Unit A Reproduction

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Chapter Preview As you look around your neighbourhood, you will notice the incredible variety of traits among the different organisms. Where did all this variety come from? Most organisms, like these dogs, demonstrate similarities as well as differences. These similarities and differences are the result of sexual reproduction. Some organisms can reproduce both asexually and sexually. What would be the advantages of this? Why do living things reproduce in so many ways? Does sexual reproduction have advantages over asexual reproduction? How does sexual reproduction occur in plants? Where are the sex cells in other organisms? Do all living things that reproduce sexually have separate males and females? If not, how do they reproduce sexually? Are there advantages to having both male and female sex cells in one individual? In this chapter, you will learn about sexual reproduction and the diversity it produces. Different species use different methods to reproduce sexually. Specialized organs in many organisms are designed to ensure successful reproduction and the survival of the offspring. Reproductive technologies allow humans to use the mechanisms of sexual reproduction to enhance the productivity of domestic organisms. TRY THIS: Observing Pollen Skills Focus: observing, recording Materials: paint brush, flowers with pollen, depression slides, cover slips, hand lens, microscope, pollen grains soaked in 10 % sucrose solution, disposable masks (as required) The male sex cells in a flower are contained in the pollen, which is found on the stamen of the flower. The sex cells in the pollen grains will fertilize the egg cells of the flower. (The parts of a flower are summarized in Figure 4 on page 85.) Students who are sensitive to pollen should wear a disposable mask. 1. Using a fine paint brush, brush some of the pollen from the flower onto a depression slide. Cover the pollen with a cover slip. Observe the slide with the hand lens and then with the microscope. 2. Using the pipette, place a drop of the pollen grain liquid onto a depression slide and cover it. Observe the slide with the hand lens and then with the microscope. A. Draw a pollen grain from each sample. B. Describe any differences you observe. NEL 73
3.1 Meiosis LEARNING TIP Scan the titles, headings, and subheadings, as well as the words in bold type. Make predictions about what you expect to learn in Chapter 3. egg sperm Figure 1 Sperm cells are much smaller than egg cells and have a very different shape. In Chapter 2, you learned that the offspring of parents that reproduce asexually are genetically identical to each other and to the parents. You also learned that asexual reproduction involves cell division and sexual reproduction involves sex cells joining to produce a zygote. In this section, you will find out how the sex cells are formed. Multicellular organisms that reproduce sexually have two types of cells: somatic cells and sex cells. Somatic cells, also called body cells, reproduce by cell division and make up the vast majority of an organism’s cells. Human somatic cells have 23 pairs of chromosomes, for a total of 46 chromosomes. The sex cells are also called gametes. Gametes have half the chromosomes of the parent cell. Gametes make up an extremely tiny fraction of an organism’s cells. Male gametes are called sperm, and female gametes are called eggs or ova (singular: ovum) (Figure 1). Chromosome Numbers In general, individuals of the same species have the same number of chromosomes. Goldfish have 94 chromosomes, fruit flies have 8, and humans have 46. If sexual reproduction involves the fusing of genetic material from two individuals, then why doesn’t the resulting offspring contain twice the number of chromosomes as the parents? To maintain the same number of chromosomes from generation to generation, there needs to be a way to reduce the number of chromosomes that are passed on from each parent. The process that produces gametes, which have half the number of chromosomes as the parent, is called meiosis. Meiosis happens only in the cells that produce gametes. These cells are called reproductive cells. Meiosis is sometimes called “reduction division” because it reduces the chromosome number by half. Each fruit fly gamete (Figure 2) contains four chromosomes—half of the genetic material of the parent. Cells with half the chromosome number of the parent are called haploid, symbolized with a single n. Cells that have a complete set of chromosomes are called diploid symbolized as 2n. 75X n n 4 2n 2n 4 4 Figure 2 Fruit fly somatic cells have eight chromosomes (diploid or 2n) and are produced by mitosis. Fruit fly gametes have four chromosomes (haploid or n) and are produced by meiosis. 8 8 4 4 4 74 <strong>Unit</strong> A <strong>Reproduction</strong> NEL
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Figure 2 The X chromosome (a) is fa
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Figure 2 The karyotype of Down synd
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4.6 Prenatal Procedures (a) (b) Fig
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4.7 Reproductive Technologies LEARN
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Sample Stakeholder Perspectives •
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CHAPTER 4 Review Human Reproduction
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Review Key Ideas and Vocabulary 1.
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UNIT A Review Reproduction Unit Sum
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15. Complete the following table to
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