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K101 LAB EXERCISE 11: MEIOSIS: GENETICS DIVERSITY AND SEXUAL REPRODUCTION Multicellular organisms like higher animals and plants result from the union of an egg and a sperm cell, each carrying one set of chromosomes that joined during fertilization to make a nucleus containing 2 complete sets of chromosomes. You received 23 chromosomes (one was an X or a Y) from your dad’s sperm cell, and 23 chromosomes (one of which was an X) from your mom’s egg to make the amazing and genetically unique creature with 46 chromosomes that became you . Thus, our 46 chromosomes are technically 23 pairs of homologous chromosomes, each of which is physically similar to the other in terms of size, and contains similar genes for the same traits. The number 46 is called our diploid (or 2n) number , as it represents 2 sets of 23 chromosomes, and the 23 chromosomes carried by each one of our egg or sperm cells is called our haploid (or n) number , where n = the n umber of chromosomes for that organism. Typically, all body (or somatic) cells are diploid, and mature germ cells (eggs or sperm) contain the haploid number of chromosomes, reduced from diploid to haploid through the process of meiosis . How is meiosis similar to and different from mitosis? Both mitosis and meiosis involve chromosome replication in S-phase followed by nuclear division. The result of mitosis is 2 genetically identical nuclei, each having the same number of chromosomes as did the parent nucleus. The purpose and result of meiosis, however, is quite different from mitosis in two major respects. First, genetic recombination occurs rapidly at the start of meiosis in a unique process called crossing over , resulting in a physical ‘shuffling’ of the DNA to create genetic diversity upon meiosis. Second, while the DNA replicates once in preparation for meiosis, the cells undergo two rounds of cell division, one to separate the homologues, and the other to separate the sister chromatids. The result : four genetically unique, haploid (n) cells from one diploid (2n) cell. The reduction in chromosome number from diploid to haploid makes the process of sexual reproduction possible. Meiosis in females creates the egg, and in males creates the sperm. The diploid chromosome number for that species is restored when two haploid nuclei unite during fertilization. Understanding meiosis is an absolute necessity for understanding the patterns of inheritance and genetics. The following activities will demonstrate the events of meiosis and the genetic basis for Gregor Mendel's laws of genetics such as independent assortment and chromosome segregation. 1 Paste your photo here (just kidding) Meiosis (1) generates genetic diversity and (2) halves the chromosome number.
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A. POP BEAD MEIOSIS: Building Homologous Chromosomes Although this activity may at first seem too basic for college biology, it is actually a very useful approach for understanding meiosis. Please take the time to go through the steps below! MATERIALS:
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