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Section 1 Chromosomes Section 1 Focus Objectives Overview Before beginning this section review with your students the objectives listed in the Student Edition. In this section, students will learn that cells must divide in order to grow, replace worn-out cells, or reproduce asexually. It is vitally important that each new cell receives the proper set of chromosomes to function normally. Students will study the structure of chromosomes, the role of chromosomes, and learn that each organism has a characteristic number of chromosomes. Bellringer Write the words haploid, diploid, and zygote on the board. Ask students to write a single sentence that reflects an understanding of each term. (Answers will vary. For example: During fertilization, haploid gametes combine to form a diploid zygote.) Bio 6E Motivate Demonstration GENERAL Show the class two pictures or photos. One picture should show a person without any visible genetic disorders. The second picture should show a person with recognizable physical abnormalities due to a genetic disorder caused by the presence or absence of a chromosome (such as Klinefelter’s or Turner’s syndrome). Discuss with students how a single chromosome contains thousands of genes that code for proteins involved in determining how a person’s body develops and functions. LS Visual TAKS 2 Bio 6C; 6F pp. 118–119 Student Edition TAKS Obj 2 Bio 4B TAKS Obj 2 Bio 6A TEKS Bio 4A, 4B, 6A, 6B, 6E, 6F Teacher Edition TAKS Obj 2 Bio 6A, 6C TAKS Obj 3 Bio 4D TEKS Bio 4D, 6A, 6C, 6E, 6F, 11D 118 ● Identify four examples of cell division in eukaryotes and one example in TAKS 2 prokaryotes. 4A 4B 6E ● ● ● ● Formation of New Cells by Cell Division About 2 trillion cells are produced by an adult human body every day. This is about 25 million new cells per second! These new cells are formed when older cells divide. Cell division, also called cell Differentiate between reproduction, occurs in humans and other organisms at different a gene, a DNA molecule, times in their life. In Figure 1, the cells of the fawn that is growing a chromosome, and a and developing and the cells in the wound that is healing are underchromatid. 6A 6E TAKS 2 going cell division. The type of cell division differs depending on the Differentiate between organism and why the cell is dividing. For example, bacterial cells homologous chromosomes, undergoing reproduction divide by one type of cell division. Eukaryautosomes, and sex TAKS 2 otic organisms undergoing growth, development, repair, or asexual chromosomes. 6A 6E reproduction divide by a different type of cell division. And the forCompare haploid and mation of gametes involves yet a third type of cell division. Gametes 6E diploid cells. are an organism’s reproductive cells, such as sperm or egg cells. Predict how changes in Regardless of the type of cell division that occurs, all of the inforchromosome number mation stored in the molecule DNA (deoxyribonucleic acid) must or structure can affect be present in each of the resulting cells. Recall from Chapter 3 that 6C 6F TAKS 2 development. DNA stores the information that tells cells which proteins to make and when to make them. This information directs a cell’s activities Key Terms and determines its characteristics. Thus, when a cell divides, the gamete DNA is first copied and then distributed. Each cell ends up with a binary fission complete set (copy) of the DNA. gene chromosome chromatid centromere homologous chromosome diploid haploid zygote autosome sex chromosome karyotype Figure 1 Cell division The cells of these organisms are undergoing some type of cell division. Repair Growth and development 118 Planner CD-ROM • Reading Organizers • Reading Strategies • Basic Skills Worksheet Length, Area and Volume • Occupational Applications Medical Sonographer GENERAL Chapter 6 • Chromosomes and Cell Reproduction INCLUSION Strategies • Learning Disability • English as a Second Language Ask students to label three sheets of notebook paper with the titles of each section of the chapter, Chromosomes, The Cell Cycle, and Mitosis and Cytokinesis. Have students take notes as they read each section. The notes should be divided into three parts, key terms, notes, and questions. In a small group or with the teacher, students may share what they learned in each section and ask for clarification on their questions. Prokaryotic Cell Reproduction A prokaryote’s single DNA molecule is circular and is attached to the inner cell membrane. Prokaryotes reproduce by a type of cell division called binary fission. Binary fission is a form of asexual reproduction that produces identical offspring. In asexual reproduction, a single parent passes exact copies of all of its DNA to its offspring. Binary fission occurs in two stages: first, the DNA is copied (so that each new cell will have a copy of the genetic information), and then the cell divides. The prokaryote divides by adding a new cell membrane to a point on the membrane between the two DNA copies. As new material is added, the growing cell membrane pushes inward and the cell is constricted in the middle, like a long balloon being squeezed near the center. A new cell wall forms around the new membrane. Eventually the dividing prokaryote is pinched into two independent cells. Each cell contains one of the circles of DNA and is a complete functioning prokaryote. Real Life Escherichia coli cells can produce 1 million new cells in less than 7 hours. A variety of E. coli known as O157:H7 is sometimes found in raw or undercooked meat. When such meat is eaten, this bacteria can cause life-threatening intestinal bleeding and kidney failure. Thorough cooking is necessary to destroy the bacteria. Finding Information Research outbreaks of E. coli O157:H7 in your community or state. Eukaryotic Cell Reproduction The vast amount of information encoded in DNA is organized into units called genes. A gene is a segment of DNA that codes for a protein or RNA molecule. A single molecule of DNA has thousands of genes lined up like train cars. Genes play an important role in determining how a person’s body develops and functions. When genes are being used, the DNA is stretched out so that the information it contains can be used to direct the synthesis of proteins. As a eukaryotic cell prepares to divide, the DNA and the proteins associated with the DNA coil into a structure called a chromosome, as shown in Figure 2. Before the DNA coils up, however, the DNA is copied. The two exact copies of DNA that make up each chromosome are called chromatids (KROH muh tihdz). The two chromatids of a chromosome are attached at a point called a centromere. The chromatids, which become separated during cell division and placed into each new cell, ensure that each new cell will have the same genetic information as the original cell. Supercoil within chromosome Further coiling within supercoil DNA and proteins DNA double helix 119 Chapter Resource File • Lesson Plan GENERAL • Directed Reading • Active Reading GENERAL • Data Sheet for Quick Lab GENERAL READING SKILL BUILDER Interactive Reading Assign Chapter 6 of Holt Biology Guided Audio CD Program to help students achieve greater success in reading the chapter. Real Life Answer TAKS 3 Bio 4D; Bio 11D Answers will vary by community. The strain was first identified in 1982. Between 1992 and 1998, 30 outbreaks each year were reported in various communities, including areas in the Pacific Northwest; Alpine, Wyoming; Milwaukee, Wisconsin; and Indianapolis, Indiana. Demonstration Figure 2 Chromosome structure. A chromosome consists of DNA tightly coiled around proteins. The chromosomes are formed as a cell prepares to divide. Centromere Chromosome (made of 2 chromatids) Teach Transparencies TT Bellringer TT Chromosome Structure TT Chromosome Number of Various Organisms TT Karyotype Use a microprojector or microscope with a video camera to show the class a cross section of an onion root tip. The root grows because cells in the root tip undergo repeated mitotic divisions. Point out chromosomes that are visible. Tell students that before each root tip cell divides, the chromosomes are replicated and then divided equally into the two resulting cells. Explain that chromosomes consist of genes, which are segments of DNA. LS Visual TAKS 2 Bio 6A; Bio 6E READING SKILL BUILDER GENERAL Writing Reading Organizer Have students create a reading organizer to compare reproduction in bacterial cells and eukaryotic cells. A description of reproduction in eukaryotic cells is given in Sections 2 and 3. LS Verbal BIOLOGY • Unit 4—Cell Reproduction: Topics 1–4 This engaging tutorial introduces students to principles of chromosome replication and cell division Chapter 6 • Chromosomes and Cell Reproduction 119 How Chromosome Number and Structure Affect Development Teach, continued continued Teaching Tip GENERAL Missing Homologue Ask students to hypothesize what might happen if a human sperm or egg cell did not contain one member of each homologous pair. (The resulting zygote would not have a full set of chromosomes. The zygote might fail to develop. If the zygote did develop, the individual would not be normal because its cells would lack the important information contained in the missing genes.) www.scilinks.org Topic: Chromosomes Keyword: HX4042 Sets of Chromosomes Each of the 23 pairs of chromosomes consists of two homologous (hoh MAHL uh gus) chromosomes, or homologues (HOH muh logs). Homologous chromosomes are chromosomes that are similar in size, shape, and genetic content. Each