuclear membranes to opposite sides of the cell. start to form, chromosomes Cytokinesis usually begins in late begin to uncoil, and the spindle anaphase or telophase. fibers fall apart.

CRITICAL How many chromosomes does the cell have at the start of

(full page) ©Michael W. Davidson/National (full High page)Magnetic W. ©Michael Field Laboratory VIEWING mitosis? How many does it have after cytokinesis? 5A

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DATA ANALYSIS 2g

CONSTRUCTING DATA TABLES Table 1. EFFECT OF HORMONES ON Scientists use data tables to record CELL division their data. Data tables are organized Concentration of Size of Cell Clump by the independent and dependent Hormone Solution (%) After 24 Hours (mm) variables. Usually the independent independent 0 3 dependent variable is listed in the left column, variable variable and the other columns list the 25 4 dependent variables. Each separate 50 8 observation is listed in its own row. 75 9 When measurements are taken using units, they are listed in the column 100 9 headings in parentheses. All tables should have numbers and titles. observations Table 1 shows data from a hypothetical experiment in which growth hormones were added to clumps of cells in a laboratory, and the growth of the cell clumps was measured. 1. Display Data Suppose a scientist decided to measure the effect of temperature on cell division in chlorella, a type of green algae. Set up a table that could be used to record the results for the number of daily doublings of the cells: 20°C, 3 doublings; 30°C, 7 doublings; 40°C, 12 doublings; 50°C, 0 doublings. 2. Apply Label the independent and dependent variables on your table.

During cytokinesis in plant cells, the membrane cannot pinch inward because of the cell wall. Instead, a cell plate forms between the two nuclei. It is HMDScience.com made by the Golgi apparatus, which supplies the new plasma membrane. A new

GO ONLINE wall then grows as cellulose and other materials are laid down. Typically, cyto- Phases of Mitosis plasm is divided evenly between daughter cells in both plant and animal cells. The formation of new cells is critical in both multicellular and single-celled organisms. Single-celled organisms use cell division to reproduce, whereas multicellular organisms use it for growth, development, and repair. Contrast How does cytokinesis differ in animal and plant cells?

Self-check Online HMDScience.com 5.2 Formative Assessment GO ONLINE Reviewing Main Ideas Critical thinking CONNECT TO 1. Draw what a chromosome looks like 3. Contrast How do prophase and Protein Synthesis during metaphase. Identify the telophase differ? 5a 5. For a cell to make proteins, chromatids and the centromere. 4. Apply Using a light microscope, you enzymes must access its genes. When histones are 2. Briefly explain why the daughter cells observe a cell that has no nucleus. modified with acetyl groups resulting from mitosis are genetically What features would you look for to (-COCH ), their positive identical to each other and to the determine whether it is a eukaryotic 3 charge is neutralized, so they original cell. 5a cell undergoing mitosis or a prokaryotic cell? 5a wrap DNA less tightly. How might this affect the rate of protein synthesis? 4B

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Get Growing

VIRTUAL Lab BIOZINE Investigating Bacterial Phases of Mitosis Stem Cell Research— Growth Find out which Test your skills at recognizing the Potential Solutions, strains of bacteria need phases of mitosis, and then explore Practical Challenges Catch oxygen to grow. what happens when errors occur. the latest headlines about cells, including stories about cloning and stem cell research. (r) ©Chris Stewart/San Francisco Chronicle/Corbis

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5.3 Regulation of the Cell Cycle

5A, 5B, 5C, 5D, Key Concept Cell cycle regulation is necessary for healthy 9C growth. VOCABULARY MAIN IDEAS growth factor Internal and external factors regulate cell division. apoptosis Cell division is uncontrolled in cancer. cancer benign malignant Connect to Your World metastasize Have you ever watched a movie in which people play with the elements of nature? carcinogen They might bring back dinosaurs or make a newfangled robot. And have you noticed that these movies are always scary? That’s because things go out of control. The

