KEY CONCEPTS AND PROCESS SKILLS. 1. Genes are the units of information for inherited traits that parents transmit to their offspring.

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1 Copycat 40- to 1 50-minute sessions ACTIVITY OVERVIEW 57 R EA D I N G SUMMARY Asexual and sexual reproduction are introduced. Differences between the two prepare students to understand the mechanisms of heredity in sexually reproducing organisms. KEY CONCEPTS AND PROCESS SKILLS 1. Genes are the units of information for inherited traits that parents transmit to their offspring. 2. Reproduction can be sexual or asexual. 3. Some organisms reproduce by only one method of reproduction, while others can use both methods. 4. Asexual reproduction produces individuals who are genetically identical to the parent. These individuals are called clones of the parent. Artificial clones can now be made of some animals that normally reproduce only sexually. 5. Sexual reproduction involves the union of two sex cells and produces unique individuals that show a combination of traits inherited from both parents. KEY VOCABULARY asexual reproduction cell clone egg cell fertilization mutation nucleus offspring sexual reproduction sperm cell heredity D-35

2 Activity 57 Copycat TEACHING SUMMARY Getting Started 1. Introduce the concept of heredity in the context of reproduction. Doing the Activity 2. Students do the interactive reading. Follow-Up 3. The class discusses the differences between sexual and asexual reproduction. INTEGRATIONS Health Depending on your existing health and sexuality curriculum, this activity offers an opportunity to reinforce or extend what students have learned about human reproduction. BACKGROUND INFORMATION Asexual reproduction Asexual reproduction results in two identical, or nearly identical, offspring individuals from one parent organism. In single-celled organisms, this process is equivalent to cell division, which is the same process (often referred to as mitosis) responsible for cell proliferation within multicellular organisms during growth and development. This process is also used to replenish cell populations (such as red blood cells). Though reproduction is exclusively asexual in bacteria and some protists, such organisms maintain their species identity by sharing genes between cells (in bacteria, this is called conjugation). Many multicellular organisms can also reproduce asexually, using more complicated mechanisms (such as budding, runners, bulbs, and regeneration). Asexual reproduction is accomplished by the splitting of an adult organism (as in sea stars and planaria). In addition, lost or sacrificed body parts (such as lizard tails) can be regrown through regeneration. Sexual reproduction In humans and other vertebrates, a two-stage process of cell division called meiosis occurs in diploid adults, resulting in the formation of haploid gametes (sperm and D-36 Science and Life Issues

3 Copycat Activity 57 egg), each of which has only half the genetic information of an adult. Two gametes combine (fertilization) to produce a new diploid organism, which dominates the life cycle. Note that cell division and fertilization, in particular the role of the chromosomes, are the focus of Activity 63, Show Me the Genes! At this point in the unit, the key concept is that sexual reproduction is useful for producing new gene combinations (each offspring contains a mixture of genes from its two parents, but does not look like a simple blend of its parents). However, sexual reproduction does not necessarily involve two parents. Numerous plant species can self-fertilize. While some hermaphroditic animal species, such as earthworms, must copulate in order to reproduce (both worms thereby have their eggs fertilized), others can self-fertilize as well. Clones The term clone is used to refer to two completely different processes, which both result in the production of identical offspring. In nature, prokaryotic clones are the identical products of cell division. In the lab technique of cloning, bacteria are used as biotechnological tools for the reproduction of DNA sequences. A clone of a eukaryotic organism is an identical offspring that can be produced only by human intervention: by manipulating the reproductive process at the cellular level, asexual reproduction is forced to occur within a species that normally can reproduce only sexually. The description of sheep cloning in the Student Book is somewhat idealized: getting the egg cell to accept the new nucleus and triggering it to divide and develop into an adult organism has been a significant challenge for scientists. Mutations While it is useful to contrast sexual and asexual reproduction by stating that asexual reproduction (cell division) produces identical offspring cells, this statement is not exactly true. Randomly occurring mutations are mentioned in passing in the reading. As will be explored in Unit F, Evolution, in Science and Life Issues, mutations, though rare, occur often enough to provide the raw material that fuels all evolutionary change. Mutations will be introduced more fully in the Student Book in Activity 63, Show Me the Genes! Mutations occur during DNA replication. They increase genetic diversity in offspring when they occur in the germline, and they can contribute to the development of cancer in an individual when they occur in somatic (body) cells. D-37

