Meiosis Division of Sex Cells

Meiosis A process of reduction division in which the number of chromosomes is cut in half through separation of homologous pairs in a diploid cell. Diploid – 2 sets of chromosomes Haploid – 1 set of chromosomes Homologous – pairs of matching chromosomes that come from each parent

Meiosis

Meiosis Stages Meiosis involves 2 distinct cell divisions: Meiosis I Meiosis II

Prophase I Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. There are 4 chromosomes in a tetrad. The pairing of homologous chromosomes is the key to meiosis. Crossing-over may occur here Crossing-over is when chromosomes overlap and exchange portions of their DNA

Prophase I

Metaphase I Spindle fibers attach to the chromosomes Homologous pairs line up side by side

Metaphase I

Anaphase I The fibers pull the homologous chromosomes toward opposite sides.

Anaphase I

Telophase I & Cytokinesis Nuclear membranes form. The cell separates into 2 cells.

Telophase I

End of Meiosis I Meiosis I results in two haploid (N) cells. Each cell has half the number of chromosomes as the original cell.

Prophase II Chromosomes re-condense and nuclear membrane breaks down again (similar to Prophase of Mitosis)

Prophase II

Metaphase II The chromosomes line up similar to metaphase in mitosis.

Metaphase II

Anaphase II Sister chromatids separate and move to opposite sides.

Anaphase II

Telophase II and Cytokinesis Meiosis II results in 4 GENETICALLY DIFFERENT haploid cells. (gametes)

Telophase II

Gamete Formation In males, meiosis results in 4 sperm cells (spermatogenesis)

Gamete Formation In females, meiosis results in 1 egg cell and three polar bodies, which are not used in reproduction. (Oogenesis)

Mitosis vs Meiosis Mitosis Meiosis Results in 2 Diploid Cells (2N) 4 Haploid Cells (N) Cells are Genetically Identical Genetically Different Occurs in Somatic (Body) Cells Sex Cells