homologue in a pair of homologous chromosomes comes from one of the two parents, as shown in Figure 3. Thus, the 46 chromosomes in human somatic cells are actually two sets of 23 chromosomes. One set comes from the mother, and one set comes from the father. A human chromosome is shown in Figure 4. TAKS 2 Bio 6C; Bio 6E SKILL BUILDER Each human somatic cell (any cell other than a sperm or egg cell) normally has two copies of 23 different chromosomes, for a total of 46 chromosomes. The 23 chromosomes differ in size, shape, and set of genes. Each chromosome contains thousands of genes that play important roles in determining how a person’s body develops and functions. For this reason, a complete set of all chromosomes is essential to survival. GENERAL Figure 3 Fertilization Math Skills Prokaryotic chromosomes are hundreds of times longer than the cell that contains them. For example, if a chromosome in an E. coli bacterium were fully extended, it would measure about 1 mm in length. The cell itself is only about 0.002 mm in length. Have students calculate how much longer E. coli’s chromosome is than the cell itself. (1 mm/0.002 mm 500 times longer than the cell) When haploid gametes fuse, they produce a diploid zygote. LS Logical Bio 4A Using the Figure Fertilization GENERAL Use Figure 3 to point out how fertilization restores the diploid number of chromosomes. Ask students why sexually reproducing organisms possess an even number of chromosomes in the diploid state. (If students do not realize that 2n must be an even number, then ask them to calculate the haploid number in each gamete as if the diploid number in humans were 47 and not 46. It cannot be done without a “half” chromosome.) LS Visual Bio 6E pp. 120–121 Student Edition TAKS Obj 2 Bio 4B TAKS Obj 2 Bio 6A TEKS Bio 4B, 6A Teacher Edition TAKS Obj 1 Bio/IPC 2C TAKS Obj 2 Bio 6C TEKS Bio 3D, 3F, 4A, 6C, 6E, 6F TEKS Bio/IPC 3C 120 Egg cell n = 23 Sperm cell n = 23 Zygote 2n = 46 120 Cultural Awareness Mormons and Gene Mapping The Human Genome Project is an effort to map the over 100,000 genes in human cells to each chromosome and to sequence each gene. Scientists throughout the world are now engaged in this effort. One group has traced genetic markers through three generations of 60 Mormon families living in Utah. Their work with the Mormon families has resulted in the mapping of almost 500 genes. Bio/IPC 3C Chapter 6 • Chromosomes and Cell Reproduction IPC Benchmark Mini Lesson Biology/IPC Skills TAKS 1 Bio/IPC 2C Organize, analyze, evaluate, make inferences, and predict trends from data. Activity Have students imagine several possible uses for the information gained by the Human Genome Project. Have students write an essay describing their ideas and how they feel about research on the human genome. All of the cells in the body, other than gametes, are somatic cells. When a cell, such as a somatic cell, contains two sets of chromosomes, it is said to be diploid (DIHP loyd). Unlike somatic cells, human gametes contain only one set of chromosomes (23 total). When a cell, such as a gamete, contains one set of chromosomes, it is said to be haploid (HAP loyd). Biologists use the symbol n to represent one set of chromosomes. The haploid number in a human gamete can be written as n 23. The diploid number in a somatic cell can be written as 2n 46. The fusion of two haploid gametes— a process called fertilization—forms a diploid zygote, as shown in Figure 3. A zygote (ZY goht) is a fertilized egg cell, the first cell of a new individual. As seen in Table 1, each organism has a characteristic number of chromosomes. The number of chromosomes in cells is constant within a species. Fruit flies, for example, have only eight chromosomes in each cell. Although most species have different numbers of chromosomes, some species by chance have the same number. For example, potatoes, plums, and chimpanzees all have 48 chromosomes in each cell. Many plants have far more chromosomes. Some ferns have more than 500. A few kinds of organisms— such as the Australian ant Myrmecia, the plant Haplopappus (a desert relative of the sunflower), and the fungus Penicillium (from which the antibiotic penicillin is obtained)—have only one pair of chromosomes. Magnification: 12,542ⴛ Teaching Tip Figure 4 Human chromosome. As many as 500 chromosomes lined up end to end would fit in a 0.2 cm space—about the thickness of a nickel. The chromosome above has replicated and consists of two identical chromatids. Bio 6E Activity Table 1 Chromosome Number of Various Organisms Organism 1–4 Saccharomyces (yeast) 18 Mosquito 6 Housefly 12 Garden pea 14 Corn 20 Adder’s tongue fern 1,262 Frog 26 Human 46 Orangutan 48 Dog 78 GENERAL Karyotypes Obtain copies of normal human karyotypes from the library or on-line resources. Enlarge them so that each chromosome is no smaller than 2.5 cm (1 in.) in length. Paste the copies of the karyotype on a poster board. Cut out each chromosome and mix up the pieces. Allow the students to sort the chromosomes into pairs using size, length, position of centromere, and banding patterns. Have them compare the karyotypes with each other and with the karyotype shown in Figure 5. English Language Number of chromosomes Penicillium Prefixes Have students use the prefixes in the words haploid and diploid to help them remember the meanings of these words. Have them relate the words to the algebraic terms n and 2n. Haploid (n) represents one set of chromosomes, as seen in gametes. Diploid (2n) represents two sets of chromosomes, as in somatic cells. Tell students that the prefix haplmeans “single” and that the prefix dipl- means “double.” Add that the prefix poly- means “many.” Then have students predict the nature of a polyploid cell. (It would have multiple sets of English Language Learners chromosomes.) LS Logical Learners Bio 6F 121 REAL WORLD CONNECTION Invite a genetic counselor to speak to the class. Before the speaker attends, have the students research chromosomal abnormalities that result from too few or too many chromosomes. Have the students prepare a list of questions based on their findings. Ask the speaker to bring sample karyotypes of the abnormalities the students researched, if possible. LS Interpersonal Bio 3D, 6F HISTORY CONNECTION In the 1920s, a biologist reported the diploid number in human cells to be 48. For many years following this report, textbooks described the normal diploid number of human chromosomes as 48. It was not until 1956 that J. H. Tjio and A. Levan showed 46 to be the correct number. These two scientists worked out a procedure to culture white blood cells. Their methods increased the number of cells undergoing mitosis and improved the spreading of the chromosomes, which allowed the scientists to accurately count the chromosomes. Bio 3F, Bio/IPC 3C Chapter 6 • Chromosomes and Cell Reproduction 121 Sex Chromosomes Teach, continued continued SKILL BUILDER GENERAL Math Skills Combined, the 22 somatic chromosomes and one sex chromosome consist of 3,000,000,000 DNA nucleotide pairs. Ask students how many total nucleotides are in a human diploid cell? (6,000,000,000) LS Logical The word chromosome is from the Greek chroma, meaning “color,” and soma, meaning “body.” Chromosomes were so named because they absorbed a colored dye that made them more visible under a microscope. TAKS 2 6C, 6D On the Trail of a Bio (grade 10 only) Chromsomal Deletion Teaching Strategies Researchers have found that life threatening defects in the aorta can result when a patient has two defective copies of the gene Tbx1. Patients with one normal and one defective copy of Tbx1 usually experience less serious abnormalities. Discussion How did researchers narrow the search from 25 to 15 genes? (They found out that the chromosome in mice with the same disorder only contained 15 genes.) Researchers now know that the gene Tbx1 controls other genes. What types of genes might it control? (those involved in development of the heart, blood, face and immune system) Of the 23 pairs of chromosomes in human somatic cells, 22 pairs are called autosomes. Autosomes are chromosomes that are not directly involved in determining the sex (gender) of an individual. The sex chromosomes, one of the 23 pairs of chromosomes in humans, contain genes that will determine the sex of the individual. In humans and many other organisms, the two sex chromosomes are referred to as the X and Y chromosomes. The genes that cause a fertilized egg to develop into a male are located on the Y chromosome. Thus, any individual with a Y chromosome is male, and any individual without a Y chromosome is female. For example, in human males, the sex chromosomes are made up of one X chromosome and one Y chromosome (XY). The sex chromosomes in human females consist of two X chromosomes (XX). Because a female can donate only an X chromosome to her offspring, the sex of an offspring is determined by the male, who can donate either an X or a Y. The structure and number of sex chromosomes vary in different organisms. In some insects, such as grasshoppers, there is no Y chromosome—the females are characterized as XX and the males are characterized as XO (the O indicates the absence of a chromosome). In birds, moths, and butterflies, the male has two X chromosomes and the female has only one. Change in Chromosome Number Each of an individual’s 46 chromosomes has thousands of genes. Because genes play an important role in determining how a person’s body develops and functions, the presence of all 46 chromosomes is essential for normal developFigure 5 A human karyotype ment and function. A person must Karyotypes are used to examine an individual’s chromosomes. have the characteristic number of To prepare a karyotype, chromosomes in his or her cells. photographs of the Humans who are missing even one chromosomes are cut out, arranged in pairs from largest of the 46 chromosomes do not surto smallest, and numbered. vive. Humans with more than two copies of a chromosome, a condition called trisomy (TRY soh mee), will not develop properly. Abnormalities in chromosome number can be detected by analyzing a karyotype (KAR ee uh tiep), a photo of the chromosomes in a dividing cell that shows the chromosomes arranged by size. Figure 5 shows a typical karyotype. A portion of a karyotype from an individual with an extra copy of chromosome 21 is also shown in Figure 5. This condition is called People with Down syndrome have Down syndrome, or trisomy 21. three copies of chromosome 21 in Short stature, a round face with their karyotype. 