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MODULE 9 Cellular Reproduction and Sexual Reproduction 1 Cellular Reproduction BEFORE YOU READ Think about the life cycle of a human

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MODULE 9 Cellular Reproduction and Sexual Reproduction 1 Cellular Reproduction BEFORE YOU READ Think about the life cycle of a human. On the lines below, write some of the stages that occur in the life cycle of a human. In this lesson, you will learn about the life cycle of a cell. WHAT YOU’LL LEARN • the stages of the cell cycle • the stages of interphase • the events of each stage of mitosis • the difference between a normal cell cycle and an abnormal cell cycle • the role of apoptosis READ TO LEARN Cell Size Limitations Most cells are smaller than the period at the end of this sentence. In this section, you will learn why cells are so small. ? FOCUS As you read, underline or highlight the main ideas in each paragraph. Copyright © McGraw-Hill Education What is the surface area to volume ratio? Recall that all cells are surrounded by a plasma membrane. All substances moving into or out of the cell must cross the plasma membrane. The surface area of the cell is the area covered by the plasma membrane. The volume of a cell is the space taken up by the inner contents. Because cells are small, their surface area is high in relation to their volume. This relationship is called the ratio of surface area to volume. To understand this ratio, imagine a cube-shaped bacterial cell with sides measuring one micrometer (µm) in length. Surface area can be calculated as length times width times the number of sides, or 1 µm × 1 µm × 6 sides = 6 µm2. Volume is length times width times height, or 1 µm × 1 µm × 1 µm, which equals 1 µm3. The ratio of surface area to volume of this cell is 6:1. What happens as the cell grows larger? Imagine a cube-shaped bacterial cell that is 2 µm per side. The surface area is now 24 µm2, and the volume is 8 µm3. The ratio of surface area to volume is 3:1. Notice that the ratio of the larger cell is lower than that of the smaller cell. APPLYING MATH 1. Explain Show the equations used to calculate 24 µm2 for the surface area and 8 µm3 for the volume of the larger cell. Reading Essentials • Cellular Reproduction and Sexual Reproduction 93 Get It? 2. Explain why a high ratio of surface area to volume benefits a cell. What are the benefits of a high surface area to volume ratio? As cells grow in size, the ratio of surface area to volume gets smaller. A low ratio means a cell might have trouble bringing nutrients into and moving wastes out of the cell. Small cells also have an easier time moving substances around inside the cell. Substances are moved by diffusion or by motor proteins pulling them along the cytoskeleton. Movement of substances over long distances is slow and difficult, so cells remain small to maintain efficiency. Cells use signaling proteins to communicate. Signaling proteins move around the cell to relay messages. In larger cells, communication becomes slow because signaling proteins have to move over longer distances. Chromosomes If a DNA strand 140 million nucleotides long was laid out in a straight line, it would be about five centimeters long. How does all of this DNA fit into a tiny cell? In prokaryotes, the DNA molecule is contained in the cytoplasm. It consists mainly of a ring of DNA and associated proteins. In eukaryotes, the DNA strand is wound up in a tight coil called a chromosome. What are chromatin and chromosomes? The Cell Cycle A cell’s cycle of growing and dividing is called the cell cycle. The cell cycle has three main stages. They are described in the table on the next page. During interphase, the cell grows, carries out cellular functions, and copies its DNA. Interphase is followed by mitosis (mi TOH sus), the period when the nucleus divides. Cytokinesis (si toh kih NEE sis) follows mitosis and is the stage when the cytoplasm divides and two cells are created. Reading Essentials • Cellular Reproduction and Sexual Reproduction 94 Copyright © McGraw-Hill Education DNA is found in the nucleus of eukaryotic cells. DNA in the nucleus can take two forms. Chromatin (KROH muh tun) is the relaxed form of DNA. DNA is not relaxed at all times. Chromosomes (KROH muh sohmz) are condensed structures that contain the DNA that are visible during mitosis. Chromosomes are passed from generation to generation of cells. Chromosomes are made of coiled chromatin. Chromatin