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63 40 2 50 -m i n ut e s e ss io G - to ns Show Me the Genes! REA DI N ACTIVIT Y OVERVIEW SUMMARY A reading describes the behavior of chromosomes during sexual reproduction and its consistency with basic patterns of inheritance. In addition, the function of DNA and the effects of randomly occurring mutations are introduced. KEY CONCEPTS AND PROCESS SKILLS 1. Sexual reproduction involves the union of two sex cells and produces unique individuals that show a combination of traits inherited from both parents. 2. An allele is one of the two or more forms of a gene present in a population. 3. Genes are located on chromosomes, which are located in the nucleus of every cell in humans and other organisms with nucleated cells. 4. The behavior of genes is related to the behavior of chromosomes. 5. Each individual obtains half of its chromosomes (and roughly half of its genes) from each parent. 6. DNA is the molecule that contains the information carried by the genes. 7. Mutations result when random errors occur during the copying of DNA before cell division. KEY VOCABULARY allele fertilization chromosome mutation cell division nucleus DNA sperm cell egg cell trait gene Teacher’s Guide D-113 Activity 63 • Show Me the Genes! TEACHING SUMMARY Getting Started 1. Discuss students’ ideas about the location of the genes. Doing the Activity 2. Students read about and discuss the role of chromosomes in cell division. 3. Students read about and discuss the role of chromosomes in inheritance. 4. Students begin to consider the role of DNA mutations in genetics. Follow-Up 5. Reflect upon the correspondence between chromosome behavior and the transmission of genes. Extension Students watch a video excerpt that relates genetic mutations to the development of cancer. D-114 Science and Life Issues Show Me the Genes! • Activity 63 BACKGROUND INFORMATION Chromosomes, Cell Division, and Reproduction The chromosomal basis of inheritance was first proposed by a young scientist named Walter Sutton in the early 1900s. His group’s work not only established the chromosomal location of the genes, but also explained why the independent assortment of many genes (see Mendel’s work) applies only to genes that are located on different chromosomes. The emphasis of the student reading is that chromosomes occur in pairs and that for each gene, the two alleles are found on a single pair of chromosomes. The reading stresses the observation that the total (diploid) number of chromosomes stays constant during normal cell division (mitosis), but is halved in the formation of (haploid) egg and sperm cells (meiosis). The details of the stages of mitosis and meiosis are typically taught in high school. Knowledge of the stages is not necessary for an understanding of the outcomes. However, the “pairs of socks” analogy used in the reading may help students realize that if two parents, such as Skye and Poppy, each contribute a tail-color allele to an offspring, they must each contribute a similar chromosome containing that tail-color allele. Chromosomes can be visible only in cells arrested before cell division, when the chromosomes are in replicated form or are migrating into the two daughter cells, as shown in Figure 2 on page D-49 in the Student Book. In preparation for either mitosis or meiosis, the chromatin (DNA and associated proteins) is in condensed form. Note that the last frame of Figure 4, on page D-50 in the Student Book, shows a pair of chromosomes as they would look immediately after mitosis or the second division of meiosis, i.e. as single chromosomes ready to decondense again. This frame represents not a “pair of chromosomes,” but two identical copies of a single chromosome. Each copy will migrate into a separate daughter cell. (In reality, these copies are not truly identical, due to the effects of recombination, which further shuffles gene combination during sexual reproduction.) Teacher’s Guide D-115 Show Me the Genes! • Activity 63 TEACHING SUGGESTIONS GETTING STARTED 1. growth and replenishment of dead cells in multicellular organisms. In the second passage, students explore the appearance of chromosomes under the microscope and the need for cells to copy their Discuss students’ ideas about the location chromosomes before division, so that the number of the genes. of chromosomes within cells stays constant. Begin by asking students if they have any ideas n Teacher’s Note: Basic cellular division is referred about where the genes are located. Listen to their to as mitosis, whereas meiosis is the division process answers and use them to address misconceptions as that produces haploid sex cells. Because these terms the activity proceeds. Scientists about 100 years ago are not necessary for students to understand the realized that the sperm contains very little cyto- basic concept that division to produce the sex cells plasm—the only common feature of both egg and results in half of the normal chromosome number sperm was the nucleus. Therefore, the nucleus and genes, they are not included as vocabulary. The seemed a likely candidate for the location of the steps of mitosis and meiosis are often included in hereditary material. You may want to ask students, the high school curriculum and can be explored in What scientific advances enabled scientists to more depth at that time. draw this conclusion? The improvement of microscopes and the development of the cell theory were precursors to this understanding of fertilization and the role of the sex cells. 