Quick Review Diploid Haploid diploid haploid 23 What terms are used to describe cells containing the normal number of chromosomes? What terms are used to describe cells containing half the number of chromosomes? Mitosis produces ________ cells. Meiosis produces ________ cells. How many pairs of chromosomes do human body cells have? ___ pairs Diploid Haploid diploid haploid 23

Chromosome Numbers of Some Common Organisms Body Cell (2n) Gamete (n) Human 46 Garden Pea 7 Fruit Fly 8 Tomato 12 Dog 78 Chimpanzee 24 Leopard Frog 26 Corn 10 Apple 17 Indian Fern 1260 23 14 4 24 39 48 13 20 34 630

Meiosis I During prophase I, the ____________ coil up and the ______ fibers form. Then each pair of homologous chromosomes come together to form a four-part structure called a _____ (tetra = what number? __). A tetrad consists of two homologous chromosomes, each made up of two sister chromatids. These 4 chromatids in a ______ are held very close and tightly together. In fact, they are so close that the arms of non-sister chromatids can wind and twist around each other, and exchange genetic information. This is called ________ ____. Crossing over results in new combinations of alleles. chromosomes spindle tetrad 4 tetrad crossing over

The Phases of MEIOSIS Before meiosis begins: Interphase Before meiosis begins, what must happen to the cell’s DNA/ chromosomes?____________________________ Meiosis occurs in sexual reproduction when 1 diploid germ cell produces 4 haploid cells that can mature to become _______ (sperm or ova). They must be copied/ duplicated gametes

Prophase I Like prophase of mitosis: The ______ ________ disappears. ________ move to the opposite poles and the _____ _____ form between the centrioles. _________ condenses into sister _________ nuclear membrane Centrioles spindle fibers Chromatin chromatids Unlike prophase of mitosis: Pairs of homologous chromosomes form ______ ______ _____ may occur. tetrads Crossing over

Metaphase I Spindle fibers attach to the ____________________. Tetrads line up at the _________ plate. kinetochore or centromere metaphase Anaphase I Tetrads, which consist of two _________ __________, separate. homologous chromosomes

Telophase I / Cytokinesis I ______ _______ reappears. ______ ____ disappears. __________, the division of the cytoplasm and the organelles begins and ends forming two cells. Nuclear envelope Spindle fiber Cytokinesis

Meiosis II The purpose of meiosis II is to separate _______ ________ of each chromosome into separate cells. sister chromatids Prophase II The _____ _________ disappears. ______ ______ form. ________ condenses into sister _________ nuclear membrane Spindle fibers Chromatin chromatids

Metaphase II Sister chromatids attach to the ______ ____ Sister chromatids line up at the _________ plate. spindle fibers metaphase

Anaphase II Sister chromatids separate into ___________. Chromosomes move to the opposite _____. chromosomes poles

Telophase II ______ _______ reforms. 4 _______ are formed. Nuclear envelope gametes