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Chapter Menu Chapter Introduction Cell Division and Reproduction 12.1 Asexual Reproduction 12.2 Chromosome Numbers 12.3 Meiosis and the Production of Gametes Sexual Reproduction 12.4 Sexual Reproduction in Microorganisms 12.5 Sexual Reproduction in Plants 12.6 Sexual Reproduction in Animals Reproduction in Humans 12.7 Egg Production and the Menstrual Cycle 12.8 Sperm Production 12.9 Secondary Sex Characteristics 12.10 Infertility and Contraception Chapter Highlights Chapter Animations Chapter Menu Contents

By the end of this chapter you will be able to: Learning Outcomes By the end of this chapter you will be able to: A Describe reproduction in plants and animals. B Explain the importance of meiosis in maintaining chromosome numbers and identify the stages of meiosis. C Infer the advantages of a dominant diploid stage in the life cycle of plants and animals. D Relate the process of fertilization in flowering plants to their successful domination of land environments. E Compare external and internal fertilization. F Discuss the influence of hormones on the human male and female reproductive systems. G Discuss causes of infertility and methods of contraception. Learning Outcomes

Reproduction This photo shows a bee visiting the disc florets of a flower in the Asteraceae family. What benefit does the flower gain from the activity of this bee? How might a cluster of flowers give this species a reproductive advantage? Chapter Introduction 1

Reproduction This photo shows a bee visiting the disc florets of a flower in the Asteraceae family. For a species to survive, it must reproduce successfully in every generation. Some organisms reproduce individually; their offspring are genetically identical to the parent. Others reproduce sexually, producing offspring that receive genetic information from two parents. Chapter Introduction 2

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Cell Division and Reproduction 12.1 Asexual Reproduction Asexual reproduction requires a single parent where one or more of their cells form a genetically identical offspring. A group of genetically identical cells or organisms produced through asexual reproduction is called a clone. 12.1 Asexual Reproduction 1

Cell Division and Reproduction 12.1 Asexual Reproduction (cont.) Prokaryotes and one-celled eukaryotes reproduce by simply dividing in two, a process called binary fission. Other eukaryotic organisms that reproduce asexually include many fungi and simple animals. 12.1 Asexual Reproduction 2

Cell Division and Reproduction 12.1 Asexual Reproduction (cont.) fragmentation in Planaria Mitotic cell division in Paramecium caudatum (color added, x400) Budding in Hydra viridis (color added, x8) 12.1 Asexual Reproduction 3

Cell Division and Reproduction 12.1 Asexual Reproduction (cont.) Some plants reproduce through fragmentation. Asexual reproduction in plants, or vegetative reproduction, is efficient in filling an area with plants, but it is less successful in quickly spreading plants to new locations. 12.1 Asexual Reproduction 4

Cell Division and Reproduction 12.1 Asexual Reproduction (cont.) Fragments of the cactus known as jumping cholla (Opuntia bigelovii) break off easily and are carried by wind or animals to new locations. The fragments form new plants where they land. Leaves of Bryophyllum produce little plantlets, each of which may fall to the ground near the parent and grow as a new plant. 12.1 Asexual Reproduction 5

Cell Division and Reproduction 12.2 Chromosome Numbers Each species has a characteristic number of chromosomes. Prokaryotes generally have only one major chromosome, consisting of a single circle of DNA. The number of chromosomes varies among eukaryotes. 12.2 Chromosome Numbers 1

Cell Division and Reproduction 12.2 Chromosome Numbers (cont.) Cells of most organisms that reproduce sexually have pairs of similar chromosomes. Each parent provides one member of each pair. Cells that carry a double set of chromosomes are called diploid (2n). Cells with just one set of chromosomes are called haploid (n). 12.2 Chromosome Numbers 2

Changes in chromosome number in a typical sexual life cycle. 12.2 Chromosome Numbers 3

Cell Division and Reproduction 12.2 Chromosome Numbers (cont.) In diploid organisms, the two chromosomes of a pair are called homologous. Homologous chromosomes, or homologues, with the exception of the sex chromosomes, are similar in structure. Homologues carry the same genes, though differences in their DNA sequences produce the variety you see among members of the same species. 12.2 Chromosome Numbers 4

Cell Division and Reproduction 12.2 Chromosome Numbers (cont.) In asexual reproduction, the cells of parent and offspring carry identical sets of chromosomes. In sexual reproduction, the reproductive cells that fuse during sexual reproduction must each be haploid. If reproductive cells were diploid like somatic cells, the number of chromosomes would double in each generation. 12.2 Chromosome Numbers 5

