Ways to show the number of chromosomes in a cell. 2n 2 copies of each chromosome Body cells n 1 copy of each chromosome Sex cells DIPLOIDHAPLOID

Meiosis Definition: Cell division where cells divide twice. Done by: Sex cells in multi- cellular organisms (diploid) Results in: 4 cells with ½ DNA (haploid cells) n n += 2n Zygote (baby) Variation: Crossing over – genes switch between homologs. Independent assortment – homologous chromosomes line up differently every time. Mutations Sexual reproduction

What Happens During Meiosis?

MEIOSIS I A diploid (2N) cell divides to produce 2 daughter cells. A diploid (2N) cell divides to produce 2 daughter cells. Creates genetic variation. Creates genetic variation.

Prophase I Homologous chromosomes pair up to form tetrads (groups of four chromatids). Homologous chromosomes pair up to form tetrads (groups of four chromatids). Crossing over occurs when homologous chromosomes overlap. Crossing over occurs when homologous chromosomes overlap.

Metaphase I Tetrads line up at the center of the cell. Tetrads line up at the center of the cell.

Anaphase I One homologous chromosome is pulled to each side of the cell. One homologous chromosome is pulled to each side of the cell.

Telophase I Two daughter nuclei are formed. Two daughter nuclei are formed. Followed by cytokinesis and then immediately meiosis II. Followed by cytokinesis and then immediately meiosis II. NO replication! NO replication!

MEIOSIS II Two cells each divide to form four haploid (N) gametes. Two cells each divide to form four haploid (N) gametes. In females, one viable egg + three “polar bodies” In females, one viable egg + three “polar bodies” In males, four viable sperm In males, four viable sperm

Prophase II Chromosomes condense in each cell. Chromosomes condense in each cell.

Metaphase II In each cell, chromosomes line up in the middle. In each cell, chromosomes line up in the middle.

Anaphase II In each cell, sister chromatids are pulled apart. In each cell, sister chromatids are pulled apart.

Telophase II 4 new nuclei form, 2 in each cell. 4 new nuclei form, 2 in each cell. Followed by cytokinesis, resulting in 4 haploid (N) gametes. Followed by cytokinesis, resulting in 4 haploid (N) gametes.