1 Meiosis

2 To help you understand about DNA…. How is DNA passed from parent to offspring? There are two main ways that DNA is passed from parent to offspring:  1. Asexual reproduction – process by which a single parent reproduces by itself Mitosis is a form of asexual reproduction. In this process, the parent and offspring will be genetically identical. C jpeg

3  2. Sexual reproduction – process by which two cells from different parents unite to produce the first cell of the new offspring Meiosis is the process used to create genetically different gametes (sperm/eggs) used in sexual reproduction. In this process, the parents and offspring will be genetically unique. Mother_and_baby_Elephas_maximus.JPG

4 What is meiosis? Meiosis is a type of cell division that results in four genetically different haploid cells.  Diploid (2N) – a cell with ‘2 sets’ of chromosomes  Haploid (1N) – a cell with ‘1 set’ of chromosomes Meiosis is used to produce haploid gametes (sperm and eggs) in organisms that reproduce sexually. Human gametes:  In men, meiosis produces 4 equally sized sperm cells.  In women, meiosis produces 1 egg and 3 polar bodies that later disintegrate.

5 The Phases of Meiosis The phases of meiosis are the same as the phases of mitosis, but in meiosis each phase occurs twice. Meiosis I includes Interphase I, Prophase I, Metaphase I, Anaphase I, and Telophase I, followed by Cytokinesis.  Meiosis I results in 2 haploid daughter cells. sis.asf

6 Meiosis I In Prophase I, homologous chromosomes pair up and form a tetrad.  Homologous chromosomes – Refers to two similar chromosomes in a diploid cell. One chromosome is derived from the father gamete cell and the other from the mother  Crossing over – homologous chromosomes exchange pieces of DNA to produce genetically different gametes. This yields genetic variation which ultimately plays a role in evolution.

7 A Sketch of Chromosomes Crossing Over (Prophase I) Fq%3Dcrossing%2Bover%2Bchromosomes%26gbv%3D2%26svnum%3D10%26hl%3Den%26safe%3Dactive%26sa%3DG

8 Meiosis II Includes Prophase II, Metaphase II, Anaphase II, and Telophase II, followed by Cytokinesis. Each of the haploid daughter cells from Meiosis I divide again to make 4 haploid cells.

9 Comparing Mitosis and Meiosis The processes of mitosis and meiosis are very similar, but with some important differences: Meiosis Includes 2 cycles of cell division (Meiosis I and Meiosis II) Occurs only in gametes (sex cells) Chromosomes cross over in Prophase I Results in four genetically different haploid cells Metaphase I: 2 duplicated chromosomes per spindle fiber Mitosis Includes only 1 cycle of cell division Occurs in body cells No crossing over of chromosomes Results in two genetically identical diploid cells Metaphase: 1 duplicated chromosome per spindle fiber