122 IPC Benchmark Fact pp. 122–123 Student Edition TAKS Obj 2 Bio 4B TAKS Obj 2 Bio 6A TAKS Obj 2 Bio 6C, 6D TEKS Bio 4A, 4B, 6A, 6C, 6D, 6E, 6F Teacher Edition TAKS Obj 2 Bio 6B TAKS Obj 2 Bio 6C, 6D TAKS Obj 5 IPC 5B TEKS Bio 3D, 3F, 6C, 6D 122 Tell students the colored dye that makes chromosomes more visible is the result of the reflection of light. Indeed it is the bouncing back of waves to our eyes that makes any object visible. Although there is a tendency to think of reflection as images we can see in a mirror, any object that we can observe with our eyes, whether or not that image is aided with a scientific instrument, is caused by light that bounces back or is reflected from an object. In this instance, the dye increases the amount of reflected light thus making the chromosomes more visible. TAKS 5 IPC 5B (grade 11 only) Chapter 6 • Chromosomes and Cell Reproduction Career GENERAL Cytogenetic Technologist Cytogenetic technologists aid physicians in diagnosis of genetic disorders by preparing specimens, such as karyotypes. Have students use library references or on-line resources to research the training requirements and duties of a cytogenetic technologist. (Technologist may prepare cells, prepare slides in the correct mitotic stage, stain slides, and prepare accurate karyotypes from photographic prints or computer images.) Bio 3D upper eyelids that cover the inner corners of the eyes, and varying degrees of mental retardation are characteristics of people with Down syndrome. In mothers younger than 30, Down syndrome occurs in about 1 in 1,500 births. In mothers 37 years old, the incidence doubles to 1 in 290 births. In mothers over 45, the risk is as high as 1 in 46 births. Older mothers are more likely to have a baby with Down syndrome because all the eggs a female will ever produce are present in her ovaries when she is born, unlike males who produce new sperm throughout adult life. As a female ages, her eggs can accumulate an increasing amount of damage. Because of this risk, a pregnant woman over the age of 35 may be advised to undergo prenatal testing that includes fetal karyotyping. What events can cause an individual to have an extra copy of a chromosome? When sperm and egg cells form, each chromosome and its homologue separate, an event called disjunction (dihs JUHNK shuhn). If one or more chromosomes fail to separate properly—an event called nondisjunction—one new gamete ends up receiving both chromosomes and the other gamete receives none. Trisomy occurs when the gamete with both chromosomes fuses with a normal gamete during fertilization, resulting in offspring with three copies of that chromosome instead of two. In Down syndrome, nondisjunction involves chromosome 21. Teaching Tip TAKS 2 Bio 6C; Bio 6D (grade 10 only) www.scilinks.org Topic: Genetic Disorders Research in Texas Keyword: HXX4008 O A Faulty Chromosome Karyotypes of people with DiGeorge syndrome show that they have one normal and one faulty 22nd chromosome. The faulty chromosome is missing a small region that contains 25 genes. To understand how this chromosomal deletion results in Finding the Crucial Gene Using a technology called chromosome engineering, the Baylor TAKS 2 Bio 6C; Bio 6D (grade 10 only) Teacher’s Notes Cut note card pieces prior to the lab. TAKS 2 DiGeorge syndrome, researchers at Baylor College of Medicine in Houston have been studying the disorder in mice. First, the researchers found that they could produce similar heart defects in mice by deleting a part of mouse chromosome 16. When these mice were bred with mice that had a duplication of the same part of chromosome 16, their offspring had no heart defects. Because the deleted part contained only 15 genes, the search for the cause of DiGeorge syndrome was narrowed from 25 genes to 15 genes. Modeling Chromosomal Mutations Skills Acquired Modeling, sequencing, interpreting On the Trail of a Chromosomal Deletion ne of every 4,000 babies is born with a genetic disorder called DiGeorge syndrome. This disorder causes serious heart defects that must be surgically corrected within a few days after birth. Children born with DiGeorge syndrome can also have blood ailments, facial abnormalities, a deficient immune system, and other problems. GENERAL Everyone Needs an X Tell students that most nondisjunctions involving autosomes are lethal, whereas most involving sex chromosomes are not. However, a YO combination in humans is lethal, leading biologists to suspect that at least one X chromosome is necessary for development and survival. Answers to Analysis researchers eventually zeroed in on a gene called Tbx1. They showed that deleting Tbx1 on one chromosome 16 in mice causes the heart defects of DiGeorge syndrome. Tbx1 is also required for the development of other embryonic structures besides the heart. Thus, the results of research on DiGeorge syndrome may provide clues about the genetic causes of other birth defects. Answers will vary based on the type of mutation: deletion: cell would be missing a gene, which could prove fatal; duplication: cell would have an extra gene, which could prove fatal or result in malfunctioning of the cell; inversion: cell may not be able to use gene because it is located in a different area on the chromosome, which could prove fatal; translocation: cell may not be able to use the gene because it is located on a different chromosome, which could prove fatal. 123 Cultural Awareness Cultural Attitudes Toward Genetic Defects From the Middle Ages through the nineteenth century, most Europeans believed that genetic defects reflected inner corruption. However, many other cultures, including the Celtic people of Europe and Native Americans, thought individuals with such defects had a special insight and a closer connection with nature. These unique members of the community were given responsibility as tribal leaders or healers. Many physically disabled leaders made careful astronomical observations of the sun and moon that helped them advise their communities on the optimum times for planting, harvesting, or migrating to another area. Bio 3F, TAKS 2 Bio 6C Chapter 6 • Chromosomes and Cell Reproduction 123 Change in Chromosome Structure Changes in an organism’s chromosome structure are called mutations. Breakage of a chromosome can lead to four types of mutations. In a deletion mutation, a piece of a chromosome breaks off completely. After cell division, the new cell will lack a certain set of genes. In many cases this proves fatal to the zygote. In a duplication mutation, a chromosome fragment attaches to its homologous chromosome, which will then carry two copies of a certain set of genes. A third type of mutation is an inversion mutation, in which the chromosome piece reattaches to the original chromosome but in a reverse orientation. If the piece reattaches to a nonhomologous chromosome, a translocation mutation results. Close Reteaching Ask students to differentiate each of the following terms by defining and sketching them: chromosomes, chromatids, DNA, and genes. Students should label their sketches. (chromosomes: coiled DNA and associated proteins; chromatids: identical copies of a given chromosome; DNA: chain of nucleic acids containing genetic code; genes: sequences of DNA that code for specific proteins or RNA.) English Language LS Visual Modeling Chromosomal Mutations You can use paper and a pencil to model the ways in which chromosome structure can change. Procedure 1. Write the numbers 1–8 on note-card pieces (one number per piece). Tape the pieces together in numerical order to model a chromosome with eight genes. GENERAL True or False: 1. At cell division, each chromosome consists of two chromatids attached at the centromere. (True) 2. The normal diploid number for humans is 23. (False) 3. A person with the sex chromosomes XX would be female. (True) GENERAL Have each student prepare a complete human karyotype based on Figure 5. Each chromosome should be drawn and then cut out. Allow students to choose the sex of their karyotype and determine whether there are mutations, deletions, or extra chromosomes. Have students exchange their diagrams with a partner. Each student must now match homologues and determine the sex of the individual, as well as identify any possible abnormalities. LS Interpersonal TAKS 2 Bio 6C, Bio 6D (grade 10 only); Bio 6F pp. 124–125 Student Edition TAKS Obj 2 Bio 4B TAKS Obj 2 Bio 6A, 6D TAKS Obj 2 Bio 6C TEKS Bio 4A, 4B, 6A, 6C, 6E, 6F Teacher Edition TAKS Obj 2 Bio 4B, 6A, 6B, 6C, 6D TEKS Bio 4A, 4B, 6A, 6B, 6C, 6D, 6E, 6F 124 3 4 5 Original chromosome 14 note-card pieces, pencils, tape Learners Alternative Assessment 2 Materials TAKS 2 Bio 6A; Bio 4A Quiz 1 TAKS 2 Bio 6D (grade 10 only) 6C 2. Use the “chromosome” you made to model the four alterations in chromosome structure discussed on this page and illustrated at right. For example, remove the number 3 and reconnect the remaining chromosome pieces to represent a deletion. 