Cell Division and Reproduction 12.2 Chromosome Numbers (cont.) In sexual reproduction, each parent produces haploid gametes. Male gametes are sperm, and female gametes are ova (singular: ovum), or eggs. During fertilization, male and female gametes join, and their nuclei fuse. A new individual develops from the diploid fertilized egg, or zygote. 12.2 Chromosome Numbers 6

Cell Division and Reproduction 12.2 Chromosome Numbers (cont.) A special cell-division process, meiosis, produces the haploid gametes that must generally either fuse with another gamete or die. In fungi and simple plants, meiosis produces different types of haploid cells called spores. Most spores can develop into haploid organisms without fertilization. 12.2 Chromosome Numbers 7

Cell Division and Reproduction 12.2 Chromosome Numbers (cont.) A few species of plants and animals have lost the ability to reproduce sexually. Populations of desert whiptail lizards, Cnemidophorus neomexicanus, consist entirely of females. The factors that led to natural selection for asexual reproduction in these populations are not yet clear. 12.2 Chromosome Numbers 8

12.3 Meiosis and the Production of Gametes 1 Cell Division and Reproduction 12.3 Meiosis and the Production of Gametes Meiosis differs from mitosis in three important ways: 1. Cells divide twice during meiosis, but the chromosomes are not duplicated after the first division. 2. Meiosis distributes a random mixture of maternal and paternal chromosomes to each gamete resulting in new genetic combinations. 12.3 Meiosis and the Production of Gametes 1

12.3 Meiosis and the Production of Gametes 2 Cell Division and Reproduction 12.3 Meiosis and the Production of Gametes (cont.) 3. Homologous chromosomes pair up side by side during the first meiotic division, often exchanging corresponding pieces of DNA. This exchange is called crossing-over. Crossing-over changes each chromosome into a mixture of maternal and paternal genes adding to the genetic variety of the gametes. 12.3 Meiosis and the Production of Gametes 2

12.3 Meiosis and the Production of Gametes 3

12.3 Meiosis and the Production of Gametes 4 Cell Division and Reproduction 12.3 Meiosis and the Production of Gametes (cont.) Meiosis involves two nuclear divisions—meiosis I and meiosis II—that produce four haploid cells. Meiosis is a complex process because it does three important things: 1. reduces chromosomes to the haploid number 2. provides genetic variation 3. ensures the correct distribution of chromosomes into the resulting cells 12.3 Meiosis and the Production of Gametes 4

12.3 Meiosis and the Production of Gametes 5 Cell Division and Reproduction 12.3 Meiosis and the Production of Gametes (cont.) Meiosis does not always divide the cytoplasm equally between daughter cells. In most male animals, including humans, meiosis produces four equal-sized sperm. 12.3 Meiosis and the Production of Gametes 5

12.3 Meiosis and the Production of Gametes 6 Cell Division and Reproduction 12.3 Meiosis and the Production of Gametes (cont.) In females, most of the cytoplasm remains in one cell which becomes the ovum. In animal species, the two small cells, called polar bodies, usually break down and disintegrate. In flowering plants, one polar body survives and remains diploid, eventually developing into the endosperm that nourishes the embryo. 12.3 Meiosis and the Production of Gametes 6

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12.4 Sexual Reproduction in Microorganisms 1 Prokaryotes reproduce asexually through a process of cell fusion called conjugation, which promotes genetic variation but does not produce offspring. In conjugation, a tube of cytoplasm temporarily connects cells which allows the exchange DNA. Conjugation also occurs in many unicellular eukaryotes. Two strands of the conjugating colonial alga Spirogyra elegans exchange DNA through temporary cytoplasmic bridges, x190. 12.4 Sexual Reproduction in Microorganisms 1

12.4 Sexual Reproduction in Microorganisms 2 12.4 Sexual Reproduction in Microorganisms (cont.) The life cycles of organisms that reproduce both asexually and sexually include both haploid and diploid stages, a pattern called alternation of generations. A diagram of alternation of generations shows that the haploid and diploid stages of the same organism often look completely different. 12.4 Sexual Reproduction in Microorganisms 2

12.4 Sexual Reproduction in Microorganisms 3 12.4 Sexual Reproduction in Microorganisms (cont.) Many fungi and other microorganisms switch from asexual to sexual reproduction in response to changes in their environment. 12.4 Sexual Reproduction in Microorganisms 3

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