5A describe the stages of robots take over, or the dinosaurs start eating humans. If cell growth goes out of the cell cycle, including control in your body, the result can be even scarier. Cancer is uncontrolled cell deoxyribonucleic acid (DNA) replication and mitosis, and the growth and results from many factors that affect the cell cycle. So how does your importance of the cell cycle to the body regulate all the millions of cell divisions happening in your body? growth of organisms; 5B examine specialized cells, including roots, stems, and leaves of plants; and animal cells such as blood, muscle, and epithelium; 5C describe the roles of DNA, ribonucleic acid (RNA), and environmental factors in cell Main Idea 5a, 5B, 5C, 9C differentiation; 5D recognize that disruptions of the cell cycle lead to diseases such as cancer; 9C identify Internal and external factors regulate and investigate the role of enzymes cell division. Both external and internal factors regulate the cell cycle in eukaryotic cells. External factors come from outside the cell. They include messages from nearby cells and from distant parts of the organism’s body. Internal factors come from inside the cell and include several types of molecules found in the cytoplasm. Both types of factors work together to help your body control the process of cell division. FIGURE 3.1 Normal animal cells External Factors (top) respond to external factors and stop dividing when they touch External factors that help regulate the cell cycle include physical and chemical each other. Cancer cells (bottom) signals. One example of a physical signal is cell-to-cell contact. Most mammal fail to respond and form clumps. cells grown in the laboratory form a single layer on the bottom of a culture Normal cell growth dish, as shown in Figure 3.1. Once a cell touches other cells, it stops dividing. The exact reason for this phenomenon is unknown. One hypothesis is that receptors on neighboring cells bind to each other and cause the cells’ cyto skeletons to form structures that may block the signals that trigger growth. Many cells also release chemical signals that tell other cells to grow. For Cancerous cell growth example, growth factors are a broad group of proteins that stimulate cell division. Growth factors bind to receptors that activate specific genes to trigger cell growth. In general, cells grow and divide in response to a combination of different growth factors, not just one.

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(tr) ©Anatomical Travelogue/Photo Researchers, Inc.; (br) ©Royalty-Free/Corbis child’s development? th P redict love to put mouths. intheir Most babies are with born little unwebbed fingers and they toes Before ababy cells typically go isthose born, through apoptosis. embryos have webbing fingers their between and or toes, digits. example of apoptosis. In early the stages of development, human use inbuildinguse other molecules. gobble up apoptotic the and cell recycle its chemical for parts immune cells inthe tidily specialized system. cells These very cell tends to shrink and break apart, and the cell is recognized by process.this What is known is that nucleus the of an apoptotic produce enzymes. Many self-destructive questions remain about internal when occurs or activate externalsignals genes that help die. Just as some to cells need grow and divide, other to cells need Apoptosis cells bind when cycle cell to each other. twoThese factors help advance acell to different stages of the rapidly made and destroyed at points certain cycle. cell inthe activated by cyclins. Cyclins are agroup of proteins that are that cell.in the kinases Those help control are cycle cell the have many of and types are kinases, they almost always present ofenergy target the or molecule changes its shape. Your cells target tomolecule aspecific molecule. This action typically increases the is an enzyme that, activated, when transfers aphosphate group from one factors involved are cycle cell eukaryotic inthe and kinases cyclins. Akinase that cycle. cell the Two affect of most the important and well-studied internal When externalfactors bind to receptors, their internal can trigger they factors FactorsInternal protein and fat metabolism. particular, growth hormone results inbone growth your and affects also oxygen levels. factor would increase number the of cells and red blood raise your blood oxygen levels would cause your to produce body more That erythropoietin. air the because pressure is lower at higheraltitudes. The inblood decrease from coast the to mountains, the your oxygen blood levels would lower be oxygen. If cells carry blood you cells. becomeRed red blood will moved (ih-rihth-roh-poy-EE-tihn)tin stimulates production the only of cells that growth factors have more targets. specific For instance, erythropoie- repairbody wounds by growth the triggering of many Other types. cell help stop bleeding. Platelets store of atype growth factor that helps your st at did not work properly. How do you think this would affect the icky fragments of bone marrow form cells. They clots that Var Some growth factors many affect of types cells. For example, platelets are Apoptosis

Suppose a child was born with growth hormone receptors ious hormones may stimulate also growth the In of types. cell certain (ap-uhp-TOH-sihs) is programmed death. cell It 5a, 5C 5a, Figure 3.2 Figure shows aclassic webbed

fin gers unwebbed fingers andtoes. a result, thebaby isbornwith later stages ofdevelopment. As digits undergo apoptosis during Thecellsopment. between the webbed digitsearly intheirdevel- Figure 3.2 Figure Chapter