1 3. Reconstruct the original chromosome before modeling a duplication, an inversion, and a translocation. Use the extra note-card pieces to make the additional numbers you need. Describe how a cell might be affected by each mutation if the cell were to receive a chromosome with that mutation. 4 5 Deletion 1 2 1 2 3 4 5 Duplication 1 Analysis 2 2 4 3 5 Inversion 1 2 9 3 4 5 Translocation Section 1 Review Summarize how prokaryotic cells divide by 4A 4B 6E binary fission. Critical Thinking Evaluating Conclusions Do you agree or disagree that homologous chro6A 6E mosomes are found in gametes. Explain. Identify the point in a eukaryotic cell cycle TAKS Test Prep How does the karyotype of a person with Down syndrome differ from a normal 4B 6F karyotype? A It lacks a chromosome. B It has two sex chromosomes. C It has a damaged chromosome. D It has an extra copy of a chromosome. at which DNA coils up to form 6A 6E chromosomes. Summarize the difference between a haploid 6E cell and a diploid cell. 124 Answers to Section Review 1. DNA is first copied. Then the cell divides into equal halves by adding new cell membrane between the two DNA copies. The growing cell membrane pushes inward, and the cell constricts to form two new, identical cells. A new cell wall forms around each new membrane. TAKS 2 Bio 4B 2. Chromosomes become visible in a eukaryotic cell when the cell prepares to divide (during prophase). TAKS 2 Bio 6A 3. A haploid cell (n) contains one set of chromosomes. A diploid cell (2n) contains two sets of chromosomes. Bio 6E Chapter 6 • Chromosomes and Cell Reproduction 4. Students should disagree. Homologous chromosomes are pairs of similar chromosomes. Because gametes are haploid (n) they contain only one set of chromosomes. Thus, homologous chromosomes are not normally found in gametes. TAKS 2 Bio 6A 5. A. Incorrect. Lacking a chromosome is usually a fatal abnormality. B. Incorrect. All normal individuals have two sex chromosomes. C. Incorrect. Damaged chromosomes can lead to death or abnormalities, but do not cause Down syndrome. D. Correct. People with Down syndrome have an extra copy of chromosome 21. TAKS 2 Bio 4B The Cell Cycle Section 2 Section 2 Focus The Life of a Eukaryotic Cell Objectives Cell division in eukaryotic cells is more complex than cell division in bacteria because it involves dividing both the cytoplasm and the chromosomes inside the nucleus. Many internal organelles must be correctly rearranged before the eukaryotic cell can properly divide and form two fully functioning cells. The Cell Cycle The life of a eukaryotic cell is traditionally shown as a cycle, as illustrated in Figure 6. The cell cycle is a repeating sequence of cellular growth and division during the life of an organism. A cell spends 90 percent of its time in the first three phases of the cycle, which are collectively called interphase. A cell will enter the last two phases of the cell cycle only if it is about to divide. The five phases of the cell cycle are summarized below: ● Describe how the cell cycle is controlled in TAKS 2 eukaryotic cells. 4B 6E ● Relate the role of the cell cycle to the onset 4B 6C of cancer. 3. Second growth (G2) phase. In the G2 phase, preparations are made for the nucleus to divide. Hollow protein fibers called microtubules are assembled. The microtubules are used to move the chromosomes during mitosis. 4. Mitosis. The process during cell division in which the nucleus of a cell is divided into two nuclei is called mitosis (mie TOH sihs). Each nucleus ends up with the same number and kinds of chromosomes as the original cell. 5. Cytokinesis. The process during cell division in which the cytoplasm divides is called cytokinesis (SIET oh kih nee sihs). Mitosis and cytokinesis produce new cells that are identical to the original cells and allow organisms to grow, replace damaged tissues, and, in some organisms, reproduce asexually. TAKS 2 Key Terms cell cycle interphase mitosis cytokinesis cancer 1. First growth (G1) phase. During the G1 phase, a cell grows rapidly and carries out its routine functions. For most organisms, this phase occupies the major portion of the cell’s life. Cells that are not dividing remain in the G1 phase. Some somatic cells, such as most muscle and nerve cells, never divide. Therefore, if these cells die, the body cannot replace them. 2. Synthesis (S) phase. A cell’s DNA is copied during this phase. At the end of this phase, each chromosome consists of two chromatids attached at the centromere. Overview ● Identify the major events that characterize each of the five phases of the cell cycle. 4B 6E TAKS 2 Bellringer Figure 6 The eukaryotic cell cycle. The cell cycle consists of phases of growth, DNA replication, preparation for cell division, and division of the nucleus and cytoplasm. INTE RP HA SE S • Lesson Plan GENERAL • Directed Reading • Active Reading GENERAL Ask students to finish the following sentence: A typical eukaryotic cell spends 90 percent of its time in ________. Write the following choices on the board: mitosis, anaphase, interphase, and cytokinesis. (interphase) Bio 4A Motivate (DNA synthesis) G1 Demonstration (Cell growth) G2 (Growth and preparation for mitosis) Cytokinesis Mitosis 125 Chapter Resource File Before beginning this section review with your students the objectives listed in the Student Edition. In this section, students will examine the three phases that take place in the life cycle of a cell, which are collectively known as the cell cycle. They will learn that cells spend most of their time in interphase, during which cells grow and DNA is replicated. The cell enters mitosis as it prepares to divide, then divides into two cells during cytokinesis. Students will learn that cancer may result if the controls for the cell cycle break down. Planner CD-ROM • Reading Organizers • Reading Strategies • Portfolio Project Genetics Project GENERAL Display either a model or a photograph of a human brain. Point out that once the brain is fully formed, most of the nerve cells do not divide, again. These cells remain in the G1 phase of the cell cycle. Then display either a model or a photograph of a human bone. Point out that red blood cells are produced from cells in the marrow of long bones. An average red blood cell lives for about 120 days. Each second, about 2 million red blood cells are produced by cell division in the bone marrow. Cells in the marrow, unlike those in the brain, continue going through the cell cycle as long as a person lives. TAKS 2 Bio 4B Transparencies TT Bellringer Chapter 6 • Chromosomes and Cell Reproduction 125 Control of the Cell Cycle Teach Teaching Tip GENERAL Interphase Have students draw a Graphic Organizer like the one below to summarize the events that occur during the phases of interphase. Have them use arrows between the phases, pointing to the right, to indicate the sequence of events. TAKS 2 Bio 4B; Bio 3E Teaching Tip If a cell spends 90 percent of its time in interphase, how do cells “know” when to divide? How is the cycle controlled? Just as traffic lights control the flow of traffic, cells have a system that controls the phases of the cell cycle. Cells have a set of “red light–green light” switches that are regulated by feedback information from the cell. The cell cycle has key checkpoints (inspection points) at which feedback signals from the cell can trigger the next phase of the cell cycle (green light). Other feedback signals can delay the next phase to allow for completion of the current phase (yellow or red light). The cell cycle in eukaryotes is controlled by many proteins. Control occurs at three principal checkpoints, as shown in Figure 7. Reviewing Information Learn the stages of interphase by reviewing the steps numbered 1–5 on the previous page. You can see in Figures 6 and 7 that the cell cycle is a repeating series of three steps followed by mitosis and cytokinesis. 