is made of units of DNA coiled around protein, called nucleosomes. The time it takes a cell to complete the cell cycle varies depending on the type of cell. A typical animal cell takes 12–24 hours to complete the cell cycle. Some cells might complete the cycle in eight minutes. Other cells might take as long as a year to complete one cycle. Stage Description Interphase The cell grows in size, performs normal functions, and copies its DNA. Mitosis The cell nucleus divides, and the chromosomes separate into the two nuclei. Cytokinesis The cytoplasm of the cell divides, forming two daughter cells. Number of Cells one cell TAKE A LOOK 3. State Complete the table by writing the number of cells present at the end of each stage. The first one has been done for you. Copyright © McGraw-Hill Education What happens during interphase? The stages of the cell cycle are shown in the figure below. Most of the cell cycle is taken up by interphase. During interphase the cell grows, performs normal cell functions, and copies its DNA in preparation for cell division. Interphase is divided into three stages: G1, S, and G2, also called Gap 1, Synthesis, and Gap 2. After Gap 2, the cell begins mitosis. As soon as a cell is finished dividing, it enters the G1 stage of interphase. During this stage, a cell grows, performs normal cell functions, and prepares to copy its DNA. Some cells, such as muscle and nerve cells, exit the cycle at this stage and do not divide again. The second stage of interphase is the synthesis, or S stage. During this stage, the cell copies its DNA. The G2 stage is after the S stage. This is the period when the cell prepares for the division of its nucleus. A protein that makes microtubules for cell division is synthesized at this time. The cell also makes sure that it is ready to move forward in the cell cycle. When preparations are complete, the cell enters mitosis. Reading Essentials • Cellular Reproduction and Sexual Reproduction 95 TAKE A LOOK 4. Label Circle the stage of interphase during which DNA is copied. S—synthesis; DNA is replicated G2—Gap 2; cell prepares for mitosis G2 S Mitosis M Interphase (G1, S, G2) G1 Cytokinesis C G1—Gap 1; cell grows and performs normal functions What happens during mitosis? Get It? 5. Explain the role of mitosis in the processes of growth and repair. Recall that during interphase, the cell copies its DNA in preparation for cell division. Mitosis follows interphase. During mitosis the two identical copies of DNA separate. Each copy will become part of a new cell, called a daughter cell. Daughter cells are genetically identical because they each have the same DNA. Mitosis increases the number of cells as a young organism grows to its adult size. Mitosis also replaces damaged cells, such as skin cells that are damaged when you get a cut. What are the stages of mitosis? Reading Essentials • Cellular Reproduction and Sexual Reproduction 96 Copyright © McGraw-Hill Education Like interphase, mitosis is divided into stages. These four stages are prophase, metaphase, anaphase, and telophase. The figure on the next page shows the four stages of mitosis. Follow the diagram as you read about each stage. Copyright © McGraw-Hill Education What happens during prophase? The first and longest stage of mitosis is called prophase. Before prophase, DNA is in a relaxed, or unwound, form known as chromatin. During prophase, chromatin becomes tightly wound, or condenses, into chromosomes. During prophase, each chromosome is X-shaped. The left half of the X is one chromosome copy. The right half is an identical copy. Each half of the X is a sister chromatid containing identical copies of DNA. The sister chromatids are attached near the center of the chromosome by a structure called the centromere. The centromere ensures that a complete copy of the DNA becomes part of each daughter cell at the end of the cell cycle. TAKE A LOOK 6. Highlight the steps that occur in prophase. THINK IT OVER 7. Identify How are the sister chromatids attached? Reading Essentials • Cellular Reproduction and Sexual Reproduction 97 What happens at the end of prophase? Make a layered-look book using the titles shown. Use it to organize your notes on phases of mitosis. Mitosis Phases and Cytokinesis Prophase Metaphase Anaphase Telophase Cytokinesis As prophase continues, the nucleolus seems to disappear. Structures called spindle fibers form in the cytoplasm. In animal and protist cells, centrioles migrate to opposite ends, or poles, of