Division to produce Sex Cells Meiosis Division to produce Sex Cells

Meiosis- A process of cell division in which a diploid cell produces haploid gametes by separating homologous pairs of chromosomes. Diploid – 2 sets of chromosomes Haploid – 1 set of chromosomes Gamete – sex cells, (sperm, egg, pollen, ovum) Homologous – pairs of matching chromosomes that come from each parent (numbered 1-23 in humans)

Meiosis

Meiosis Stages Meiosis involves 2 distinct cell divisions: Meiosis I (P,M,A,T) Meiosis II Meiosis Animation

Prophase I Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. There are 4 chromosomes in a tetrad (2 pairs of sister chromatids) 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 (genes) “CROSSING OVER” is like “shuffling the genetic deck”- called RECOMBINATION

Prophase I

Metaphase I Homologous pairs line up side by side in middle of cell It is RANDOM which side the maternal and paternal chromosomes line up on “RANDOM ASSORTMENT” is a second “shuffling of the genetic deck”

Anaphase I Homologous pairs pulled apart. (Not sister chromatids!!)

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

End of Meiosis I Meiosis II Meiosis I results in two haploid (1N) cells. Each cell has half the number of chromosomes as the original cell. Meiosis II Starts immediately after Meiosis I (no interphase) Meiosis II is basically just like Mitosis

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

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

Anaphase II Sister chromatids separate and move to opposite sides.

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

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