1. Cell growth (G1) checkpoint. This checkpoint makes the decision of whether the cell will divide. If conditions are favorable for division and the cell is healthy and large enough, certain proteins will stimulate the cell to begin the synthesis (S) phase. During the S phase, the cell will copy its DNA. If conditions are not favorable, cells can typically stop the cell cycle at this checkpoint. The cell cycle will also stop at this checkpoint if the cell needs to pass into a resting period. Certain cells, such as some nerve and muscle cells, remain in this resting period permanently and never divide. GENERAL Busy Cells Remind students that during interphase, a cell is not only growing, it is also producing the proteins and carrying out the functions that are characteristic of that type of cell. TAKS 2 Bio 4B Group Activity Writing Cell Cycle Phases Have students read Section 2, then write all the activities and events described during the cell cycle on index cards (one event per card). Draw a large version of Figure 6 at the front of the room. Then collect the students’ cards. Allow one student at a time to come to the front of the room, read a card out loud, and then tape the card to the proper phase of the cell cycle. LS Verbal TAKS 2 Bio 4B 2. DNA synthesis (G2) checkpoint. DNA replication is checked at this point by DNA repair enzymes. If this checkpoint is passed, proteins help to trigger mitosis. The cell begins the many molecular processes that are needed to proceed into mitosis. Figure 7 Control of the cell cycle. The cell cycle in eukaryotes is controlled at three inspection points, or checkpoints. Many proteins are involved in the control of the cell cycle. G1 checkpoint 3. Mitosis checkpoint. This checkpoint triggers the exit from mitosis. It signals the beginning of the G1 phase, the major growth period of the cell cycle. When Control Is Lost: Cancer INTE RP HA S Certain genes contain the information necessary to make the proteins that regulate cell growth and division. If one of these genes is mutated, the protein may not function, and regulation of cell growth and division can be disrupted. Cancer, the uncontrolled growth of cells, may result. Cancer is essentially a disorder of cell division. Cancer cells do not respond normally to the body’s control mechanisms. Some mutations cause cancer by overproducing growth-promoting molecules, thus speeding up the cell cycle. Others cause cancer by inactivating the control proteins that normally act to slow or stop the cell cycle. E S G1 G2 Cytokinesis Mitosis G2 checkpoint Mitosis checkpoint 126 Graphic Organizer Use this graphic organizer with Teaching Tip: Interphase on this page. G1 pp. 126–127 Student Edition TAKS Obj 2 Bio 4B TAKS Obj 2 Bio 6C TEKS Bio 4A, 4B, 6C, 6E, 6F Teacher Edition TAKS Obj 2 Bio 4B, 6C TEKS Bio 3E, 4B, 6C 126 Cell growth Chapter 6 • Chromosomes and Cell Reproduction Interphase S DNA synthesis G2 Growth and preparation for cell division Exploring Further TAKS 2 Cancer Although all cancers are not curable, great progress has been made in cancer research over the last 30 years. We now know that cancer results from damage to a small set of genes that, in normal cells, limits the ability of cells to divide. What causes this damage? Certain environmental factors appear to be associated with cancer. For example, the incidence of cancer per thousand people is not uniform throughout the United States. Rather, it is higher in cities and in the Mississippi delta, suggesting that pollution and pesticide runoff may contribute to cancer. When pollutants, radiation, and other environmental factors associated with cancer are analyzed, a clear pattern emerges. Most cancer-causing agents are powerful mutagens—that is, they readily damage DNA. The conclusion that cancer is caused by mutation of a cell’s DNA is now supported by a very large body of evidence. How many mutations are required to produce cancer? Research in the last several years indicates that mutation of only a few genes can transform normal cells into cancerous ones. All of these cancer-causing genes are involved with regulating how fast cells grow and divide. How is cell division regulated? As a crude analogy, imagine a car parked on the side of a road. To get it going, you must step on the accelerator and release the brake. Stepping on the Accelerator A cell divides when it receives a signal to do so. A “divide” signal is usually in the form of a chemical substance released by another cell. The substance is bound by a protein on the surface of the receiving cell. This binding activates a second protein inside the cell—relaying the signal from the outside of the cell to the inside. Here, a family of proteins then relay the signal inward to the nucleus. One protein molecule passes the signal to the next like a baton in a relay race. The genes for these signal-carrying proteins are called oncogenes (onco is Greek for “mass” or “tumor.”). If oncogenes are changed by mutation to become more active, cancer can result. Like stepping on Melanoma cells the accelerator of a car, an increase in the activity of these proteins amplifies the “divide” signal. This causes the cell to divide more often. Understanding Cancer Releasing the Brakes At the nucleus, the divide signal overrides a set of genes that act as “brakes.” These braking genes—called tumor supressor genes—prevent cell division from occurring too often. In cancer, these tumor suppressor genes are damaged. Like removing pressure from the brakes of a car increases a car’s speed, decreasing the activity of tumor suppressors speeds up cell division. Cells have three kinds of tumor suppressors, all of which must be disabled before cancer can occur. First, cells have proteins that inhibit DNA replication for limited periods. In cancer cells they are permanently inactivated. Second, cells have errorcorrecting proteins that detect damage to genes. In most cancers this error-detection has been disabled. Third, cancer cells rebuild the tips of their chromosomes. A little is lost from the ends of chromosomes at each replication, limiting the number of www.scilinks.org times a normal cell Topic: Cancer Cells can divide. Adding Keyword: HX4030 the deleted material back to the tips removes this limit to a cell’s life span. Section 2 Review Differentiate between the G1, G2, and S phases of the eukaryotic cell cycle. 4B 6E Relate what occurs at each of the three principal checkpoints in the cell cycle. 4B 6E Critical Thinking Evaluating Information TAKS 2 Bio 4B Reteaching Assign students to groups. Have each group visually interpret a stage in the cell cycle. Have each group display their graphics in the appropriate space in a circle graph that represents the cell cycle. TAKS Test Prep In the cell cyle of typical cancer cells, mutations have caused 4B 6C A slower growth. C uncontrolled growth. B a failure in mitosis. D a halt in cell division. Quiz GENERAL 1. The three phases of the cell cycle are interphase, mitosis, and ________. (cytokinesis) TAKS 2 Bio 4B 2. When normal control of the cell 3. Chromosomes are formed right before a cell divides in mitosis. During most of interphase, the DNA exists as chromatin, which is more elongated and harder to see with a microscope. TAKS 2 Bio 4B 2. Cell growth checkpoint (G1): determines if cell is ready to undergo division; DNA synthesis checkpoint (G2): DNA repair enzymes check progression; mitosis checkpoint: triggers exit from mitosis at the beginning of the G1 cycle. Close LS Visual Co-op Learning Answers to Section Review 1. G1 phase: cell grows rapidly and carries out routine functions; G2 phase: mitochondria and other organelles replicate, microtubules are assembled; S phase: DNA is copied Teaching Strategies • Tell students that most anticancer drugs interfere with the cell cycle of cancer cells. Unfortunately, the drugs also interfere with healthy cells, which is why the patient suffers side effects from the medications. Discussion • Where in the cell cycle could scientists target anticancer drugs? (Answers will vary but may mention proteins involved in the three checkpoints, DNA replication, or cytokinesis.) • What type of environmental factors have been associated with the onset of cancer? (Answers may include diet, UV radiation, hormones, and environmental pollution.) Why are individual chromosomes more difficult to see during interphase than during mitosis? 4B 6E 127 TAKS 2 Bio 4B, 6C 4. cycle fails, ________ may develop. (cancer) TAKS 2 Bio 4B TAKS 2 Bio 4B Alternative Assessment A. Incorrect. Cancer is associated with accelerated cell growth. B. Incorrect. Cancer does alter the cell cycle, but not by halting mitosis. C. Correct. Uncontrolled cell growth is the defining characteristic of cancer. D. Incorrect. Cancer cells continue to divide. Have students write and illustrate a description of the cell cycle in the style of a children’s book called “A Cell’s Life.” Be sure students include each phase, including G1, S, G2, mitosis, and cytokinesis. LS Verbal TAKS 2 Bio 4B, 6C Chapter 6 • Chromosomes and Cell Reproduction 127 Section 3 Section 3 Mitosis and Cytokinesis Focus Chromatid Separation in Mitosis Objectives Overview Before beginning this section review with your students the objectives listed in the Student Edition. In this section, students will examine the processes of mitosis and cytokinesis in more detail. During mitosis, spindle fibers pull the chromatids to opposite ends of the cell, ensuring that each new cell receives the proper assortment of chromosomes. During cytokinesis, the cytoplasm is divided by a new cell membrane or cell wall. Every second about 2 million new red blood cells are produced in ● Describe the structure and your body by cell divisions occurring in the bone marrow. These cells function of the spindle during mitosis. 4B 6E TAKS 2 have received the signal to divide. The cells continue past the G2 phase and enter into the last two phases of the cell cycle—mitosis and cytokinesis. During mitosis the nucleus divides to form two nuclei, each containing a complete set of the cell’s chromosomes. During cytokinesis the cytoplasm is divided between the two resulting cells. ● Differentiate cytokinesis in During mitosis, the chromatids on each chromosome are physi4B 6E animal and plant cells. TAKS 2 cally moved to opposite sides of the dividing cell with the help of the spindle, shown in Figure 8. Spindles are cell structures made up Key Terms of both centrioles and individual microtubule fibers that are spindle involved in moving chromosomes during cell division. ● Summarize the events of the four stages of mitosis. 