Chapter 8 Section 3 Meiosis • Objectives: List and describe the phases of meiosis. • Compare the end products of mitosis with those of meiosis. • Explain crossing-over and how it contributes to the products of unique individuals. • Summarize the major characteristics of spermatogenesis and oogenesis. Cells undergoing meiosis have a duplicate set of chromosomes, just as cells beginning mitosis do. Cells undergoing meiosis divide twice . Diploid cells that divide meiotically become 4 haploid cells rather than two diploid cells. Prophase 1 Every chromosome lines up next to its homologue. The pairing of homologous chromosome is called synapsis. Each pair of homologous chromosomes is called a tetrad. During synapsis the chromatids within a homologous pair twist around one another. Portions of chromatids may break off and attach to adjacent chromatids on the homologous chromosome – a process called crossing over. Crossing over causes the exchange of genetic material between maternal and paternal chromosmes. Metaphase 1 Tetrads line up randomly along the midline of the dividing cell. Anaphase 1 Each homologous chromosome move s to an opposite pole of the dividing cell. Independent assortment results in genetic variation. Telophase 1 The chromosomes reach the opposite ends of the cell and cytokinesis begins. Meiosis II The steps continue without an interphase and result in four new cells. Each new cell contains half of the original cell’s number of chromosomes. Formation of Gametes In animals, meiosis produces haploid reproductive cells, called gametes. Meiosis occurs only in reproductive organs. In humans, meiosis occurs in the testes and in the ovaries. Spermatogenesis It occurs in the testes. Four haploid cells called spermatids are formed. Oogenesis Oogenesis is the production of matrue egg cells or ova. During cytokinesis I and II of oogenesis, the cytoplasm of the original cell is divided unequally between new cells. As a result, one egg cell is produced by meiosis. The other three products of meiosis, called polar bodies, degenerate. Asexual reproduction- production of offspring from one parent. Offspring are geneticall y identical to the parent. Sexual reproduction – production of offspring through meiosis and the union of sperm an egg. Offspring are genetically different from their parents.