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SExual reproduction continued Spermatogenesis and Oogenesis

Cell division: a comparison Mitosis Meiosis Function of Process Genetic Variability Number of Divisions Number of Cells Produced Number of Chromosomes Per Cell to produce somatic cells to produce sex cells Yes - crossing over during Prophase I None One Two Two Four 46 Diploid 2n 23 Haploid n

gamete formation (Gametogenesis) Meiosis takes place in the testes and ovaries (haploid sperm [n] and egg cells [n] respectively are produced) Haploid gametes unite to form diploid (2n) zygote known as fertilization

oogenesis Begins with a diploid cell called an oogonium Cytoplasm of female gametes doesn’t divide equally after nuclear division One daughter cell receives most of the cytoplasm after the division. That cell continues through meiosis I and II to form ONE viable egg. This cell contains a large quantity of nutrients that will support a zygote after fertilization The other 3 polar bodies die and the body absorbs the nutrients. Begins with a diploid cell called an oogonium Did you know..... Before birth, the oogonia reproduce by mitosis, and they begin meiosis, but stop at prophase I. Meiosis I will continue for one cell each month beginning at puberty

Up to 1 billion sperm cells produced / day spermatogenesis Begins with diploid cell called spermatogonium Equal division of cytoplasm with sperm 4 cells produced via meiosis Capable of many mitotic divisions before meiosis Up to 1 billion sperm cells produced / day

Sources of genetic variation 1. Crossing Over 2. Independent Assortment

Crossing over In prophase 1, non-sister chromatids can exchange pieces of chromosomes As a result, individual chromosomes can contain genes of maternal origin and some genes of paternal origin Increases genetic diversity of the gametes produced

independent assortment In metaphase 1, homologous chromosomes line up in the middle of the cell Depending how the homologous chromosomes line up, a number of different combinations of chromosomes may be found in the gametes

Homework Read pages 173-175 Pg. 181 # 1, 4, 6, 9, 11