the cell. Star-shaped aster fibers come out of the centrioles. Spindle fibers, centrioles, and aster fibers are all made of microtubules. These structures form the spindle apparatus which helps move and organize the chromosomes before cell division. Centrioles are not present in plant cells. As prophase ends, the nuclear envelope disappears. The spindle fibers attach to the sister chromatids of each chromosome on both sides of the centromere and attach to opposite poles of the cell. One spindle fiber connects the centromere to one pole of the cell. The other spindle fiber connects the centromere to the opposite pole of the cell. This ensures that each new cell gets one copy of the DNA. What happens during metaphase and anaphase? C09_002A-145262 THINK IT OVER 8. Describe What happens to the chromatids during metaphase? Get It? What happens during telophase? Telophase is the final stage of mitosis. During telophase, the chromosomes arrive at the poles of the cell and begin to change back into chromatin. Two new nuclear membranes form around each set of chromosomes, the nucleoli reappear, and the spindle apparatus is taken apart, as shown below. TAKE A LOOK 10. Identify What does the cell still need to do at the end of telophase? Reading Essentials • Cellular Reproduction and Sexual Reproduction 98 Copyright © McGraw-Hill Education 9. Summarize the role of the spindle apparatus during cell division. During the second stage of mitosis, metaphase, the chromatids are pulled by motor proteins along the spindle apparatus toward the center of the cell. The chromatids line up in the middle, or the equator, of the cell. If metaphase is completed successfully, each daughter cell will have a copy of each chromosome. During anaphase, the sister chromatids are pulled apart. The microtubules of the spindle apparatus shorten and pull at each centromere to separate into two identical chromosomes. At the end of anaphase, the microtubules move each identical chromosome toward the poles of the cell. What happens during cytokinesis? Near the end of mitosis, cytokinesis begins. During cytokinesis, the cytoplasm divides. The result of cytokinesis is two daughter cells, each with an identical nucleus. In animal cells, cytokinesis is accomplished by using microtubules to constrict, or pinch, the cytoplasm of the cell in half. The area where constriction occurs is called the furrow. The cell splits into two daughter cells. Furrow THINK IT OVER 11. Name t he cell structure that pinches the cytoplasm in half. TAKE A LOOK 12. Identify What features in the figure can you use to identify this cell as an animal cell? How is cytokinesis different in plant cells? Copyright © McGraw-Hill Education Plant cells complete cell division a different way. Recall that plant cells are surrounded by a rigid cell wall. During cytokinesis, plant cells form a new structure, called the cell plate, between the two daughter nuclei. New cell walls then form on either side of the cell plate, dividing the cell into two identical daughter cells. Reading Essentials • Cellular Reproduction and Sexual Reproduction 99 Cell Cycle Regulation The timing and rate of cell division are important to the health of an organism. The rate of cell division varies depending on the type of cell. A mechanism involving proteins and enzymes controls the cell cycle. The cell cycle in eukaryotic cells is controlled by a combination of two substances that signals the cellular reproduction process. Proteins called cyclin bind to enzymes called cyclin-dependent kinases (CDKs) in the stages of interphase and mitosis to trigger the various activities that take place in the cell cycle, as shown below. TAKE A LOOK 13. Name When are cyclin/ CDK combinations active? Is the DNA replicating correctly? Yes. Has all DNA been replicated? Yes. G2 S M C Is the DNA damaged? No. Are chromosomes properly attached to spindles? Yes. G1 14. Summarize the role of cyclins. How do cells use cyclin/CDK combinations? Cells use different combinations of cyclin/CDK to control activities at different stages in the cell cycle. For instance, in the G1 stage, one cyclin/CDK combination signals the start of the cell cycle. Other cyclin/CDK combinations signal other activities, including DNA replication, protein synthesis, and nuclear division. Cyclin/CDK combinations also signal the end of the cell cycle. Reading Essentials • Cellular Reproduction and Sexual Reproduction 100 Copyright © McGraw-Hill Education Get It? How do quality control