4B 6E TAKS 2 Forming the Spindle Animal cells usually have one pair of centrioles, with the centrioles at right angles to each other. During the G2 phase of the cell cycle, the centriole pair is replicated so that the cell has two pairs of centrioles as it enters the mitotic phase. When a cell enters the mitotic phase, the centriole pairs start to separate, moving toward opposite poles of the cell. As the centrioles move apart, the spindle begins to form. Centrioles and spindle fibers are both made of hollow tubes of protein called microtubules. Each spindle fiber is made of an individual microtubule. Each centriole, however, is made of nine triplets of Bellringer Draw a football field on the board with the players lined up at midfield and the goal posts at each end. Have students write a short paragraph describing how the players, the midfield line, and the goal posts compare to the structures of a cell involved in mitosis and cell division. (The players represent the chromosomes, the midfield line represents the equator of the cell, and the goal posts represent the centrioles at the poles of the cell.) Figure 8 The spindle The spindle, made up of centrioles and spindle fibers, helps move chromosomes apart during mitosis. Microtubule triplets TAKS 2 Bio 4B; Bio 3E, 6E Centromere Cell Chromatids Motivate Activity GENERAL Mitosis Set up several microscopes with prepared slides of cells at various stages of mitosis, but presented out of order. Have students examine the slides and try to place each slide into the proper order of events. If microscopes and slides are not available, the various stages can be drawn on the board, unlabeled English Language Learners and out of order. TAKS 2 Bio 4B; Bio 6E pp. 128–129 Student Edition TAKS Obj 2 Bio 4B TEKS Bio 4B, 6E Teacher’s Edition TAKS Obj 2 Bio 4B TEKS Bio 3E, 3F, 4B, 6E 128 Spindle fibers Centrioles Each centriole is composed of nine triplets of microtubules arranged in a circle. 128 Chapter Resource File • Lesson Plan GENERAL • Directed Reading • Active Reading GENERAL • Data Sheet for Math Lab • Data Sheet for Quick Lab Chapter 6 • Chromosomes and Cell Reproduction Transparencies TT Bellringer TT Stages of Mitosis GENERAL Planner CD-ROM GENERAL • Reading Organizers • Reading Strategies • Supplemental Reading The Lives of a Cell microtubules arranged in a circle. Unlike animal cells, plant cells do not have centrioles, but they form a spindle that is almost identical to that of an animal cell. Teach 18 493 Some of the microtubules in the spindle interact with each other. Others attach to a protein structure found on each side of the centromere. The two sets of microtubules extend out toward opposite poles of the cell. Once the microtubules attach to the centromeres and poles, the two chromatids in each chromosome can be separated. The chromatids are moved to each pole of the cell in a manner similar to bringing in a fish with a fishing rod and reel. When the microtubule “fishing line” is “reeled in,” the chromatids are dragged to opposite poles. The reeling in occurs because the ends of the spindle fibers are broken down bit by bit at each of the poles. As the fibers become shorter, the chromatids they are pulling move closer and closer to the poles. As soon as the chromatids separate from each other they are called chromosomes. When the chromosomes finally arrive, each pole has one complete set of chromosomes. x 2+ 6x -7-0 -7-0 Calculating the Number of Cells Resulting from Mitosis TAKS 2 Bio 4B; Bio 6E Teacher’s Notes Review numbers in scientific notation before starting the lab. Review how to cancel units and set up equal unit proportions. For example, 60 seconds/ 1 minute is an equal proportion. Analysis Answers 0 2 5 < x 2+ 6x 0 2 5 Skills Acquired Calculating, predicting 8 493 76 < Separation of Chromatids by Attaching Spindle Fibers 1. 9.0 1010 cells 90,000,000,000 cells 2. 2.16 1012 cells 3. Factors that might increase or decrease the rate of mitosis include mutated genes, diet, and exposure to ultraviolet light and tobacco products. Calculating the Number of Cells Resulting from Mitosis 4B 6E TAKS 2 Background MATH TAKS Obj 9 8.3B; Obj 10 8.14A Scientists investigating cancer might need to know the number of cells produced in a certain amount of time. In the human body the rate of mitosis is about 25 million (2.5 107) cells produced every second! You can calculate the number of cells produced by mitosis in a given amount of time. 1. Calculate the number of cells produced by mitosis in the given time. For example, to find the number of cells produced in 3 minutes, determine how many seconds there are in 3 minutes (since the rate is given in seconds). 60 seconds 3 minutes 180 seconds 1 minute 2. Multiply the rate of mitosis by the time (in seconds) asked for in the problem (180 seconds). 2.5 107 cells 180 seconds 4.5 109 cells (4,500,000,000 cells) second Analysis 1. Calculate the number of cells that would be produced in 1 hour. 2. Calculate the number of cells that would be produced in 1 day. 3. Critical Thinking Predicting Patterns Identify factors that might increase or decrease the rate of mitosis. 129 did you know? Fly Chromosomes The fruit fly Drosophila melanogaster has been used to study chromosomal mutations since 1933. The salivary glands of these flies, as in many insects, are composed of cells that do not divide during the larval stage. However, the chromosomes continue to replicate, producing many copies. The copies of each chromosome are closely aligned, resulting in thick chromosomes that are easy to study with a microscope. Bio 3F Chapter 6 • Chromosomes and Cell Reproduction 129 Mitosis and Cytokinesis Although mitosis is a continuous process, biologists traditionally divide it into four stages, as shown in Figure 9. Teach, continued continued SKILL Mitosis BUILDER Vocabulary To help students master vocabulary, relate the word mitosis to its Greek origin, mitos, meaning “thread.” Ask students to explain why mitosis is associated with thread. (The hereditary material consists of long, threadlike molecules.) Using the Figure Lead students through each stage shown in Figure 9, focusing on the behavior of chromosomes. Point out that the various stages are not of equal duration. Using movie film as an analogy, help students avoid the misconception that mitosis “jumps” from stage to stage. Although a movie consists of individual frames of film, the images on the film appear to change continuously. Explain that mitosis progresses in a similar fashion. IO B graphic Step Metaphase During metaphase the chromosomes move to the center of the cell and line up along the equator. Spindle fibers link the chromatids of each chromosome to opposite poles. Step Anaphase Centromeres divide during anaphase. The two chromatids (now called chromosomes) move toward opposite poles as the spindle fibers attached to them shorten. Step Telophase A nuclear envelope forms around the chromosomes at each pole. Chromosomes, now at opposite poles, Stages of Mitosis The chromosome copies in the nucleus of a dividing cell are separated into two nuclei. 1 2 Prophase • Chromosomes become visible • Nuclear envelope dissolves • Spindle forms Chromosome (already copied) The chromosomes replicate during interphase. INTER PH AS E GENERAL Have students hypothesize about how a cell’s ATP use changes during mitosis. (The events of mitosis require a lot of additional energy, which is supplied by ATP.) TAKS 2 Bio 4B; Bio 6E pp. 130–131 Student Edition TAKS Obj 2 Bio 4B Teacher Edition TAKS Obj 2 Bio 4B TEKS Bio/IPC 3C TEKS Bio 3E, 3F, 4A, 4B, 6E Centrioles S Prefixes Tell students that the prefixes of the different stages of mitosis describe the order of events or the events themselves. Pro- means “earlier than;” meta- means “later than” or “after;” ana- means “up” or “back” and describes the movement of the chromosomes “up” toward the poles; and telo- means English Language Learners “end.” Spindle fibers G1 Cy to ki n es i s Magnification: 567ⴛ G2 Mitosis 130 MEDICINE CONNECTION When cultured, cancer cells can divide indefinitely if they are given a continual supply of nutrients. One cell line has been continuously cultured since 1951. The cells in this cell line are called HeLa cells because they were originally from a tumor removed from a woman named Henrietta Lacks. The patient suffered from uterine cervical carcinoma. Cells from this cell line are still used by researchers around the world, especially in research on viruses. Bio/IPC 3C; Bio 3F Chapter 6 • Chromosomes and Cell Reproduction Metaphase • Chromosomes line up along equator Nucleus Teaching Tip 130 Prophase Chromosomes coil up and become visible during prophase. The nuclear envelope dissolves and a spindle forms. Figure Figure 96-9 IO B graphic TAKS 2 Bio 4B; Bio 6E Teaching Tip Step uncoil and the spindle dissolves. The spindle fibers break down and disappear. Mitosis is complete. Teaching Tip Belt of protein threads Cytokinesis As mitosis ends, cytokinesis begins. During cytokinesis, the cytoplasm of the cell is divided in half, and the cell membrane grows to enclose each cell, forming two separate cells as a result. The end result of mitosis and cytokinesis is two genetically identical cells where only one cell existed before. During cytokinesis in animal cells and other cells that lack cell walls, the cell is pinched in half by a belt of protein threads, as shown in Figure 10. 