Meiosis Is a form of cell division that halves the number of chromosomes when forming specialized reproductive cells (gametes).

Meiosis Involves two divisions of the nucleus – meiosis I and Meiosis II Starts with one diploid cell, and ends with four haploid cells.

Review of Terms Haploid (n)-- one set chromosomes Diploid (2n)-- two sets chromosomes (46 chromosomes) Chromatid – Two exact copies of DNA (each single copy called a chromatid) makes up a chromosome Centromere – point at which the two chromatids are attached Homologous chromosomes – chromosomes similar in size, shape, and genetic content.

Meiosis

Meiosis I Prophase I Metaphase I Anaphase I Telophase I

Prophase I Homologous Chromosomes synapse (pair together) Each pair is called a tetrad because there are 4 chromatids. Crossing – over occurs when portions on one homologous chromosome are broken and exchanged with the corresponding portion on one of the chromatids of the other homologous chromosome.

Crossing - Over

Meiosis II Prophase II Metaphase II Anaphase II Telophase II

Between Meiosis I & II The chromosomes do NOT replicate between Meiosis I and Meiosis II. Start of Meiosis I Start of Meiosis II DNA copy made during Interphase

Prophase I & II A new spindle forms around the chromosomes Spindles – are cell structures made up of both centrioles and individual microtubule fibers that are involved in moving chromosomes during cell division Centrioles - two cylindrical cellular structures that form at the poles of a cell during meiosis.

Metaphase I & II Chromosomes line up in the center of the cell. In metaphase I, chromosome tetrads line up, each with their sister chromatids attached.

Anaphase I & II Centromeres divide Chromatids move to opposite poles of the cell In anaphase I, the tetrad chromosomes with attached sister chromatide moves to poles. * Remember centromeres are what hold the two chromatids together to make a chromosome.

Telophase I Nuclear membrane forms around each set of chromosomes Cytokinesis begins -division of cytoplasm Left with two identical haploid daughter cells.

Telophase II Nuclear membrane forms around each set of chromosomes Cytokinesis begins Left with four haploid cells.

Male vs Female Meiosis Spermatogenesis Oogenesis

Spermatogenesis (Males) Cytoplasm divided equally between all 4 cells during cytokinesis Results in 4 haploid sperm cells

Oogenesis (Female) Cytoplasm divided unequally between 4 cells during cytokinesis; 1 cell receives more cytoplasm than other cells Results in 1 mature egg and 3 polar bodies

Sexual vs. Asexual Reproduction 2 Parents required Offspring are similar to parents (USES MEIOSIS) Advantages -genetic variation. - Allows species to rapidly adapt to changing environment. Only 1 parent required Offspring identical (NO MEIOSIS) Ex: Binary Fission & Budding Advantages - Less energy required - Faster reproduction time