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tumor is relativelybe harmless and can probably cured by be removing it. However, ifa cancerthe cells typically remain clustered together. This means tumor the may the bloodstream or lymphatic system to other parts of the body, as shown in (mih-TAS-tuh-syz), from the tumor. These breakaway cells can be carried in healthy cells. As aresult, divide much they more often than do healthy cells. divide absence inthe of many of growth the factors required for division in surroundedeven when by neighboring cells. Cancer continue cells can also to Unlike healthy cells, cancer cells grown inaculture dish continue to divide, trolled division. cell It regulation ariseswhen of is cycle cell disrupted. the carcinogens f Cancer the developmentthe of cancer are called stances known to produce or promote ultraviolet radiation from sunlight. Sub- are due to DNA damage caused by chemicals. For example, some skincancers causedcan by be exposure to radiation or errors genes. mutations Other inspecific cancers appear caused to by be inherited Mutations genes inthese inherited. can be For instance, some breast oncogenes, accelerate as cell cycle.act cell the The second type mutations,cells carry or errors, of intwo types genes. called One type, genes that help make proteins involved incell metastasizes, it is much more difficult to entirely rid the body of tumors. continue to grow unchecked, eventually organism. the will they kill for space, and some regions unable be to will properly. function If cancer cells ing organs. For instance, atumor growing inside cramp skull the will brain the Infunction. addition, agrowing tumor can exertgreat pressure on surround- of supply and blood food ahearty but that contribute nothing to body’s the Therefore, has large body the clumps of rapidly dividing cells that require lots carefullythe ordered messages to interpret needed electrical information. exchange oxygen and carbon dioxide. In brain, the do not they transmit byfunctions needed body. the In lung, the for example, cancer cells do not such virus can cause cervical cancer.such can cause virus cervical oncogenes are byone even carried viruses; with lung cancer. Some mutated forms of air pollutants, which are associated both includeThese tobacco smoke and certain M igure 3.3, ai Can Can But w n I n is common the name for aclass characterized of by diseases uncon- cer cells form disorganized clumps tumors. called In a cer cells come from normal cells that have suffered damage to the d malignant, where they can form more tumors, called metastases. Once a tumor hy are tumors Cancer harmful? cells do not specialized perform the e a (kahr-SIHN-uh-juhnz). some of cancer the cells can break away, or 5a, 5 5a, d ‑ cy Figure 3.4 Figure may metastasize quickly. an example ofaskin cancer, which cle regulation. Most cancer

This cancerous mole is benign ‑ cy metastasize cle brakes. tumor,

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Q U I C K L A B observing 5b Cancer In this lab, you will compare normal cells with cancerous cells and observe the differences between them. Materials • microscope Problem How do normal and cancerous cells compare? • slides of normal cells • slides of cancerous cells Procedure 1. Examine the slides of normal cells under the microscope. Draw and describe your observations. 2. Repeat step 1 with slides of cancer cells. Analyze and Conclude 1. Compare How does the structure of the normal cells compare with the structure of the cancerous cells for each of the slides you viewed? 2. Infer Cancer cells not only appear different from normal cells but they also divide more rapidly. Why do you think chemotherapy, a common treatment for cancer, results in the loss of hair?

Standard cancer treatment often involves both radiation and chemotherapy. Radiation therapy is the use of radiation to kill cancer cells and shrink tumors. It works by damaging a cell’s DNA so much that the cell cannot Web divide. Radiation is usually localized—that is, its use is targeted to a specific HMDScience.com region—because it can also hurt healthy cells. Chemotherapy uses certain GO ONLINE drugs, often in combination, to kill actively dividing cells. Like radiation, it Skin Cancer kills both cancerous and healthy cells. However, chemotherapy is systemic— drugs travel throughout the entire body. Medical researchers use laboratory-grown cancer cells in their search for cancer treatments. Much of what is known about the cell cycle has come from studies that use cancer cells. The most famous cancer cells used for research are called HeLa cells. HeLa cells were originally obtained in 1951 from a cervical tumor removed from a woman named Henrietta Lacks. This cell line continues to be grown and studied in laboratories all over the world. Analyze HeLa cells are also used to study cell signaling processes. What might be a disadvantage of using cancer cells to study processes occurring in healthy cells? 5d Self-check Online HMDScience.com 5.3 Formative Assessment GO ONLINE Reviewing Main Ideas Critical thinking CONNECT TO 1. Describe what a growth factor is and 3. Contrast How do benign and Cell Organelles how it influences the cell cycle. malignant tumors differ? 5. Some anticancer drugs 5a 4. Hypothesize Suppose chromosomes prevent microtubules from 2. Explain how cancer cells differ from in a skin cell are damaged by ultravio- forming spindle fibers. Why healthy cells. 5a, 5d let radiation. If the damaged genes do do you think these drugs not affect cell cycle regulation, do might be effective treat- you think the cell will become ments for cancer? 5a, 5d cancerous? Explain. 5A, 5d

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5.4 Asexual Reproduction

Key Concept VOCABULARY Many organisms reproduce by cell division. asexual reproduction MAIN IDEAS binary fission Binary fission is similar in function to mitosis. Some eukaryotes reproduce through mitosis.

Connect to Your World In this flashy world of ours, you may think that the humble bacterium would have little chance of finding a mate. No dazzling smile, no fancy hair products, no shiny car, and—if we are brutally honest—not even a brain. With all of these limitations, it may seem that our bacteria friends would be destined to die out. And yet, bacteria are found in abundance and live just about everywhere on Earth. How can there be so many bacteria?

