Meiosis Objectives 1. Describe the stages of meiosis. 2. Explain why meiosis occurs. 3. Compare and contrast mitosis and meiosis Pg 275-278

Think! Think!! Think!!! Meiosis is the process in which one cell divides into four daughter cells that have half the number of chromosomes as the parent cell. Why would your body produce cells that have half the number of necessary chromosomes?

Think! Think!! Think!!! Your body has two types of cells: body cells and gametes. Body cells – have 46 or 23 pairs of chromosomes Gametes – sperm and egg cells that have only 23 chromosomes Gametes join with other gametes and together they have 46 chromosomes.

Meiosis Meiosis is the process in which one cell divides into four daughter cells that have half the number of chromosomes as the parent cell. In addition, the daughter cells are different from each other and different from the parent cell. Diploid – two sets of chromosomes (body cell) Haploid – one set of chromosomes (gamete)

Stages of Meiosis The end result of meiosis is four cells, so there must be two divisions. First one cell divides into two (Meiosis I) and then each of those cells divide into two (Meiosis II). Meiosis I consists of Prophase I, Metaphase I, Anaphase I and Telophase and Cytokinesis I. Meiosis II consists of Prophase II, Anaphase II and Telophase II and Cytokinesis II.

Meiosis http://www.sumanasinc.com/webcontent/animations/content/meiosis.html

Mitosis vs. Meiosis Mitosis Meiosis Creates two cells from one, identical to each other and identical to the parent cell Creates four cells from one, different from each other and from the parent cell Makes diploid daughter cells Makes haploid daughter cells Occurs in body cells Occurs in gametes Has only one set of phases Has two sets of phases Sister chromatids split apart and one chromosome of each pair goes to each daughter cell Homologous sister chromatids pair up to form a tetrad and two sister chromatids go to each daughter cell, then the sister chromatids split apart Chromosomes do not “cross over” Chromosomes do “cross over”

Meiosis Homologous – similar Homologous chromosomes pair up. They are homologous because one was inherited from mom and one was inherited from dad. Tetrad – two sets of homologous sister chromatids joined together.

Crossing Over When the tetrad is formed, the “legs” switch pieces.

Mitosis vs. Meiosis http://www.pbs.org/wgbh/nova/body/how-cells-divide.html

Meiosis During meiosis, the chromosomes are “shuffled” like a deck of cards because of crossing over. This means that every child a parent has will be different from all the other children that could possible be conceived. Crossing over spreads biological variation throughout the population. Variation makes a population strong!