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Cell Reproduction Mitosis – asexual reproduction – 1 cell produces 2 identical cells Meiosis – sexual reproduction – 1 parent cell produces 4 cells with ½ the number of chromosomes as the parent Why Cells Divide Growth Repair Reproduction When Cells Divide Surface Area/Volume Ratio determines the maximum size of any cell. Cells can’t get bigger, so they divide. Terminology Parent cells for new daughter cells Chromosomes contain DNA & genes Homologous Chromosomes (a pair) Phases of Mitosis Interphase Chromosomes are uncoiled DNA replicates Centrioles replicate DIVISION INTERPHASE Prophase Chromosomes coil - appear double stranded Nucleolus & nuclear membrane disappear Centromere and chromatids are visible Centrioles form aster rays & spindle fibers as they move to the poles Metaphase Double stranded chromosomes individually line up on the equator Centromeres attach to a spindle fiber CENTROMERE CENTRIOLE CHROMATID Anaphase Chromosomes split & separate and are pulled to opposite poles of the cell by the spindle fibers Telophase Cytoplasm divides (cytokinesis) New nuclear membranes form Cell Division and Reproduction Sexual Reproduction – 2 parents needed. – Offspring are similar to, but not identical to the parents. – Genetic variations contribute to evolution of species. Chromosome Number Each species has a specific chromosome number. Homologous chromosomes - 2 of a same pair. Have the same number and types of genes. One from egg, other from sperm. Gametes - reproductive cells. Have half the chromosome number. They have only one chromosome of each pair. Chromosome Number Haploid - half chromosome number Sex cells (n) Diploid - pairs of chromosomes Body cells (2n) Meiosis - reduction division reduces chromosome number to haploid. Phases of Meiosis One DNA replication followed by two cell divisions. Interphase I DNA replicated Centrioles replicated Prophase I Chromosomes coil Centrioles move to poles Spindle fibers and asters form Nuclear membrane disappears Homologous chromosomes synapse & crossing over occurs Metaphase I Homologous chromosomes line up at the equator in pairs Crossing over occurs between homologs Anaphase I Homologous chromosomes separate and move to poles Telophase I Cytokinesis occurs forming 2 haploid cells Cells contain double stranded chromosomes at this point Interphase II No DNA replication Chromosomes uncoil Centrioles replicate Rest same as Mitosis Prophase II - Chromosomes coil. Metaphase II - Line up on equator. Anaphase II - Split at centromere and move to opposite poles. Telophase II - Cytokinesis results in 4 haploid cells. Spermatogenesis Sperm cell formation 4 equal sized gametes form 23 23D 23 46 46D 23 23D 23 Oogenesis Egg cell formation 1 large egg + 3 polar bodies (die) 23 23 D 46 46D 23 23 23D 23 Mitosis vs. Meiosis Mitosis 46 Zygote Diploid Cells Fertilization 23 23D 23 Meiosis 46 23D Haploid Cells 23 23 Gametes 23 Genetic Variations Sexual reproduction (fertilization) Meiosis - independent assortment Meiosis - crossing over Random Mutations