3 Figure 10 Cytokinesis in animal cells. The cell membrane is pinched in half by a belt of protein threads. LS Visual TAKS 2 Bio 4B; Bio 6E Using the Figure 4 Anaphase • Centromeres divide • Chromatids (now called chromosomes) move toward opposite poles GENERAL Stages of Mitosis Have students draw pictures of each stage of mitosis on four note cards. On one side, have students write the name of the stage. On the opposite side, have them draw an accurate sketch of the stage. They can then use these as flash cards to help each other learn the stages of mitosis. Two genetically identical cells Tell students that the belt of protein threads that are labeled in Figure 10 is a ring of the proteins actin and myosin, the same two proteins that interact to contract muscle cells. The furrowing of the cell membrane occurs perpendicular to the long axis of the spindle. Cytokinesis usually begins in anaphase but is not completed until after the two nuclei have formed. Telophase • Nuclear envelope forms at each pole • Chromosomes uncoil • Spindle dissolves • Cytokinesis begins TAKS 2 Bio 4B; Bio 4A, 6E Group Activity GENERAL Performing Mitosis and Cytokinesis Have students work in groups of three or four students. Ask each group to design a “performance” using the entire class as the cast, as well as props, that would demonstrate the process of mitosis and cytokinesis. Ask groups to present their designs to the class, and have the class decide which “show” is best. If time and materials permit, allow students to stage their mitosis and cytokinesis show. LS Interpersonal Co-op Learning TAKS 2 Bio 4B; Bio 3E, 6E 131 MISCONCEPTION ALERT Mitosis vs. Cytokinesis Students often think that mitosis is the same as cell division. Be sure they understand that mitosis refers strictly to the division of the chromosomes, whereas cytokinesis refers to the division of the cytoplasm. Remind students that cell division is just one of the four events that make up the cell cycle. TAKS 2 Bio 4B; Bio 6E Trends in Cell Biology Turning Off Cancer Currently, researchers at the National Cancer Institute are working to find the proteins that “turn on” or “turn off” gene ECT2, an oncogene that appears to be a critical regulator of cytokinesis. Eventually, researchers may be able to “turn off” the mechanism that causes cancer cells to divide uncontrollably. Bio/IPC 3C Chapter 6 • Chromosomes and Cell Reproduction 131 In plant cells and other cells that have rigid cell walls, the cytoplasm is divided in a different way. In plant cells, vesicles formed by the Golgi apparatus fuse at the midline of the dividing cell and form a cell plate. A cell plate is a membranebound cell wall that forms across the middle of the plant cell. A new cell wall then forms on both Cell wall sides of the cell plate, as shown in Figure 11. When Nucleus complete, the cell plate separates the plant cell into two new plant cells. In both animal and plant cells, offspring cells are about equal in size. Each offspring cell receives an identical copy of the original cell’s chromosomes. Each offspring cell also receives about one-half of Forming the original cell’s cytoplasm and organelles. Teach, continued continued Observing Mitosis and Cytokinesis TAKS 2 Bio 4B; Bio 6E Skills Acquired Comparing, inferring Answers to Analysis cell plate 1. Prophase: chromosomes are visible as dark threads; metaphase: chromatids line up along the equator; anaphase: chromosomes appear to pull toward opposite poles; telophase: chromosomes are at opposite poles 2. Answers will vary but should indicate more cells in interphase than in the other stages. 3. Cells spend the majority of their time in interphase. Observing Mitosis and Cytokinesis TAKS 2 You can identify the stages of mitosis and the process of cytokinesis by observing slides of tissues undergoing mitosis using a compound microscope. Materials compound microscope, prepared slide of mitosis, paper, pencil Analysis Procedure 1. View a prepared slide of cells undergoing mitosis under low power of a compound microscope. 2. Move the slide until you find a section where different stages of mitosis are visible. Close Reteaching 2 Provide students with yarn to TAKS Bio 4B represent chromosomes, nuclear envelopes, and cell membranes; tie tabs to represent centromeres; and string to represent spindle fibers. Have them recreate on their desktops a cell undergoing mitosis. English Language LS Kinesthetic Figure 11 Cytokinesis in plant cells. A cell wall forms in the center of the dividing cell. Learners Quiz GENERAL 1. In mitosis, the chromatids move toward opposite poles during ________. (anaphase) TAKS 2 Bio 4B 2. During mitosis, the chromosomes line up along the equator during ________. (metaphase) TAKS 2 Bio 4B 3. Switch to high power. Use the photos in Figure 9 to help you locate and identify cells in interphase and in each stage of mitosis. 4. On a separate piece of paper, sketch an example of each stage. Label each sketch with the following terms where appropriate: chromosomes, cell membrane, cytoplasm, nucleus, spindle, and cell wall. 5. Switch to low power, and estimate how many cells are clearly in interphase and how many cells are in one of the stages of mitosis. Student Edition TAKS Obj 2 Bio 4B TAKS Obj 2 Bio 6C TEKS Bio 4B, 6C, 6E Teacher Edition TAKS Obj 2 Bio 4B, 6C TEKS Bio 4B, 6C 132 2. Compare the number of cells in interphase with the number of cells in one of the stages of mitosis. 3. Critical Thinking Inferring Relationships What does your answer to item 2 indicate about the relative length of interphase? Section 3 Review Describe the function of the microtubules Compare how cytokinesis occurs in plant cells during anaphase. with how it occurs in animal cells. 4B 6E Describe the events that occur during each of the four stages of mitosis. 4B 6E 4B 6E TAKS Test Prep Mitosis could not proceed if a mutation interrupted the assembly of 4B 6C A the cell wall. C the cell membrane. B spindle fibers. D the nuclear envelope. 132 Answers to Section Review 1. The microtubules, attached to the centromeres, shorten and pull the chromatids to opposite poles, similar to a fishing line reeling in a fish. TAKS 2 Bio 4B pp. 132–133 1. Describe the activity of chromosomes in each stage of mitosis. 2. Prophase: chromosomes become visible, nuclear envelope dissolves, spindle forms; metaphase: chromosomes line up at the equator, spindle fibers attach to each chromatid; anaphase: centromeres divide, chromatids move to opposite poles due to shortening spindle fibers; telophase: nuclear envelope forms around the chromatids at each pole, chromosomes uncoil, spindle fibers break down and disappear TAKS 2 Bio 4B Chapter 6 • Chromosomes and Cell Reproduction 3. In plant cells, vesicles formed by the Golgi apparatus fuse at the equator and form the cell plate. A new cell wall forms on both sides of the cell plate. In animal cells, the cell is pinched in half by a belt of protein threads. TAKS 2 Bio 4B 4. A. Incorrect. The cell wall does not play a key role in mitosis. B. Correct. Without the spindle fibers, chromosomes cannot segregate properly into two complete nuclei. C. Incorrect. The cell membrane does not play a key role in mitosis. D. Incorrect. The nuclear envelope breaks down before mitosis and reforms after the completion of mitosis. TAKS 2 Bio 4B, 6C Study CHAPTER HIGHLIGHTS ZONE Key Concepts Alternative Assessment Key Terms Section 1 1 Chromosomes ● Cell division allows organisms to reproduce asexually, grow, replace worn-out or damaged tissues, and form gametes. ● Bacteria reproduce by binary fission. ● Before cell division, DNA coils tightly around proteins and forms chromosomes. At cell division, each chromosome consists of two chromatids attached at the centromere. ● Each organism has a characteristic number of chromosomes. ● Human somatic cells are diploid, with 23 pairs of homologous chromosomes. Human gametes are haploid, with 23 chromosomes. ● Sex chromosomes carry information that determines an organism’s sex. ● Changes in chromosome number or structure can cause abnormal development. Karyotypes are used to examine an individual’s chromosomes. gamete (118) binary fission (119) gene (119) chromosome (119) chromatid (119) centromere (119) homologous chromosome (120) diploid (121) haploid (121) zygote (121) autosome (122) sex chromosome (122) karyotype (122) ● The life of a eukaryotic cell—the cell cycle—includes interphase, mitosis, and cytokinesis. ● Interphase consists of 3 phases: growth, DNA synthesis (replication), and preparation for cell division. A cell about to divide enters the mitosis and cytokinesis phases of the cell cycle. ● The cell cycle is carefully controlled; failure of cellular control can result in cancer. Set up a lab practical with various stations. Include microscopes with slides at various stages of mitosis, pictures of chromosomes, and karyotypes for interpretation. Chapter Resource File • Science Skills Worksheet GENERAL • Critical Thinking Worksheet • Test Prep Pretest GENERAL • Chapter Test GENERAL IPC Benchmark Review Section 2 2 The Cell Cycle GENERAL To prepare students for the TAKS, have students review Changes in Matter: Physical Changes and Chemical Changes TAKS Obj 4 IPC 8A on p. 1052 of the IPC Refresher in the Texas Assessment Appendix of this book. cell cycle (125) interphase (125) mitosis (125) cytokinesis (125) cancer (126) Section 3 3 Mitosis and Cytokinesis ● During mitosis, spindle fibers drag the chromatids to opposite poles of the cell. A nuclear envelope forms. Each resulting nucleus contains a set of the original cell’s chromosomes. ● Cytokinesis in animal cells occurs when a belt of protein threads pinches the cell membrane in half. Cytokinesis in plant cells occurs when vesicles from the Golgi apparatus fuse to form a cell plate. spindle (128) Unit 4—Cell Reproduction Use Topics 1–4 BIOLOGY in this unit to review the key concepts and terms in this chapter. 