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

2. 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?

3. 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.

4. 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)

5. 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.

6. Meiosis

7. 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”

8. 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.

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

10. Mitosis vs. Meiosis

11. 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!