Meiosis

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

2. Sexual reproduction – process by which two cells from different parents unite to produce the first cell of the new offspring 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. 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. In this process, the parents and offspring will be genetically unique. and-baby.jpg Mother_and_baby_Elephas_maximus.JPG

What is meiosis? Meiosis is a type of cell division that results in four genetically different haploid cells. Meiosis is a type of cell division that results in four genetically different haploid cells. Diploid (2N) – a cell with ‘2 sets’ of chromosomes (humans = 46)‏ Diploid (2N) – a cell with ‘2 sets’ of chromosomes (humans = 46)‏ Haploid (1N) – a cell with ‘1 set’ of chromosomes (humans = 23)‏ Haploid (1N) – a cell with ‘1 set’ of chromosomes (humans = 23)‏ Meiosis is used to produce haploid gametes (sperm and eggs) in organisms that reproduce sexually. Meiosis is used to produce haploid gametes (sperm and eggs) in organisms that reproduce sexually. Human gametes: Human gametes: In men, meiosis produces 4 equally sized sperm cells. In men, meiosis produces 4 equally sized sperm cells. In women, meiosis produces 1 egg and 3 polar bodies that later disintegrate. In women, meiosis produces 1 egg and 3 polar bodies that later disintegrate.

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

Prophase I In prophase I, homologous chromosomes pair up and form a tetrad. In prophase I, homologous chromosomes pair up and form a tetrad. Homologous chromosomes – carry paired alleles for a gene that were donated from the sperm (Dad) and the egg (Mom). Homologous chromosomes – carry paired alleles for a gene that were donated from the sperm (Dad) and the egg (Mom). Crossing over – homologous chromosomes exchange pieces of DNA to produce genetically different gametes. Crossing over – homologous chromosomes exchange pieces of DNA to produce genetically different gametes.

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

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

Comparing Mitosis and Meiosis The processes of mitosis and meiosis are very similar, but with some important differences: 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

Interphase DNA replication occurs. DNA replication occurs. DNA is in an uncondensed form known as chromatin. DNA is in an uncondensed form known as chromatin.

Prophase I DNA condenses to form chromosomes. DNA condenses to form chromosomes. Homologous chromosomes pair up to form a tetrad. Homologous chromosomes pair up to form a tetrad. Crossing over occurs. Crossing over occurs.

Metaphase I Homologous pairs line up along center of cell Homologous pairs line up along center of cell 1 homologous chromosome and its duplicate are attached to each spindle fiber 1 homologous chromosome and its duplicate are attached to each spindle fiber

Anaphase I Homologous pairs are separated, move to opposite ends of the cell. Homologous pairs are separated, move to opposite ends of the cell.

Telophase I Nuclear membrane will reappear. Nuclear membrane will reappear. Spindle fibers disappear. Spindle fibers disappear.

Cytokinesis Cytoplasm of the two cells separate. Cytoplasm of the two cells separate. Meiosis I results in 2 haploid cells that are genetically different. Meiosis I results in 2 haploid cells that are genetically different.

Prophase II The 2 haploid cells from Meiosis I will divide again. The 2 haploid cells from Meiosis I will divide again.

Metaphase II Chromosomes (set of sister chromatids) line up along the center of the cell. Chromosomes (set of sister chromatids) line up along the center of the cell. 1 sister chromatid is attached to each spindle fiber. 1 sister chromatid is attached to each spindle fiber.

Anaphase II Centromeres split and sister chromatids are pulled apart. Centromeres split and sister chromatids are pulled apart.

Telophase II and Cytokinesis The cytoplasmic division occurs, resulting in 4 genetically different haploid cells. The cytoplasmic division occurs, resulting in 4 genetically different haploid cells.