Main Idea Binary fission is similar in function to mitosis. Reproduction is a process that makes new organisms from one or more parent organisms. It happens in two ways—sexually and asexually. Sexual reproduction involves the joining of two specialized cells called gametes (eggs and sperm cells), one from each of two parents. The offspring that result are genetically unique; they have a mixture of genes from both parents. In contrast, asexual reproduction is the production of offspring from a single parent and does not involve the joining of gametes. The offspring that result are, for the most part, genetically identical to each other and to the single parent. Binary Fission and Mitosis Most prokaryotes reproduce through VISUAL VOCAB CONNECT TO binary fission (BY-nuh-ree FIHSH- Binary fission is the asexual reproduc- Cell Structure uhn), the asexual reproduction of a tion of a single-celled organism by Recall from the chapter Cell single-celled organism by which the division into two roughly equal parts. Structure and Function that cell divides into two cells of the same many scientists hypothesize that parent cell mitochondria and chloroplasts size. Binary fission and mitosis have were originally free-living similar results. That is, both processes prokaryotes. One piece of form two daughter cells that are DNA duplicates evidence that supports this genetically identical to the parent cell. hypothesis is the fact that these cell begins to divide two organelles replicate much as However, the actual processes are bacteria do, through fission. different in several important ways. daughter cells As you already learned, prokaryotes such as bacteria do not have nuclei. They also do not have spindle fibers. And although they have DNA, prokaryotes have much less DNA than do most eukaryotes. The DNA of most bacteria is in the form of a single circular chromosome.

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©Dr. Gopal Murti/Photo Researchers, Inc. Summarize separation into two daughter cells. formswall two the chromosomes, between which completes the undergoes cytokinesis. The membrane pinches inward, and cell anew away from each other. When is cell about the its twice size, original it membrane. As grows cell the and gets longer, chromosomes the move chromosome is copied. chromosomes Both are attached to cell the to attract mates. Organisms that reproduce do not asexually have costs. these amatefind but many also structures, and signals, behaviors that have evolved attract amate. This effort involves not only to and time the needed energy females canoffspring. bear In addition, reproducing sexually organisms must ism reproduces sexually, having five females and five males, only five the asexually, ten offspring all can have offspring of own. their If other the organ- Suppose two organisms each have ten offspring. If one organism reproduces For example, reproducing asexually all organisms can potentially reproduce. more efficient; rather, associated the costs reproduction of sexual are greater. chancethe that inchanging some individuals survive will conditions. off. In contrast, reproduction sexual increases genetic diversity, which raises tothem reproduce inachanged environment, entire the population could die ment same inthe way. If population members lack traits that enable conditions. identical offspring Genetically respond will to environ- the more than harmful helpful. In other words, ifit ain’t broke, don’t it. fix arethey well suited to environment, the genetic variation could be that reproduction asexual results ingenetically identical offspring. If reproduction more can be reproduction. efficient than sexual Recall situation. In favorable environments that do not change much, asexual Very often, something whether is helpful or dependson harmful the Advantages Disadvantages and Asexual of Reproduction Keep inm How Binary fission,Binary shown in 1 ever, reproduction asexual may adisadvantage be inchanging

How i 2 ind, however, that ofreproduction act the asexual itself is not s asexual reproduction an advantage in some conditions? some in advantage an reproduction s asexual figure 4.1, Binary Fission GO ONLINE starts when the bacterial bacterial the when starts HMDScience.com 3 Biology show (colored TEM;magnification 60,000 separating thetwo cells (3). meets, andanew cell wall forms, inward the cell membrane pinches and theDNAisreplicated. Next, fission. First,acell elongates (1), each at adifferent stage ofbinary F igure 4.1 s three individual bacteria,

(2). Final F cell division. of 1024 cells after only 10rounds of One bacterium can result inatotal igure 4.2

Number of bacteria 1000 1200 This micrograph 800 600 400 200 0 ly, themembrane 10 9 8 7 6 5 4 3 2 1 resistance onto theiroffspring. passing thegenes for antibiotic antibiotics can reproduce quickly, The bacteria not killed by in multidrug-resistant bacteria. misuse ofantibiotics hasresulted Viruses and Prokaryotes, the As you learn will inthechapter Evolu B Chapter 5:Cell Growth andDivision CO ac Cycles ofcell division NNE te ria t io C 3 T G )

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(hydra: LM,magnification 12 can reproduce by budding. F colored SEM,magnification 3,200 igure 4.3 Unit 2: CellsUnit 2: 5.4 2. 1. Re

