Meiosis Division to produce Sex Cells Meiosis A

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 § (P, M, A, T) 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 I results in two haploid (1 N) 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 Results in Cells are Occurs in Mitosis 2 Diploid Cells (2 N) Genetically Identical Somatic (Body) Cells Meiosis 4 Haploid Cells (N) Genetically Different Sex Cells