Meiosis

1. Meiosis An example of reduction division

2. During sexual reproduction, a gamete (23 chromosomes, X or Y) from the male organism and a gamete from the female organism (23 chromosomes) fuse to create a new cell. United Streaming: Mitosis and Meiosis Part 9

3. Prophase I • Most of the significant processes of Meiosis occur during Prophase I • The chromosomes condense and become visible • The centrioles form and move toward the poles • The nuclear membrane begins to dissolve • The homologs pair up, forming a tetrad • Each tetrad is comprised of four chromotids - the two homologs, each with their sister chromatid • Homologous chromosomes will swap genetic material in a process known as crossing over • Crossing over serves to increase genetic diversity by creating four unique chromatids United Streaming: Mitosis and Meiosis Part 10-11

4. Crossing Over • Genetic material from the homologous chromosomes is randomly swapped • This creates four unique chromatids • Since each chromatid is unique, the overall genetic diversity of the gametes is greatly increased

5. Metaphase I • Microtubules grow from the centrioles and attach to the centromeres • The tetrads line up along the cell equator United Streaming

6. Anaphase I • The centromeres break and homologous chromosomes separate (note that the sister chromatids are still attached) • Cytokinesis begins United Streaming

8. Telophase I • The chromosomes may decondense (depends on species) • Cytokinesis reaches completion, creating two haploid daughter cells United Streaming

9. In Cytokinesis I, the cells finally split, with one copy of each chromosome in each one. • Prophase II, Metaphase II, Anaphase II, and Telophase II repeats the same steps as Prophase I-Telophase I, with half as much genetic material. • Cytokinesis II is the final step of meiosis, where each cell splits into two daughter cells, for a total of four gametes, each with half the number of chromosomes.

11. Notice the “crossing over” of genetic material