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ism can produce and cells. eggs both sperm have separate males and females. In other anemone same the organ- species, offspring oftenThe stay connected to organism, original the through struc- modification of astem or underground structures of parent the organism. vegetative reproduction. In general, vegetative reproduction involves the mentation. Many plants, including and strawberries potatoes, reproduce via gr yeast reproduce by budding. Examples are shown in dently or attached of as part acolony. For instance, hydras and of some types ism, forming aseparate individual. new The organism new may live indepen- including budding, fragmentation, and vegetative reproduction. multicellularboth and unicellular eukaryotes. It forms, can several take reproduction common is especially insimpler plants and animals. It in occurs duction and are therefore genetically same the as parent the organism. Mitotic armthe of another one? organisms new These are result the of mitotic repro- grown plant anew from astem star growing cutting? sea anew Or seen from reproduceSome eukaryotes also asexually, through mitosis. Have you ever M reproduce by andof sexually makingSome eggs species anemone sperm. tures runners, for called example. ow into organism. anew Flatworms stars and reproduce sea both by frag- ai In fragmentation, aparent organism splits into pieces, each of which can In bu half, by breaking pieces off small from its or base, by budding. It can also

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5.5 Multicellular Life

5b, 5c, 10c Key Concept Cells work together to carry out complex functions. VOCABULARY MAIN IDEAS tissue Multicellular organisms depend on interactions among different cell types. organ Specialized cells perform specific functions. organ system Stem cells can develop into different cell types. cell differentiation stem cell Connect to Your World Each of us enters this world as a helpless infant. At first, your ability to eat solid 5B examine specialized cells, including roots, stems, and foods or take your first steps elicits a great deal of praise. Over time, however, your leaves of plants; and animal cells development of normal skills gets far less attention. By the time you reach the age of such as blood, muscle, and epithelium; 5C describe the roles of 18, people want to know what you plan to do with your life. Will you build houses or DNA, ribonucleic acid (RNA), and design clothing or treat patients? What will your specialty be? Cells, too, undergo environmental factors in cell differentiation; 10C analyze the specialization to carry out the complex functions required by the body. levels of organization in biological systems and relate the levels to each other and to the whole system Main Idea 10c Multicellular organisms depend on interactions among different cell types. Within multicellular organisms, cells communicate and work together in groups that form increasingly larger, more complex structures. This arrange- ment progresses from cells to tissues to organs to organ systems, as shown in Figure 5.1. Tissues are groups of cells that work together to perform a similar function. Groups of tissues that work together to perform a specific function or related functions are called organs. For instance, plants have photosynthetic tissues made of chlorophyll-containing cells. Conductive tissues transport sugars, water, and minerals to and from other parts of the plant. Protective tissues help prevent water loss. Together, these and other tissues form a leaf, the plant’s food-producing organ. Organs that carry out similar functions are further grouped into organ systems. In plants, the shoot system is above the ground. It includes stems that support the plant, leaves that capture radiant energy, and flowers CONNECT TO that aid reproduction. Beneath the ground, the root system has different types Homeostasis of roots and root hairs that anchor the plant and absorb water and minerals. As you learned in the chapter As organ systems work together, they help an organism maintain homeo- Biology in the 21st Century, stasis. For example, plants need to maintain a certain level of water within homeostasis is the maintenance of a stable internal environment. their cells, otherwise they will wilt and die. They absorb water through their Both an organism’s physiology roots and expel it as water vapor through openings in their leaves called and its behavior help it achieve stomata. Stomata are controlled by special cells called guard cells, which close homeostasis. the stomata when a plant’s water intake cannot keep up with its water loss. Apply Suppose your family goes out of town and forgets to ask your neighbor to water the plants. Do you think the plants’ stomata will be open or closed? Explain.

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they form gradients. distributed throughout thecell; these molecules are not evenly an embryo to grow. Manyof molecules that are necessary for stocked withorganelles and Meiosis andMendel,theeggis As you learn will inthechapter G Unit 2: CellsUnit 2: Cells work together ingroups that form larger, specialized structures. F am CO IGURE SEM; magnification 200 tube-shaped cells. (colored Vessel elements are c e NNE ll et og C 5.1 T

