Meiosis Section 7-1

Meiosis Forms Haploid (n) Cells Some organisms reproduce by joining gametes to form the first cell of a new individual Gametes are haploid which means they contain 1 set of chromosomes Meiosis – form of cell division that halves the number of chromosomes when forming specialized reproductive cells (gametes or spores)

Meiosis involves 2 divisions of the nucleus – meiosis I and meiosis II – with each divided into prophase, metaphase, anaphase and telophase. Before meiosis begins, the DNA in the original cell is replicated. Meiosis starts with homologous chromosomes.

Stages of Meiosis Meiosis is two rounds of mitosis essentially PI – prophase I PII – prophase II MI – metaphase I MII – metaphase II AI – anaphase I AII – anaphase II TI – telophase I TII – telophase II

Prophase I Chromosomes condense Nuclear envelope breaks down Homologous chromosomes pair all along their length and then cross-over Crossing-over – occurs when portions of a chromatid on one homologous chromosome are broken and pair with the corresponding portion on one of the chromatids on the other homologous chromosome

Metaphase I Pairs of the homologous chromosomes moved by the spindle to the equator of the cell Homologous chromosomes (2 chromatids) remain together

Anaphase I Homologous chromosomes separate Chromosomes of each pair pulled to opposite poles of cell by spindle fibers Chromosomes DO NOT separate at their centromeres – 1 chromosome is still 2 chromatids

Telophase I Individual chromosomes gather at each pole Cytoplasm divides – now 2 cells

Prophase II New spindle forms around chromosomes

Metaphase II Chromosomes line up along equator Chromosomes attached to spindle fibers at their centromeres

Anaphase II Centromeres divide Chromatids (now called chromosomes) move to opposite poles of cell.

Telophase II Nuclear envelope forms around each set of chromosomes Spindle breaks down Cells undergo cytokinesis

After all eight phases are complete, will have FOUR haploid (n) cells.

Meiosis Contributes to Genetic Variation Meiosis allows for the rapid generation of new genetic combinations Three Mechanisms for genetic variation 1. independent assortment 2. crossing-over 3. random fertilization

Independent Assortment Chromosome pairs separate independently Defined as the random distribution of homologous chromosomes during meiosis

Crossing-over and Independent Assortment DNA exchange that occurs during crossing-over adds even more recombination to the independent assortment of chromosomes that occurs later in meiosis Unlimited combinations Joining of two gametes, and each gamete produced independently gives approximately 64 trillion possibilities

Importance of Genetic Variation Meiosis and joining of gametes is essential to evolution No genetic process generates variation more quickly

Gamete Formation in Male and Female Animals Involves Meiosis Gametogenesis – meiosis is the primary event in the formation of gametes Spermatogenesis – process by which sperm are produced in male animals. Occurs in testes. Oogenesis – process by which gametes are produced in female animals . Occurs in ovaries.

Oogenesis After meiosis I, cytoplasm divides unevenly and forms a polar body. May or may not divide again, but WILL NOT survive. Larger cell – will undergo meiosis II. Larger cell develops into an ovum or egg. Polar bodies die.