133 Answer to Concept Map The following is one possible answer to Performance Zone item 15. Cell cycle includes interphase mitosis cytokinesis includes divides divides synthesis phase chromosomes cytoplasm first growth phase second growth phase Chapter 6 • Chromosomes and Cell Reproduction 133 Performance ZONE CHAPTER 6 ANSWERS Using Key Terms 1. b Bio 6E 2. a TAKS 2 Bio 4B; Bio 6E 3. c Bio 4A, 6E 4. d TAKS 2 Bio 4B; Bio 6E 5. a. Gametes, or reproductive cells, are haploid, or n, and contain one set of chromosomes. When gametes unite, they form a zygote, which is diploid, or 2n, and contains two sets of chromosomes. b. Of the 23 pairs of chromosomes, 22 pairs are called autosomes. The individual’s sex is determined by the 23rd pair, the sex chromosomes. A karyotype is a photograph of chromosomes arranged by size. c. A person’s sex is determined by their sex chromosomes. An individual with two X chromosomes (XX) is female; an individual with XY chromosomes is male. Understanding Key Ideas 6. b TAKS 2 Bio 4B 7. d TAKS 2 Bio 6C 8. c TAKS 2 Bio 4B 9. a Bio 6E 10. b TAKS 2 Bio 4B; Bio 6E 11. a TAKS 2 Bio 4B; Bio 6E 12. Normal cells can become cancer cells if a genetic mutation impairs a cell’s ability to regulate cell growth and division. The mutation leads to cancer, or uncontrolled cell growth. TAKS 2 Bio 6C CHAPTER REVIEW 8. When the cell cycle is not controlled, Using Key Terms 1. Prokaryotes reproduce asexually by a. disjunction. c. cytokinesis. b. binary fission. d. mitosis. 2. The stage of the cell cycle in which a cell’s DNA is copied is called the _______ phase. 4B 6E a. S c. G2 b. G1 d. mitosis each chromosome. d. structures that move chromosomes during mitosis. 4. The process in which the cytoplasm of a cell is divided is called 4B 6E a. disjunction. c. binary fission. b. interphase. d. cytokinesis. 134 mosomes present in the original cell. b. receives most of the chromosomes from the original cell. c. donates a chromosome to the original cell. d. receives exactly half the chromosomes from the original cell. 10. During the metaphase stage of mitosis, 4B 6E a. the cell membrane folds inward. b. chromosomes line up at the cell’s equator. c. spindle fibers shorten, pulling chromo- somes to the poles of the cell. d. chromosomes are at opposite ends of 5. For each set of terms, write one or more sentences summarizing information learned in this chapter. a. haploid, diploid, gametes, zygote b. autosome, sex chromosome, karyotype c. XX chromosomes, XY chromosomes the cell. 11. How does cell division differ between animal and plant cells? 4B 6E a. Plant cells do not have centrioles. b. Animal cells form a cell plate. c. Plant cells are always haploid. d. Animal cells do not have centrioles. Understanding Key Ideas 6. In humans, females have _______ sex chromosomes. a. XY b. XX 12. Summarize how normal cells can become 4B cancer cells. c. YY d. XO 1 2 c. inversion d. duplication 3 4 fragment before the cytoplasm divides. (Hint: See Chapter 3, Section 2.) 4B 6E 2 1 2 3 14. How did researchers narrow the search for genes involved in DiGeorge syndrome to 15 genes? 6C 15. Concept Mapping Make a concept map that shows the events in the cell cycle. Try to include the following words in your map: cell cycle, interphase, synthesis phase, chromosomes, cytokinesis, mitosis, second growth phase, and first growth phase. 3E 5 Original chromosome 1 6C 13. List five organelles that must divide or 7. The diagram below represents a(n) _______ mutation. 6C a. deletion b. translocation 4B 9. As a result of mitosis, each resulting cell 6E a. receives an exact copy of all of the chro- 3. Chromatids are 4A 6E a. dense patches within the nucleus. b. prokaryotic chromosomes. c. two exact copies of DNA that make up 4 5 ? mutation 134 13. Answers will vary, but should include organelles such as lysosomes, the Golgi apparatus, the endoplasmic reticulum, ribosomes, and vacuoles. TAKS 2 Bio 4B; Bio 6E 14. When the mice had two copies of those genes, DiGeorge syndrome did not appear. TAKS 2 Bio 6C pp. 134–135 Review and Assess TAKS Obj 1 Bio/IPC 2B, 2D TAKS Obj 2 Bio 3D, 4B, 6C, 6E TEKS Bio 3D, 4B, 6C, 6E TEKS Bio/IPC 2B, 2D _______ may result. a. Down syndrome b. binary fission c. cancer d. a spindle 6E 15. The answer to the concept map is found at the bottom of the Study Zone page. Bio 3E Chapter 6 • Chromosomes and Cell Reproduction Assignment Guide Section 1 2 3 Questions 1, 3, 5 ,6, 14, 17, 19 2, 7, 8, 12, 15, 17, 18, 20, 21, 22 4, 9, 10, 11, 13 Critical Thinking Alternative Assessment 16. Predicting Outcomes Predict the result 20. Finding and Communicating Information if cytokinesis were omitted from the cell cycle? 4B 6E 17. Inferring Relationships Explain the relationships between mitosis in eukaryotic cells and binary fission in prokaryotes. 4A 4B 6E 18. Evaluating Conclusions Damage to the brain or the spinal cord is usually permanent. Use your knowledge of the cell cycle to explain why damaged cells in the brain or spinal cord are not replaced. 4B 6E 19. Evaluating Viewpoints A newspaper article describes the concern that more infants with Down syndrome will be born in the United States as more women delay having children. Do you agree or disagree with this concern? Explain. 4B 6C Critical Thinking Scientists have determined that telomeres (the tips of chromosomes) are shaved down slightly every time a cell divides. When the telomeres reach a certain length, the cell may lose its ability to divide. Find out what scientists have recently uncovered about telomeres and their association with cell division and cancer. Prepare a brief written report to share with your class. 2B 2D 4B 6E 21. Career Connection Oncologist Research the field of oncology, and write a report that includes a job description, training required, kinds of employers, growth prospects, and starting salary. 2B 3D 22. Interactive Tutor Unit 4 Cell Reproduction Write a report summarizing how different cancer-fighting drugs kill cancer cells by interrupting the cell cycle. How is cell division related to cancer? 2B 4B 6E 16. Without cytokinesis, the cell would not divide into two daughter cells. TAKS 2 Bio 4B; Bio 6E 17. Both result in new cells with the same number of chromosomes as in the original cell. TAKS 2 Bio 4B; Bio 4A, 6E 18. Most nerve cells are permanently resting in interphase G1. Because they do not undergo mitosis, most damaged nerve cells are not replaced. TAKS 2 Bio 4B; Bio 6E 19. Answers will vary, but should include statistics about how risk factors change with increasing maternal age. TAKS 2 Bio 4B, 6C Alternative Assessment TAKS Test Prep The illustration below shows the events of the cell cycle. Use the illustration and your knowledge of science to answer questions 1–3. I NT E RP HA SE S G1 1. For about what proportion of the cell cycle is the cell in interphase? 4B 6E A less than 1/2 C more than 3/4 B less than 3/4 D more than 9/10 2. List the phases of the cell cycle starting immediately after cell division. F G1, S, G2, mitosis, cytokinesis G cytokinesis, G1, S, G2, mitosis H S, G2, mitosis, cytokinesis, G1 J G1, cytokinesis, mitosis, G2, S 4B 6E TAKS 1 Bio/IPC 2B, 2D; TAKS 2 Bio 4B; Bio 6E 3. If the phase during which DNA is synthe- G2 Cytokinesis Mitosis sized did not occur, what effect would it have on the cell cycle? 4B 6E A Mitosis would immediately follow S. B Cytokinesis would immediately follow G2. C G1 would immediately follow mitosis. D G2 would immediately follow G1. Test When several questions refer to the same graph, table, drawing, or passage, answer the questions you are sure of first. 135 Standardized Test Prep 1. A. Incorrect. Cells spend the majority of time in interphase. B. Incorrect. Cells spend the majority of time in interphase. C. Correct. Cells spend most of their time in interphase. D. Incorrect. While cells spend most of their time in interphase, the processes of mitosis and cytokinesis take longer than 1/10 of the total cell cycle. TAKS 2 Bio 4B; Bio 6E 2. F. Correct. The cell cycle can be divided into recognized phases that complete the continuous process. G. Incorrect. While the phases are presented in proper order, cytokinesis does not immediately follow cell division. H. Incorrect. 20. Answers will vary. Cells from large tumors often have unusually short telomeres. Telomerase, an enzyme that catalyzes the lengthening of telomeres, stabilizes telomere length, especially in cancer cells. Researchers are focusing on telomerase as a target for cancer diagnosis and chemotherapy. While the phases are presented in proper order, S does not immediately follow cell division. J. Incorrect. The phases here are written backwards of their proper order. TAKS 2 Bio 4B; Bio 6E 3. A. Incorrect. The S phase would not exist, because DNA synthesis takes place during the S phase. B. Incorrect. Cytokinesis would continue to follow mitosis. C. Incorrect. G1 would continue to immediately follow cytokinesis. D. Correct. G2 normally follows S, the phase during which DNA synthesis takes place. 21. Oncologists treat and manage cancer patients by using radiation therapy or drugs. Training requires an M.D. or D.O. degree, passing a licensing exam, an internship, and a residency. Employers include solo, partnership, or group practices, as well as universities and government agencies. Growth prospects are excellent. Starting salary varies by region. TAKS 1 Bio/IPC 2B; Bio 3D 22. Answers will vary. Examples include vincristine and taxol, which prevent the mitosis spindle microtubules from functioning. Cancer occurs when cell division does not respond to the normal signals that regulate the cell cycle. TAKS 1 Bio/IPC 2B; TAKS 2 Bio 4B; 6E TAKS 2 Bio 4B; Bio 6E Chapter 6 • Chromosomes and Cell Reproduction 135