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ric, as shownric, in ate. In plant first cells,division the is unequal, egg or of asymmet- afertilized dishes are different, very but come all from they same the cookbook. brownies. If you to need make amain course, you might lentil fix stew. The carry out its instructions. If you to youneed make adessert, might choose dish, youmake that aspecific select recipeto and want you When cookbook. among genes the and only uses ones. certain You of can think your DNA as a genes out it specific the toits needs carry That function. is, differentiates acell of set inyour cell DNA, has afull body every of eachexpresses type cell only structure orspecific during function multicellular development. While almost organismscountless around you, cells must specialize. form intricate the structures that make up your of and body bodies the simplyegg divided to make lots of identical cells, it would not form ababy. To organism like you develop? Your If egg. the began as asingle fertilized body single bacterium can generate another bacterium. But how acomplex does It that is to easy see can askincell divide to make skincell, anew or that a

changing conditions and continue to develop throughout lifetime. their Plant cells cannot easily migrate of because but wall, cell the adapt they to provide nutrients to embryo; the it creates also growth the point for roots. the growththe point forto is stemscell and leaves. The majorbasal the of role e M How is the shape of this plant’s roots suited to suited roots plant’s this of shape the How is ai A cel Cell differentiation n I d l’s location within embryo the helps determine how differenti- it will 3 e a ) 5B, 10c 5B, f igure 5.2 5 O water andnutrients. form roots that absorb Xylem andother tissues B is process the for by whichbecomesa acell specialized , 5c rga . The apical forms cell most of embryo, the including n roots lateral S tissue vascular root primary ys te

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As the plant grows, new cells continue to differentiate based on their location. For example, cells on the outer layer of a leaf may become epidermal cells that secrete a waxy substance that helps prevent water loss. Cells on the lower leaf surface may become guard cells that control the exchange of water, air, and carbon dioxide. FIGURE 5.2 Cell Differentiation in Seed Plants seed leaves

apical cell

apical cell zygote embryo (two cells) basal cell embryo inside seed plant

In animals, an egg undergoes many rapid divisions after it is fertilized. The resulting cells can migrate to a specific area, and the cells quickly begin to differentiate. The early animal embryo generally takes the shape of a hollow ball. As the embryo develops, part of the ball folds inward, forming an inner layer and creating an opening in the outer cell layer. A middle layer of cells then forms. As shown in figure 5.3, in vertebrates, the outer cell layer differentiates to form the outer layer of skin and elements of the nervous system, such as the brain and spinal cord. The middle cell layer forms bones, muscles, kidneys, and the inner layer of skin. The inner cell layer forms internal organs, such as the pancreas, lungs, and digestive system lining. Analyze Why is regulation of the differentiation process during the early stages of Animal embryo cross section development so critical? outer FIGURE 5.3 Cell Differentiation in Animals middle inner Cell differentiation in the developing animal embryo is based on location.

Outer Skin cells help prevent Middle Bone cells form a hard Inner Intestinal epithelia have a infection and dehydration. matrix (shown) that supports and large surface area that increases (colored SEM; magnification 5003) protects organs. absorption. (colored SEM; magnification 153) (colored SEM; magnification 253) (l) ©Andrew Syred/Photo Researchers, Inc.; (c) ©Susumu Nishinaga/Photo Researchers, Inc.; (r) ©David Scharf/Photo Researchers, Inc. Inc.; (r) ©David Scharf/Photo Researchers, Inc.; (c) ©Susumu Nishinaga/Photo Researchers, (l) ©Andrew Syred/Photo Researchers,

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Main Idea 5c CONNECT TO Stem cells can develop into different cell types. VOCABULARY Stem cells are a unique type of body cell that can (1) divide and renew them- Potent comes from a Latin word meaning “to be able.” selves for long periods of time, (2) remain undifferentiated in form, and (3) The addition of prefixes defines differentiate into a variety of specialized cell types. When a stem cell divides, it the level of power or ability. forms either two stem cells or one stem cell and one specialized cell. toti- = all pluri- = more, several multi- = many stem cell 2 new stem cells

1 new stem cell + 1 specialized cell

Biology IDEO Stem Cell Classification V CL IP Stem cells can be classified by their ability, or potential, to develop into the HMDScience.com differentiated cell types of different tissues. In general, the more differentiated a GO ONLINE stem cell already is, the fewer the types of cells it can form. Stem Cells • Totipotent stem cells can grow into any other cell type. Only a fertilized egg and the cells produced by the first few divisions of an embryo are totipotent. • Pluripotent stem cells can grow into any cell type except for totipotent stem cells. • Multipotent stem cells can grow only into cells of a closely related cell family. Stem cells are also classified by their origin, as either adult or embryonic. Adult stem cells have been studied for decades, but the ability to grow human embryonic stem cells was not developed until 1998. Since that time, embryonic stem cells have attracted great attention because of their potential to form almost any cell type. Adult Stem Cells Adult stem cells are partially undifferentiated cells located among the specialized cells of many organs and tissues. They are found all over the body, in the brain, liver, bone marrow, skeletal muscle, dental pulp, and even fat. These stem cells are also found in children and in umbilical cord blood, so the term somatic stem cell is more accurate although less frequently used. A major advantage of adult stem cells is that they can be taken from a patient, grown in culture, and put back into the patient. Thus, the risk of transplant rejection by a patient’s immune system is very low. This method also avoids many ethical issues associated with using embryonic stem cells. Adult stem cells currently pose many disadvantages as well. They are few in number, difficult to isolate, and sometimes tricky to grow. They may also contain more DNA abnormalities than do embryonic stem cells. For years, much evidence suggested that adult stem cells were multipotent. This would mean that a stem cell from fat would produce only fat cells, never muscle cells.