checkpoints work? The cell cycle has built-in quality control checkpoints that monitor the cell cycle and can stop it if something goes wrong. For instance, near the end of the G1 stage of interphase, the cell monitors its DNA for damage and can stop the cell cycle before entering the S stage if something is wrong. There are other quality control checkpoints during the S stage and after DNA duplication in the G2 stage before entering mitosis. During mitosis, the cell checks the spindle fibers before it undergoes cytokinesis. If the cell detects a failure, the cell cycle stops. What is the role of apoptosis? THINK IT OVER Some cells in an organism are no longer needed. Apoptosis (a pup TOH sus) is a natural process of programmed cell death. Cells that are no longer needed are destroyed by apoptosis. Apoptosis occurs in the embryo to remove tissue between developing fingers and toes. Apoptosis also occurs in cells that are damaged beyond repair or that could turn into cancer cells. It is also part of the process by which leaves fall from trees in autumn. 15. Identify Which statement about apoptosis is correct? (Circle your answer.) a. Apoptosis can lead to cancer. b. Apoptosis can prevent cancer. How is cancer related to the cell cycle? THINK IT OVER 16. Define What is cancer? Copyright © McGraw-Hill Education Sometimes control of the cell cycle fails. When cells do not respond to control mechanisms, cancer results. Cancer is the uncontrolled growth and division of cells. Cancer cells grow and divide as long as they receive nutrients. They crowd normal cells causing tissues and organs to stop working. Cancer can kill an organism. Reading Essentials • Cellular Reproduction and Sexual Reproduction 101 What causes cancer? Cancer is caused by mutations, or changes, in segments of DNA that code for production of proteins, including those that regulate the cell cycle. Often, cells can fix mutations in DNA. If the repair system fails, cancer can result. Environmental factors can increase the risk of cancer. Substances that are known to cause cancer are called carcinogens (kar SIH nuh junz). Tobacco, tobacco smoke, alcohol, some viruses, and radiation from the Sun or X-rays are examples of carcinogens. Avoiding carcinogens can help reduce the risk of cancer. Federal laws protect people from exposure to carcinogens in the workplace and in the food supply. People can reduce their risk of cancer by avoiding all tobacco (including secondhand smoke and smokeless tobacco) and by using sunscreen to protect their skin from ultraviolet radiation from the Sun. Who can get cancer? Cancer can occur in people of all ages, but older people have a higher risk. This might be because it takes more than one DNA mutation to change an abnormal cell into a cancer cell. Older cells have had more time to accumulate the mutations that lead to cancer. Cancer runs in some families. People might inherit one or more DNA mutations from their parents, increasing their risk of developing cancer. Copyright © McGraw-Hill Education Reading Essentials • Cellular Reproduction and Sexual Reproduction 102 2 Meiosis and Sexual Reproduction BEFORE YOU READ Think about the traits that make people unique. Some people are tall, while others are short. People can have brown, blue, or green eyes. On the lines below, list a few traits that make you look different from other people. In this lesson, you will learn how meiosis rearranges genes. WHAT YOU’LL LEARN • the stages of meiosis • how meiosis provides genetic variation • how nondisjunction leads to abnormal chromosome numbers • why cellular differentiation is important • what stem cells are and how they can be used READ TO LEARN Copyright © McGraw-Hill Education Chromosome Numbers All students in your class have characteristics passed on to them by their parents. Each characteristic, such as hair color, eye color, and height, is called a trait. The instructions for each trait are found on chromosomes. Recall that chromosomes are found in the nuclei of cells. The DNA on the chromosomes is arranged in sections that control the production of proteins. These DNA sections are called genes. Each chromosome has hundreds of genes. Each gene has a role in the characteristics of the cell and how the cell works. Living things have thousands of genes. Human body cells have 46 chromosomes. Chromosomes come in pairs. You have 23 chromosomes from your father and 23 chromosomes from your mother, making 23 pairs of chromosomes. ? FOCUS After