Cell Reproduction n Mitosis – asexual reproduction –1 cell produces 2 identical cells n Meiosis – sexual reproduction –1 parent cell produces 4 cells with ½ the number of chromosomes as the parent

Why Cells Divide n Growth n Repair n Reproduction

When Cells Divide n Surface Area/Volume Ratio determines the maximum size of any cell. n Cells can’t get bigger, so they divide.

Terminology n Parent cells for new daughter cells n Chromosomes contain DNA & genes n Homologous Chromosomes (a pair)

Phases of Mitosis

Interphase n Chromosomes are uncoiled n DNA replicates n Centrioles replicate INTERPHASE DIVISION

Prophase n Chromosomes coil - appear double stranded n Nucleolus & nuclear membrane disappear n Centromere and chromatids are visible n Centrioles form aster rays & spindle fibers as they move to the poles

Metaphase n Double stranded chromosomes individually line up on the equator n Centromeres attach to a spindle fiber CENTROMERE CHROMATID CENTRIOLE

Anaphase n Chromosomes split & separate and are pulled to opposite poles of the cell by the spindle fibers

Telophase n Cytoplasm divides (cytokinesis) n New nuclear membranes form

Cell Division and Reproduction n Sexual Reproduction –2 parents needed. –Offspring are similar to, but not identical to the parents. –Genetic variations contribute to evolution of species.

Chromosome Number n Each species has a specific chromosome number. n Homologous chromosomes - 2 of a same pair. Have the same number and types of genes. One from egg, other from sperm. n Gametes - reproductive cells. Have half the chromosome number. They have only one chromosome of each pair.

Chromosome Number nHnHaploid - half chromosome number Sex cells (n) nDnDiploid - pairs of chromosomes Body cells (2n) nMnMeiosis - reduction division reduces chromosome number to haploid.

Phases of Meiosis One DNA replication followed by two cell divisions.

Interphase I n DNA replicated n Centrioles replicated

Prophase I n Chromosomes coil n Centrioles move to poles n Spindle fibers and asters form n Nuclear membrane disappears n Homologous chromosomes synapse & crossing over occurs

Metaphase I nHnHomologous chromosomes line up at the equator in pairs nCnCrossing over occurs between homologs

Anaphase I n Homologous chromosomes separate and move to poles

Telophase I n Cytokinesis occurs forming 2 haploid cells n Cells contain double stranded chromosomes at this point

Interphase II n No DNA replication n Chromosomes uncoil n Centrioles replicate

Rest same as Mitosis n Prophase II - Chromosomes coil. n Metaphase II - Line up on equator. n Anaphase II - Split at centromere and move to opposite poles. n Telophase II - Cytokinesis results in 4 haploid cells.

Spermatogenesis nSnSperm cell formation n4n4 equal sized gametes form 46D 46 23D 23

Oogenesis nEnEgg cell formation n1n1 large egg + 3 polar bodies (die) 4646D 23D 23

Mitosis vs. Meiosis D 23 Meiosis Mitosis Diploid Cells Fertilization Haploid Cells Gametes Zygote

Genetic Variations n Sexual reproduction (fertilization) n Meiosis - independent assortment n Meiosis - crossing over n Random Mutations