Chapter 4 Meiosis Lesson 23

Chapter 4 • Lesson 23 Meiosis and Genetic Variation Objective: 3,2,1 Key Words • sexual reproduction • gamete • somatic cell • mitosis • variations • haploid number • fertilization • diploid number • homologous chromosomes • meiosis • crossing-over • independent assortment • asexual reproduction ;•.................................................'................'; Getting the Idea About 7 billion people live on Earth. With the exception of identical twins, no two of those people are genetically alike. Earth's human population is tremendously diverse. For example, there are probably thousands of shades of skin color among the world's population. Many other traits also vary widely. You may have inherited your mother's blood type and your father's hair color. Such diversity, although perhaps more subtle, exists among all organisms that reproduce sexually. Sexual Reproduction and Variation Humans and most other multicellular organisms reproduce sexually. Recall that in sexual reproduction, cells from two parents join to form a new individual. Offspring produced sexually are genetically different from either parent because sexual reproduction combines two genetically unique reproductive cells, called gametes or sex cells. In multicellular organisms, such as humans, the somatic cells, or body cells, divide by mitosis. Recall from Lesson 5 that mitosis is a type of cell division in which the cell nucleus divides in two. In this process, the DMA of the original cell is divided equally between two daughter cells. Each daughter cell has the same number and kinds of chromosomes as the parent cell. If two somatic cells joined together, the resulting cell would have twice as many chromosomes as it should. This does not happen because gametes are not produced by mitosis. Instead, they are produced by a kind of cell division called meiosis. You will learn how this process occurs later in the lesson. Meiosis results in variations—differences in traits among the members of a species or population. Chromosomes Recall that chromosomes are structures that contain the cell's genetic material. Each human gamete has 23 chromosomes. The gametes of other organisms may have more or fewer chromosomes. For example, each gamete of a cat contains 19 chromosomes, a rabbit 22, a crayfish 100, and a tomato 12. The number of chromosomes in a gamete is called the haploid number. Haploid refers to "half." The chromosome number in gametes is haploid because sex cells contain half as many chromosomes as somatic cells. Sexual reproduction requires the joining of two gametes, a sperm from the male parent and an egg from the female parent. The process in which a sperm and an egg combine is called fertilization. The zygote, or fertilized egg that results from this process, develops into a new multicellular organism. During fertilization in humans, two gametes combine to form a cell with 46 chromosomes. The resulting offspring has 46 chromosomes in every somatic cell, or body cell. The chromosomes are arranged in 23 pairs. The number of chromosomes in each somatic cell is the diploid number. A zygote receives one set of chromosomes from each gamete. The chromosomes from the two parents combine to form pairs of homologous chromosomes. Homologous chromosomes are paired chromosomes that have matching genes but may have different alleles. Each gamete is different, so each organism inherits a different combination of alleles. The varied combinations of alleles give different offspring different traits. Meiosis and Gamete Formation Sperm and egg cells form by meiosis. Meiosis is a process of cell division that reduces the number of chromosomes by half. During meiosis, each sex cell divides twice, in stages called meiosis I and meiosis II. As the diagram below illustrates, meiosis begins with a single diploid cell and produces four genetically different haploid cells. During meiosis I (the first division in the diagram above), the homologous chromosomes of the parent cell pair up. While the homologous chromosomes are paired up, crossing-over can occur. Crossing-over is a process in which segments of homologous chromosomes break off and are exchanged. When the cell divides to produce two daughter cells, each daughter cell receives one chromosome from each homologous pair. In this way, crossing-over increases the number of possible genetic combinations in the offspring. Recall Mendel's law of independent assortment from the last lesson. This law states that different pairs of genes separate independently of one another when gametes form. After crossing-over, pairs of homologous chromosomes line up along the center of the cell in a random fashion called independent assortment. Like crossing-over, independent assortment leads to genetic variation. When the cell divides, each daughter cell receives a mix of chromosomes that differs from that of the original cell. The exact mix depends on how the chromosomes lined up before the cell divided. Look again at the diagram of meiosis. The stages of meiosis II (the second division in the diagram) are nearly identical to those of mitosis except that meiosis II begins with haploid cells instead of a diploid cell. The two haploid daughter cells formed by meiosis I divide to form four haploid cells in meiosis II. Each haploid cell has a unique set of chromosomes. The rearrangement of genes during sexual reproduction gives organisms combinations of genes that differ from those of their parents. Each human gamete has 23 chromosomes, containing a total of more than 20,000 genes. These genes are shuffled during meiosis I through crossing-over and independent assortment. The number of possible gene combinations is many times greater than the number of humans who have ever lived. This huge variety of possible combinations accounts for the diversity of traits that result from sexual reproduction. Comparing Methods of Reproduction Recall that asexual reproduction is the production of offspring by a single parent. Some eukaryotes reproduce asexually by mitosis. Yeasts and freshwater animals called hydras reproduce in this way. Their offspring develop from buds on the parent's body. By contrast, meiosis is used to make gametes, specialized cells used only for sexual reproduction. The steps of meiosis are similar to those of mitosis, but there are important differences. Cells that undergo mitosis divide only once, to form two genetically identical diploid cells. By contrast, cells divide twice during meiosis, in stages called meiosis I and meiosis II, to produce four genetically different haploid cells. Asexual reproduction and sexual reproduction both have advantages and disadvantages. Asexual reproduction enables an organism to produce many offspring quickly. However, because the offspring are genetically identical, a factor such as a toxin in the environment that harms one offspring can harm all of them. Sexual reproduction produces relatively few offspring. However, because the offspring are genetically diverse, they may be able to survive in more varied conditions than organisms produced by asexual reproduction. For example, suppose a disease strikes a particular type of crop plant. If the plants are genetically different, a few may have genes that enable them to resist the disease. Although many individuals will die, some resistant plants will survive and reproduce. Those plants can then pass on the genes for disease resistance to their offspring.