4 Copycat Activity 57 TEACHING SUGGESTIONS GETTING STARTED 1. Introduce the concept of heredity in the context of reproduction. Ask the class, While most of your traits can be influenced by the environment, what in your cells causes traits to be inherited? Most traits are at least partially the result of information contained in the nuclei of our cells. This information is passed on in the form of genes, from the parent s cells to the cells of the offspring. The reading in this activity gives students background on reproduction that they will need in order to understand the mechanisms of heredity in sexually reproducing organisms like humans (as well as flowering tobacco plants and critters ). Write the word reproduction on the board or an overhead transparency. Beneath it, write the words asexual and sexual. Clarify that asexual means simply nonsexual. Ask students as they read to pay attention to the special characteristics of each type of reproduction. DOING THE ACTIVITY 2. Students do the interactive reading. Have students read the Introduction on page D-16 in the Student Book. Make sure they understand the word offspring, as it will be used throughout the reading and the unit. Be prepared to note that in asexual reproduction of a single-celled organism, both daughter cells are actually offspring cells. (See Stopping to Think 1.) Teacher s Note: See the Literacy and Communication Appendix for suggestions about how to help students with science content readings and how to use Science and Life Issues to develop students literacy and communication skills. The appendix includes a complete overview of literacy and communication connections in this unit. Stopping to Think 2 provides an opportunity for students to reflect on the observation that some multicellular organisms, such as hydra and plants, can reproduce asexually as well as sexually. This ability is useful as it allows organisms to reproduce themselves and colonize adjacent regions without the need for sex. This is especially important for plants, which are nonmobile. Students often ask why some twins are identical and others are not. Stopping to Think 3 (with the paragraph that precedes it) addresses this question. It asks students to realize that identical twins arise from a single fertilized egg that splits into two embryos (and are therefore genetically identical), but fraternal twins are two siblings who develop during the same pregnancy (and therefore have only about half their genes in common). In Stopping to Think 4, students are asked to consider the paradox of cloning in mammals. A clone of a sheep is in a sense not an offspring, but an identical sibling of that parent. However, the clone and the parent sheep can not be considered identical twins, since the clone is younger than its parent. FOLLOW UP 3. The class discusses the differences between sexual and asexual reproduction. After the reading, have students help you fill in characteristics of each type of reproduction on the board or on a transparency. Use the following table, D-39

5 Activity 57 Copycat from the reading, as a starting point. Students should record this table and their additions to it in their science notebooks. Reproduction Asexual Sexual One parent Two parents Offspring (clones) Offspring inherit traits identical to parent from both parents The Analysis Questions allow students to apply their understanding of the two types of reproduction to some examples. You may wish to have students suggest additional examples of their own. Questions 1a and c address the attempts of humans to achieve desired offspring results through breeding. (Activity 58, Creature Features, uses the puzzling results of an imaginary animal breeding experiment to introduce Mendelian inheritance.) Questions 1b and e describe asexual reproduction of plants by cuttings and of flatworms by regeneration. Unlike budding and runners, which were described and compared in the reading, these methods of reproduction start not with one cell, but with a larger portion of the parent s body. SUGGESTED ANSWERS TO ANALYSIS QUESTIONS 1. Classify each of the following as either sexual or asexual reproduction. Explain each answer. a. An orange cat is bred with a black cat, in hopes of producing a tortoiseshell cat. This is an example of sexual reproduction there are two parents, and the offspring has inherited its coat color from both parents (this is true regardless of whether the breeding produces a tortoiseshell cat). b. A cutting is taken from a red-flowered geranium and placed in water to develop roots. Once roots have grown, the new plant is placed in soil and grows to produce another red-flowered geranium. This is an example of asexual reproduction the offspring is reproduced directly from a portion of a single parent, and will be genetically identical to it. c. A red-flowered geranium with dull leaves is bred with a white-flowered geranium with shiny leaves, with a goal of producing a redflowered geranium with shiny leaves. This is an example of sexual reproduction two parents produce an offspring with some traits of each parent. The offspring is not genetically identical to either parent. d. A male fish releases sperm cells into the water. One of the sperm unites with an egg from a female fish to form a new cell that grows into a new fish. This is an example of sexual reproduction two parents produce an offspring with some traits of each parent. The offspring is not identical to either parent. Though this question does not describe a breeding experiment as do Questions 1a and c, the method of reproduction is the same. e. A small worm that lives in water splits in two and each half grows to normal size. The head end grows a tail, and the tail end grows a head. D-40 Science and Life Issues

6 Copycat Activity 57 This is an example of asexual reproduction the offspring is reproduced directly from a single parent, and will be identical to it. f. Sheep reproduce only by sexual reproduction in nature. Using modern technology, a clone of an adult sheep is produced. Cloning is asexual reproduction, since genes from only one parent are involved. In this case, the organism being cloned would normally be able to reproduce only sexually. Technology must be used to enable the organism to reproduce asexually. 2. Reflection: If you were given an opportunity to clone yourself, would you do it? Explain. Expect a range of ideas from students. For example, If there were more of me, I could get more done and have more fun. But wait, I d be sharing the fun with the others of me! We wouldn t be exact clones anyway, since we would grow up differently and that would affect what my clone is like. Plus, my clone would be 13 years younger than me. It would be like having an identical twin, only even more weird. If I were cloned, scientists would want to do experiments on me. Or at least on my clone, but that wouldn t be okay since my clone would be a human being too. D-41