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Newer data suggest otherwise. Adult stem cells treated with the right combination of molecules may give rise to a completely different type of tissue. This process, called transdifferentiation, remains an active area of research. Embryonic Stem Cells Most embryonic stem cells come from donated embryos grown in a clinic. These embryos are the result of in vitro fertilization, a process by which eggs are fertilized outside a woman’s body. The stem cells are taken from a cluster of undifferentiated cells in the three-to-five-day-old embryo. These cells, called the inner cell mass, do not have the characteristics of any specific cell type. Because Figure 5.4 Harvesting Embryonic Stem cells they are pluripotent, they can form any of the 200 cell types of the body. They can also be inner cell mass grown indefinitely in culture. These qualities offer hope that many diseases will be treatable or even curable. Stem cells have long been used to treat patients with leukemia and lymphoma, fertilized egg and people with diabetes might someday be cured if nonworking cells in the pancreas are replaced with healthy, growing cells. Even damaged organs might be strengthened by an injection of healthy cells. Embryonic stem cells also have a downside. If these cells are used in treatment, a patient’s body might reject them as foreign material. A muscle cells red blood cells different possibility is that the stem cells could neurons grow unchecked in a patient’s body and form a First, an egg is fertilized by a sperm cell in a petri dish. The egg divides, tumor. The use of embryonic stem cells also forming an inner cell mass. These cells are then removed and grown raises many ethical questions. FIGURE 5.4 shows with nutrients. Scientists try to control how the cells specialize by the most common method of obtaining adding or removing certain molecules. embryonic stem cells. This method currently involves destruction of the embryo, which some people consider ethically unacceptable. Compare and Contrast List treatment benefits and risks of both types of stem cells. Self-check Online HMDScience.com 5.5 Formative Assessment GO ONLINE Reviewing Main Ideas Critical thinking CONNECT TO 1. How does communication between 4. Compare Describe how cells, Ethics in Biology cells help maintain homeostasis? tissues, organs, and organ systems 6. Explain which factor you 10c are related. 10c think is most important in 2. Why is cell differentiation an 5. Evaluate What role does the deciding whether stem cell important part of the development location of a cell in a developing research should be legal of a multicellular organism? embryo play in cell differentiation? and government-funded. 5C 3D 3. What are the defining characteristics of stem cells?

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CHAPTER G N I D A E R

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have justafter theSphase ofthecell cycle? somes initsnucleus. How manychromatids would it A A D important forimportant anorganism that reproduces asexually? environmental conditions. Whydoyou thinkthisis known to increase whenthey are understressed related to differentiation ofthat cell? (Hint: Thinkabouttheprocess ofosmosis.) how would thecells undergoing mitosis beaffected? formed cells and older cells? S S A tightly how to mightthecell DNA, cycle beaffected? P and risksthat could result from thissituation? them into new plants. What are some potential benefits budding potatoes andteach farmers how to separate vegetables to increase crop yields. Heplans to distribute A help maintain homeostasis? cells? Explain. A ynthesize ynthesize redict pply nal pply nalyze pply escribe yze

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races. Use the graph to answer the next two questions. of invasive cancers and includes males and females of all cancer from 1985 through 1997. This data is for all types expressed as percentages, of patients diagnosed with The graph below shows the five-year survival rate, A Making Connections 34. 35. 32. 33. 5- nalyzing

Y survival duringthespanoftime given? I for thedata? results would suggest. you would perform, andwhat thedifferent possible questions you would want to answer, what experiments Outline aplan for anexperiment to describe what mine whetheracertain substance was acarcinogen. labs for research uses. Suppose you wanted to deter- shown onthechapter opener, are frequently grown in D include all details oftheprocess andrelated terms. perspectives that illustrate what Besure ishappening. to first-person point ofview. Come upwithsounds or phase through mitosis. Becreative. Use humoranda Describe what happento will you from starting theS mosome aboutto undergo replication andmitosis. Write a A Relative survival rate (%) nterpr ear Canear nal esign an 60 68 66 64 50 58 56 70 54 62 52 yze

