Meiosis 1883 ~ Pierre-Hoseph van Beneden discovered the process. He proposed that some sort of process occurred that reduced the number of chromosomes in the sperm and egg.

Production of gametes Meiosis ~ the reduction of a cell’s chromosome number from diploid to haploid by two consecutive cell divisions. Meiosis is divided into Meiosis I and Meiosis II, and the phases are also designated as I or II. For animals the process of meiosis occurs in the testes in males and in the ovaries in females. These organs produce gametes through the process of meiosis The gametes are sperm in males and an ovum (egg) in females. Gametes from a diploid organism like a human are haploid

Fertilization Fertilization is when the male and female gamete unite. This restores the diploid number and forms a zygote. Zygote ~ diploid cell formed when the two haploid cells (n) from the parents unite. Through many mitotic divisions, a zygote develops into an embryo and then a baby.

Meiosis Remember meiosis is what produces the gametes.

Meiosis I 1. Prophase I ~ Chromatin condenses into chromosomes consisting of sister chromatids Synapsis occurs- Homologous chromosomes each consisting of 2 sister chromatids come together forming a tetrad. Because the chromatids are so close together during this phase, crossing-over occurs (the transfer of genetic information between non sister chromatids) Nuclear envelope breaks apart Microtubules begin moving tetrads toward the center of the cell

Meiosis I 2. Metaphase I ~ 3. Anaphase I~ 4. Telophase I ~ Tetrads line up in the equatorial plane. The side that the paternal chromosome and the maternal chromosome in each homologous pair lines up on is random, so there are lots of possibilities. 3. Anaphase I~ Homologous pairs separate, but sister chromatids stay together and are pulled to opposite polls by the centrosomes 4. Telophase I ~ Chromosomes consisting of sister chromatids arrive at poles.

Cytokinesis Cytokinesis occurs with each resulting daughter cell containing the diploid number of chromosomes.

Meiosis II The process is similar to mitosis in that it separates sister chromatids, however since there are no longer homologous pairs in the cell, the resulting cells are haploid (gametes). Still have the separate stages: Prophase II, Metaphase II, Anaphase II, Telophase II, Cytokinesis

Sexual Reproduction Meiosis produces haploid gametes that are genetically unique Union of haploid gametes results in offspring that are not genetically identical to either parent. Spermatogenesis ~ process used to form sperm. Forms 4 functional sperm. Oogenesis ~ process used to form ovum. Produces 1 mature ovum and three polar bodies. Polar bodies disintegrate. Unequal distribution of cytoplasm when the cells split.

Genetic Variation There are 8 million possible combinations of chromosomes in a human gamete because the homologous chromosomes randomly line up on the left or the right. So between the male and female gamete there are 64 trillion possible combinations. Then there is crossing over which adds even more possible combinations. There is an extremely large amount of possible gene combinations that you can inherit from your father and mother. Plus the interaction of genes from your mother and father could cause characteristics that neither your mother nor your father had (that’s what we’ll talk about in the next unit)

Mitosis vs. Meiosis Mitosis produces genetically identical daughter cells Meiosis produces haploid gametes that are genetically unique Mitosis http://www.youtube.com/watch?v=L0k-enzoeOM Meiosis http://www.youtube.com/watch?v=qCLmR9-YY7o