3. Students read about and discuss the role of chromosomes in inheritance. Part Two of the reading looks at the natural oscillation of chromosome number during sexual repro- DOING THE ACTIVIT Y 2. Students read about and discuss the role of chromosomes in cell division. duction, from body cell to sex cell to fertilization. Stopping to Think 3 asks students to consider what would happen to the chromosome number if each parent contributed all of its genetic information to This reading is most effective if done as a class read- the offspring—the number of chromosomes would aloud, with different students reading different double with each generation. Students familiar with paragraphs. Be sure to take plenty of time to discuss geometric progressions will realize that this would each figure and its relationship to the reading. Alter- eventually lead to a vast number of chromosomes— natively, you can have students read passages aloud more than any fertilized egg could hold! This is the within their groups, pausing to discuss each Stop- essential point for students to understand, as it will ping to Think question once each group has had a allow them to relate the behavior of chromosomes chance to consider it. to the basic genetic model they’ve been exploring The first passage clarifies that cell division is the for several activities. method of asexual reproduction in single-celled The next passage of the reading explores an analogy organisms, but it is also the process that enables using 23 pairs of socks to represent the chromosomes. Teacher’s Guide D-117 Activity 63 • Show Me the Genes! In responding to Stopping to Think 4, just as in the Mendelian model they have been exploring, students should not expect siblings to have the same set of chromosomes (and thus the same alleles for each gene). After all, genes are present on chromosomes, FOLLOW–UP 5. Reflect upon the correspondence between chromosome behavior and the transmission of genes. and only one chromosome of each pair is given to Reinforce the parallel between the behavior of the each offspring. Students are already familiar with this chromosomes as observed under a microscope and fact from observations of differences in traits among the model for the behavior of the pea plant genes biological siblings. However, urge them to think at developed by Mendel and simulated for critter the level of the genes and chromosomes, which will genes using coin tosses. be explored more clearly in Activity 65, “Breeding Critters—More Traits.” The probability of two siblings receiving the same 23 chromosomes from even one of the two parents is extremely small. 4. You may want students to go back to Activity 58, “Creature Features,” to think about whether they would have considered multiple hypotheses to be possible if they had had the multiple sources of evi- Students begin to consider the role of DNA dence they have since learned about. Why or why mutations in genetics. not? What evidence have they discovered that sup- Part Three of the reading describes the large number of genes present in organisms, specifically humans, and the complexity of inheritance for most traits. You may wish to use a slinky or a diagram of a coil as a simple model of the structure of DNA. Explain that this coil is very tightly packed upon itself to form a chromosome. Most importantly, Part Three introduces the idea that mutations are random changes in the DNA sequence of genes, which can affect the organism. In discussing Stopping to Think 5, explain that mutations can occur at any cell division (actually, during the copying of DNA that precedes it). (See Extension below.) However, mutations that change the entire form of an organism occur only during the formation of sex cells in the parents of that organism. D-118 Science and Life Issues ports a two-allele hypothesis more than any other? Show Me the Genes! • Activity 63 SUGGESTED ANSWERS TO ANALYSIS QUESTIONS 1. Draw a flow diagram (a series of pictures) such as the one below that shows the locations and relative sizes of DNA, genes, chromosomes, and cells in a human body. Write a paragraph to explain your diagram. ■ Teacher’s Note: Suggest to students that they start with the person, rather than with the DNA, before they begin the assessment. Students’ diagrams should show each of the body requested components, illustrating the cells as organ system organ structures within the person and chromosomes as structures within the nucleus of each cell. Each gene is on a chromosome, which is made of a tightly coiled double-strand of DNA. A sam46 (23 pairs) chromosomes in each nucleus skin cells ple response is shown here. 1 chromosome DNA gene on a chromosome Teacher’s Guide D-119