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Record your answers on a separate piece of paper. 4B, 5A MULTIPLE CHOICE 4 B 3A, 5D A 1 Scientists researching anticancer drugs treat a cell culture with a compound. Following treatment, they notice that the culture stopped growing. Untreated cells from the same culture, however, C have continued to grow. These results indicate that the compound blocks the normal cell cycle. In the diagram above, cell A is undergoing What else could have caused these results? mitosis. If cell A has 6 chromosomes, how many A The compound had degraded. chromosomes will cells B and C have? B The compound prevented cells from mutating. A none C The compound killed the treated cells. B 3 each D The compound had no effect. C 6 each D 12 each THINK THROUGH THE QUESTION The untreated cells serve as a control in this 10C experiment. Therefore, differences between the 5 treated and untreated cells should be the result of organ systems the drug. If the drug has no effect, the two groups X of cells should be the same. tissues

5C The figure above represents some levels of 2 Which describes the role of DNA in cell organization in multicellular organisms. Which differentiation? term fits in the box marked “X”? A It regulates cell differentiation. A cells B It carries the code from which each cell type B organelles expresses specific genes. C organs C It removes genes that are unneeded from D organisms differentiated cells. D It prevents a full set of DNA from being passed 5C on to stem cells. 6 Unlike stem cells, most body cells cannot form different types of cells. For example, skin cells can 5C only make skin cells, and nerve cells only make 3 One environmental factor that plays a key role in nerve cells. Which statement best explains why cell differentiation in multicellular organisms is skin cells will never become nerve cells? the location of the developing cell with respect to A Each type of cell gets a different message from other cells. In human development, from which the central DNA, which is stored in DNA cells. cell layer in the embryo will a nerve cell develop? B Each type of cell has only the part of the DNA A within the nucleus necessary for making that type of cell. B outer layer C Each cell type is determined by messages sent C middle layer from the brain, which directs development. D inner layer D Both types of cells have the same DNA, but each cell uses only part of the DNA message.

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Opinion Poll Practical Challenges A news program asks viewers to vote on- Strange Biolog y line: “Should stem cell research be banned? Yes or no?” Some people claim that stem cell therapy will revolutionize medicine. Others believe that some types of stem cell research violate ethical standards and are not justified by the potential benefits. Between these two positions exists a wide range of ideas about what is or is not acceptable. Would you know how to vote? ©Professor Miodrag Stojkovic/Photo Inc. Researchers,

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Using Stem Cells 3D • A better understanding of the properties of stem cells may give scientists more information about Stem cells are undifferentiated cells that can regener- how cancer cells replace themselves and thus help ate themselves and develop into specialized types of scientists develop more-targeted cancer therapies. cells. Stem cell research offers the hope of understanding basic cell processes and treating or even • Stem cells could be used to grow human tissues to curing many diseases. However, many technical test the effects of drugs and chemicals. challenges must be overcome before stem cell therapy • Stem cells may be used to replace healthy cells that is a realistic option, and ethical issues continue to are killed by radiation treatment for cancer. surround stem cell research. • Stem cells may be used to regenerate tissues. For example, chemotherapy kills blood-producing cells Potential Benefits 3D in bone marrow. To replace these cells, stem cells Stem cell research offers many potential benefits. could be used instead of the patient’s own marrow, • Studying adult stem cells may help scientists better which may contain cancer cells. understand how tissues develop and what goes • Stem cells may be used to treat spinal cord wrong when those tissues become diseased. injuries and neurodegenerative diseases, such as Parkinson’s. TECHNOLOGY

DNA body cell human Somatic Cell Nuclear Transfer from egg cell Somatic cell nuclear transfer (SCNT), also called therapeutic patient cloning, is a method for obtaining stem cells that has been used The egg to clone animals. The process is still under development, however, cell DNA and it has not yet been used to produce stem cells for humans. is extracted SCNT offers the hope of using a patient’s own DNA to produce and discarded. The DNA from the stem cells that can form many types of specialized cells. Many body cell is extracted SCNT studies have been done in mice and pigs; the diagram to the to be used, and the right shows how the SCNT process might be applied in human rest is discarded. cells. An unfertilized egg is taken from a female’s body, and the nucleus—containing the DNA—is removed. A cell is then heart cells red blood cells taken from a patient’s body. The nucleus is removed and spinal cord neurons inserted into the egg. cells insulin- producing The egg is given a mild electrical stimulation, which makes it cells divide. The DNA comes from the patient’s nucleus, and the materials needed for division come from the egg. An organ or tissue can be transplanted The stem cells could then be cultured and caused to into a patient without rejection.

differentiate into any tissue or organ needed by the patient. Once a stem cell line is established, in theory it can continue to grow indefinitely. Researchers could use these cell lines without having to harvest more stem cells. The cell lines also could be frozen and shipped to other researchers around the world.