you have read this lesson, create a quiz based on what you have learned. After you have completed writing the quiz questions, be sure to answer them. APPLYING MATH 1. Calculate t he approximate number of genes humans have. Show your work. (Assume that each chromosome has 1500 genes.) Reading Essentials • Cellular Reproduction and Sexual Reproduction 103 What are homologous chromosomes? The chromosomes that make up a pair, one from each parent, are called homologous (huh MAH luh gus) chromosomes. Homologous chromosomes are the same length and have the centromere in the same place. They also carry genes for the same traits at the same place. Look at the picture below, and see if you can spot the homologous pair. Homologous chromosomes are similar but not identical. For example, the gene for ear shape will be located at the same place on each homologous chromosome. Although these genes code for ear shape, the gene on one chromosome might code for one ear shape. The gene on the other chromosome might code for a different ear shape. TAKE A LOOK 2. Identify Circle a pair of homologous chromosomes. How is chromosome number maintained in a species? THINK IT OVER How is sex determined? Each cell in your body, except for gametes, contains 46 chromosomes, or 23 pairs of chromosomes. One pair of these chromosomes, the sex chromosomes, determines an individual’s gender. There are two types of sex chromosomes—X and Y. Individuals with two X chromosomes are female, and individuals with an X and a Y chromosome are male. The other 22 pairs of chromosomes are called autosomes. Reading Essentials • Cellular Reproduction and Sexual Reproduction 104 Copyright © McGraw-Hill Education 3. Identify How many chromosomes are in a human gamete? (Circle your answer.) a. 46 b. 23 c. 10 The number of chromosomes does not change from generation to generation. You have the same number of chromosomes as your parents. Gametes (GA meets), or sex cells with half the number of C10_001A_896205 chromosomes, ensure the chromosome number stays the same. The symbol n represents the number of chromosomes. In humans, n is equal to 23. A cell with n number of chromosomes is called a haploid cell. Gametes are haploid cells. The process in which one haploid gamete joins with another haploid gamete is called fertilization. After fertilization, the cell has 2n chromosomes—n chromosomes from the female parent plus n chromosomes from the male parent. A cell with 2n chromosomes is called a diploid cell. Notice that n also represents the number of chromosome pairs in an organism. Meiosis I Recall that most cells are formed by mitosis. During mitosis the chromosome number stays the same. Because sex cells need half the number of chromosomes, a different process of cell division is needed. Gametes are formed during a process called meiosis. Meiosis is a kind of cell division that reduces the number of chromosomes by half through the separation of homologous chromosomes. Meiosis takes place in the reproductive organs of plants and animals. During meiosis, there are two cell divisions. They are called meiosis I and meiosis II. What happens during interphase? Just as in mitosis, a cell goes through interphase before undergoing meiosis. A cell in interphase carries out a variety of metabolic functions, copies its DNA, and makes proteins. Get It? 4. Distinguish between homologous chromosomes and sister chromatids. What happens during prophase I? Meiosis I begins with prophase I. During prophase I, replicated chromosomes, consisting of two sister chromatids, condense. When that happens, the chromosomes become visible under a light microscope. As the homologous chromosomes condense, they begin to form homologous pairs in a process called synapsis (suh NAP sus). The homologous chromosomes are held tightly together along their lengths by a protein that acts like a zipper. Prophase I continues as the chromosomes move to opposite sides of the cell. Copyright © McGraw-Hill Education What is crossing over? During synapsis, the chromosomes often swap pieces of DNA. Crossing over occurs when a section of one chromosome changes place with a section of its homologous chromosome. This is shown in the figure below. The centrioles move to the opposite poles of the cell. Spindle fibers form and bind to the sister chromatids at the centromere. TAKE A LOOK 5. Label Circle the part of each chromosome that was swapped during crossing over. Reading Essentials • Cellular Reproduction and Sexual Reproduction 105