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Demonstrate understanding of the following terms. › Chromosome › Chromatid › Chromatin Produces offspring that are genetically different from parents. Accomplished by gametes (sex cells) – one male and one female. Formed during meiosis. Somatic cells are diploid cells – they have 2 copies of each chromosome (homologous pairs- 1 from mom, 1 from dad). Gametes (sex cells) are haploid cellsthey have only 1 copy of each chromosome from each pair. Because of meiosis, the chromosome number of a species is maintained even after fertilization. Cells go through 2 divisions › Meiosis I › Meiosis II Meiosis results in 4 new cells rather than 2 (mitosis) The chromosome number is reduced by half in the 4 new cells. DNA coils tightly into chromosomes Spindle fibers appear The nucleolus and nuclear membrane disappear. Synapsis occurs (Tetrads formed) Source: HOPES: An Introduction to DNA and Chromosomes http://hopes.stanford.edu/basics/dna/b7.html Crossing-over occurs = genetic variability The homologous chromosome pairs line up randomly along the middle of the dividing cell. Spindle fibers from one pole attach to one chromosome, while spindle fibers from the opposite pole attach to the other chromosome. Source: HOPES: An Introduction to DNA and Chromosomes http://hopes.stanford.edu/basics/dna/b7.html Each chromosome moves to opposite poles of the dividing cell. Random separation = independent assortment. Source: HOPES: An Introduction to DNA and Chromosomes http://hopes.stanford.edu/basics/dna/b7.html The chromosomes reach opposite ends of the cell and cytokinesis begins. The new cell contains half the number of chromosomes as the original cell. Pearson Biology Coach: Meiosis I Animation Pearson Biology Coach Activity Meiosis I is known as a “reduction division” because the number of chromosomes is reduced by ½. Meiosis II is known as an “equation division” because the number of chromosomes is the same at the end of meiosis II as at the end of meiosis I. No copying of DNA takes place before Meiosis II. In some species, meiosis II begins after nuclear membrane forms in the new cells. In other species, meiosis II begins right after cytokinesis. The spindle fibers appear and begin to move the chromosomes toward the middle of the cell. Source: “Cell Division, Mitosis and Meiosis Lecture Notes .” Professor Michael Muller. http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect16.htm Each chromosome is located at the middle of the cell with each chromatid facing opposite poles. Source: “Cell Division, Mitosis and Meiosis Lecture Notes .” Professor Michael Muller. http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect16.htm The chromatid separate and move toward opposite poles of the cell. Source: “Cell Division, Mitosis and Meiosis Lecture Notes .” Professor Michael Muller. http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect 16.htm Nuclear membrane re-forms around chromosomes in each of 4 new cells. Cytokinesis results in 4 new cells each which contains ½ the number of chromosomes as parent cell. Pearson Biology Coach Meiosis II Animation Source: “Cell Division, Mitosis and Meiosis Lecture Notes .” Professor Michael Muller. http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect16.htm Assembling the Stages of Meiosis II Meiosis occurs in the testes of males and the ovaries of females. In the formation of male gametes, the original cell in testes produces 4 sperm cells by meiosis. In the formation of female gametes, the original cell in the ovary produces 1 egg and 3 polar bodies by meiosis. What types of cells are made by meiosis? How many cells are made at the end? Are they the same/different as parent? What happens to chromosome number? What types of cells are made by meiosis? How many cells are made at the end? Are they the same/different as parent? What happens to chromosome number? Why is meiosis important?