MGC New Life Christian Academy

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1 A. Meiosis Main Idea: Meiosis produces haploid gametes. Key Concept: Asexual reproduction involves one parent and produces offspring that are genetically identical to each other and to the parent. Sexual reproduction involves two parents and produces offspring that is genetically unique. The sequence of stages an organism goes through from one generation to the next is its life Lesson Vocabulary asexual reproduction: reproduction that involves a single parent and results in offspring that are all genetically identical to the parent crossing-over: exchange of genetic material between homologous chromosomes when they are closely paired during meiosis I diploid: having two of each type of chromosome egg: female gamete fertilization: union of two gametes that produces a diploid zygote gamete: reproductive cell produced during meiosis that has the haploid number of chromosomes gametogenesis: development of haploid cells into gametes such as sperm and egg haploid: having only one chromosome of each type independent assortment: independent segregation of chromosomes to gametes during meiosis life cycle: series of stages a sexually reproducing organism goes through from one generation to the next meiosis: type of cell division in which the number of chromosomes is reduced by half and four haploid cells result sexual reproduction: type of reproduction that involves the fertilization of gametes produced by two parents and produces genetically variable offspring sperm: male gamete zygote: diploid cell that forms when two haploid gametes unite during fertilization High School Faculty 2

2 Real-World Reading Link: Look around your class. You might notice that the students in your class do not all look the same. They might be of different heights and have different eye color, hair color, and other features. This variety of characteristics is a result of two sex cells combining during sexual reproduction. I. Chromosomes and Chromosome Number Each student in your class has characteristics passed on to them by their parents. Each characteristic, such as hair color, height, or eye color, is called a trait. The instructions for each trait are located on chromosomes, which are found in the nucleus of cells. The DNA on chromosomes is arranged in segments that control the production of proteins. These DNA segments are called genes. Each chromosome consists of hundreds of genes, each gene playing an important role in determining the characteristics and functions of the cell. Homologous Chromosomes Human body cells have 46 chromosomes. Each parent contributes 23 chromosomes, resulting in 23 pairs of chromosomes. The chromosomes that make up a pair, one chromosome from each parent, are called homologous chromosomes. Haploid and Diploid Cells In order to maintain the same chromosome number from generation to generation, an organism produces gametes, which are sex cells that have half the number of chromosomes. Humans each gamete contains 23 chromosomes. The symbol n can be used to represent the number of chromosomes in a gamete. A cell with n number of chromosomes is called a haploid cell. Haploid comes from the Greek word haploos, meaning single. The process by which one haploid gamete combines with another haploid gamete is called fertilization. As a result of fertilization, the cell now will contain a total of 2n chromosomes n chromosomes from the female parent plus n chromosomes from the male parent. High School Faculty 3

3 A cell that contains 2n number of chromosomes is called a diploid cell. When two human gametes combine, 23 pairs of homologous chromosomes are formed. II. Meiosis I Gametes are formed during a process called meiosis, which is a type of cell division that reduces the number of chromosomes; therefore, it is referred to as a reduction division. Meiosis occurs in the reproductive structures of organisms that reproduce sexually. While mitosis maintains the chromosome number, meiosis reduces the chromosome number by half through the separation of homologous chromosomes. A cell with 2n number of chromosomes will have gametes with n number of chromosomes after meisosis. Meiosis involves two consecutive cell divisions called meiosis I and meiosis II. Stages of Meiosis I 1. Interphase Cells that undergo meiosis also go through interphase as part of the cell Cells in interphase carry out various metabolic processes, including the replication of DNA and the synthesis of proteins. 2. Prophase I Pairing of homologous chromosomes occurs, each chromosome consists of 2 chromatids. Crossing over produces exchange of genetic information. The nuclear envelope breaks down. Spindles form. Crossing over is a process during which chromosomal segments are exchanged between a pair of homologous chromosomes. 3. Metaphase I Chromosome centromeres attach to spindle fibers. High School Faculty 4

4 Homologous chromosomes line up at the equator. 4. Anaphase I Homologous chromosomes separate and move to opposite poles of the cell 5. Telophase I and Cytokinesis The spindle breaks down, and new nuclear membranes form. The cytoplasm of the cell divides, and 2 haploid daughter cells result. The daughter cells each have a random assortment of chromosomes, with one from each homologous pair. Both daughter cells go on to meiosis II. The DNA does not replicate between meiosis I and meiosis II. Meiosis II 1. Prophase II The nuclear envelope breaks down and the spindle begins to form in each haploid daughter cell from meiosis I. The centrioles also start to separate. 2. Metaphase II Spindle fibers line up the sister chromatids of each chromosome along the equator of the cell. 3. Anaphase II Sister chromatids separate and move to opposite poles. 4. Telophase II and Cytokinesis The spindle breaks down, and new nuclear membranes form. The cytoplasm of each cell divides, and 4 haploid cells result. Each cell has a unique combination of chromosomes. The Importance of Meiosis: Meiosis is important because it results in genetic variation. Recall that pairs of homologous chromosomes line up at the equator during prophase I. How the chromosomes line up at the equator is a random process that results in gametes with different combinations of chromosomes. Depending on how the chromosomes line up at the equator, four gametes with four different combinations of chromosomes can result. Genetic variation also is produced during crossing over and during fertilization, when gametes randomly combine. Mitosis and Meiosis High School Faculty 5

5 Mitosis Meiosis One division occurs during mitosis. Two sets of divisions occur during meiosis: meiosis I and meiosis II. DNA replication occurs during interphase. DNA replication occurs once before meiosis I. Synapsis of homologous chromosomes does not occur. Synapsis of homologous chromosomes occurs during prophase I. Two identical cells are formed per cell Four haploid cells (n) are formed per cell The daughter cells are genetically The daughter cells are not genetically identical. identical because of crossing over. Mitosis occurs only in body cells. Meiosis occurs in reproductive cells. Mitosis is involved in growth and repair. Meiosis is involved in the production of gametes and providing genetic variation in organisms. B. Mendelian Genetics High School Faculty 6