Meiosis

Meiosis A process of nuclear division that reduces the number of chromosomes in new cells to half the number in the original cell.

Formation of Haploid Cells Meiosis produces gametes, which are haploid reproductive cells. Cells preparing to divide by meiosis undergo the G1, S, and G2 phases of interphase. Cells begin meiosis with a duplicate set of chromosomes, just like mitosis. Cells undergoing meiosis divide twice, resulting in 4 haploid (1n) cells.

Two Stages of Meiosis First cell division = Meiosis I Prophase I, Metaphase I, Anaphase I, Telophase I and Cytokinesis I Second cell division = Meiosis II Prophase II, Metaphase II, Anaphase II, Telophase II and Cytokinesis II

Telophase I and Cytokinesis Meiosis I Meiosis I Interphase I During meiosis, the number of chromosomes per cell is cut in half through the separation of the homologous chromosomes. The result of meiosis is 4 haploid cells that are genetically different from one another and from the original cell. Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Copyright Pearson Prentice Hall

Prophase I DNA coils tightly into chromosomes. Spindle fibers appear. Nuclear membrane disassembles. Synapsis occurs - homologous chromosomes line up next to each other. Each pair of homologous chromosomes is called a tetrad because there are 4 chromatids in a tetrad. Tetrad

Tetrads Form in Prophase I Homologous chromosomes (each with two chromatids)   Join to form a TETRAD Called Synapsis

Prophase I During synapsis, the chromatids within a homologous pair twist around one another. Portions of chromatids may break off and attach to adjacent chromatids–a process called crossing-over. Genetic recombination results – genetic material between maternal and paternal chromosomes is exchanged.

Copyright Pearson Prentice Hall Crossing-over Crossing-over occurs during meiosis. (1) Homologous chromosomes form a tetrad. (2) Chromatids cross over one another. (3) The crossed sections of the chromatids are exchanged. Copyright Pearson Prentice Hall

Metaphase I Tetrads line up randomly along the middle of the cell. Spindle fibers attach to the centromere of each homologous chromosome.

Anaphase I Each homologous chromosome moves to an opposite end of the cell. Note: Each homologous chromosome still consists of two chromatids attached by a centromere. Random separation of homologous chromosomes is called independent assortment and results in genetic variation.

Telophase I and Cytokinesis I Chromosomes reach opposite ends of the cell and cytokinesis begins. A nuclear membrane forms around each set of chromosomes. Two new cells are produced, each containing one chromosome from each homologous pair (haploid – 1n).

Copyright Pearson Prentice Hall Meiosis II The two cells produced by meiosis I now enter a second meiotic division. Unlike meiosis I, neither cell goes through chromosome replication. Each of the cell’s chromosomes has 2 chromatids. Copyright Pearson Prentice Hall

Telophase I and Cytokinesis I Telophase II and Cytokinesis Meiosis II During meiosis, the number of chromosomes per cell is cut in half through the separation of the homologous chromosomes. The result of meiosis is 4 haploid cells that are genetically different from one another and from the original cell. Meiosis II Telophase I and Cytokinesis I Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Copyright Pearson Prentice Hall

Prophase II The nuclear membrane breaks down and a new spindle forms.

Metaphase II The chromosomes line up in the center of cell.

Anaphase II Chromatids separate and move toward opposite ends of the cell.

Telophase II and Cytokinesis II Telophase II – a nuclear membrane forms around the chromosomes in each of the four new cells. Cytokinesis II – 4 new cells are formed, each with half of the original cell’s number of chromosomes. http://glencoe.mcgraw-hill.com/sites/9834092339/student_view0/chapter11/stages_of_meiosis.html

Stages Of Meiosis: Meiosis I Interphase Mother cell Prophase I: Condensing Chromosomes Prophase I: Tetrad formation/ crossing over Metaphase I Anaphase I Telophase I Meiosis II

Stages Of Meiosis: Meiosis II The products of meiosis are 4 haploid cells each with a unique set of chromosomes. Telophase I Prophase II Metaphase II The products of mitosis are 2 diploid cells with identical chromosomes. Anaphase II Telophase II

Development of Gametes In animals, the only cells that divide by meiosis are those that produce gametes within the reproductive organs. In humans – testes (males) and ovaries (females)

Development of Gametes SPERMATOGENESIS In the testes – male gametes known as sperm cells or spermatozoa are produced. One diploid cell divides meiotically to form four haploid cells called spermatids – each develops into a mature sperm cell.

Development of Gametes OOGENESIS Production of mature egg cells, or ova. A diploid reproductive cell divides meiotically to produce one mature egg cell (ovum) and three polar bodies which will degenerate – cytoplasm is not evenly distributed in cytokinesis.

Sexual Reproduction Production of offspring through meiosis and the union of a sperm and an egg. Offspring are genetically different because genes are combined in new ways. Advantage: enables species to adapt rapidly to new conditions.

Comparing Mitosis and Meiosis Mitosis Meiosis Number of Cell Divisions Number of Cells Produced Homologous Chromosomes pair up (yes/no) Mother Cell Haploid (1n) or Diploid (2n) Daughter Cells Haploid (1n) or Diploid (2n) Type of cell being produced Daughter cells have identical DNA (yes/no) 1 2 2 4 No Yes Diploid (2n) Diploid (2n) Diploid (2n) Haploid (1n) Body Cells Sex Cells Yes No http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__comparison_of_meiosis_and_mitosis__quiz_1_.html