Meiosis Chapter 11 Section 11:4

Sexual reproduction Meiosis is a form of sexual reproduction where cells, REPRODUCTIVE cells, egg or sperm are produced through a series of 2 divisions, reducing the number of chromosomes to ½. ***Reproductive cells, like eggs and sperm are also called gametes. These cells produced will be genetically DIFFERENT.

II. Asexual Reproduction A. One parent, genetically identical cells and the division is mitosis.

III. Meiosis: Two Divisions A. ***Produces reproductive cells B. Two consecutive nuclear divisions . 1. Meiosis I, and Meiosis II C. DNA is NOT duplicated between divisions. D. Result is 4 haploid nuclei are formed.

Meiosis I Each homologue in the cell pairs with its partner, then the partners separate

Meiosis II The two sister chromatids of each duplicated chromosome are separated from each other two chromosomes (unduplicated) one chromosome (duplicated)

IV. Stages of Meiosis Meiosis I Meiosis II Prophase II Prophase I Metaphase I Anaphase I Telophase I Meiosis II Prophase II Metaphase II Anaphase II Telophase II

Meiosis I - Stages Prophase I Metaphase I Anaphase I Telophase I

V. Prophase I 1. DNA coils tightly into chromosomes, spindle fibers appear, nucleus and nucleolus disappear. 2. ***Homologous pairs line up, this is call Synapses. This forms a Tetrad, a. ***Tetrads – form during synapse and are a group of 2 homologous pairs or 4 chromatids.

3. ***Crossing over –portions of chromosomes break off and switch with other chromosomes, this permits the ***exchange of genetic material between maternal and paternal chromosomes.

1. Tetrads line up down the center of the cell. VI. Metaphase I 1. Tetrads line up down the center of the cell.

VII. Anaphase I 1. Chromosomes randomly move to either sides of the cell, this random distribution is called Independent Assortment.

VIII. Telophase I 1. Chromosomes reach poles. 2. The cytoplasm divides. 3. *** Meiosis I result : two haploid (double stranded) cells.

Meiosis II - Stages Prophase II Metaphase II Anaphase II Telophase II

1. Spindle fibers form and begin moving chromosomes to the center. IX. Prophase II 1. Spindle fibers form and begin moving chromosomes to the center.

1. Duplicate chromosomes move to the center of the cell. X. Metaphase II 1. Duplicate chromosomes move to the center of the cell.

1. Sister chromatids separate to become single chromosomes. XI. Anaphase II 1. Sister chromatids separate to become single chromosomes. 2. Chromosomes move to opposite poles.

XII. Telophase II 1. The chromosomes arrive at opposite ends of the cell and the nuclear membrane forms around each new set of chromosomes 2. ***Meiosis II result: After cytoplasm divides there are ***FOUR haploid single stranded cells with half the number of chromosomes.

The dividing of the cytoplasm of the 4 new reproductive cells. Cytokinesis II The dividing of the cytoplasm of the 4 new reproductive cells.

Animal Life Cycle multicelled body mitosis zygote Diploid fertilization meiosis Haploid gametes

XIII. Formation of Gametes Gametes are reproductive haploid cells, formed in the testes of males and in the ovaries of females.

2. The spermatids will eventually mature into sperm cells *Spermatogenesis 1. Testes – a diploid reproductive cell divides meiotically to form *****four haploid single stranded sperm cells called spermatids. 2. The spermatids will eventually mature into sperm cells

Spermatogenesis secondary spermatocytes (haploid) spermato- gonium (diploid male reproductive cell) primary spermatocyte (diploid) spermatids (haploid) Mitosis I, Cytoplasmic division Meiosis II, Cytoplasmic division Growth

Oogenesis 1. A diploid reproductive cells divides meiotically to produce one mature egg cell (Ovum or Ova) 2. Because the cytoplasm does not divided equally, only one egg is produced and the other 3, called ***polar bodies die because they receive little or no cytoplasm. 3. The results is ***ONE haploid egg and 3 polar bodies.

Oogenesis three polar bodies haploid) first polar body haploid) oogonium (diploid reproductive cell) primary oocyte (diploid) secondary oocyte haploid) ovum (haploid) Mitosis I, Cytoplasmic division Meiosis II, Cytoplasmic division Growth

Mitosis & Meiosis Compared Asexual Genetically the same 1 parent 1 division 2 daughter cells Meiosis Sexual Genetically different 2 parents 2 division 4 daughter cells