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Lesson Outline Introduction The Cell Cycle Mitosis: Somatic cell division Mitosis: Prophase Mitosis: Metaphase Mitosis: Anaphase Mitosis: Telophase Meiosis: Gamete formation Meiosis: Prophase I Meiosis: Metaphase I Meiosis: Anaphase I Meiosis: Telophase l Meiosis: Prophase II Meiosis: Metaphase II Meiosis: Anaphase II Meiosis: Telophase II Parallel Behavior and the Chromosome Theory

Mitosis and Meiosis and the Cell Cycle Rate Me 1 2 3 4 5 Meiosis: Gamete formation The objective of meiosis is to make four cells from a single somatic cell. The four cells each have half the chromosome number found in the somatic cell. In our human bodies, the four gametes will each have 23 chromosomes which means the 46 chromosomes in the somatic cell must replicate during interphase prior to meiosis just as they would before mitosis. Meiosis occurs in specialized cells of the body called germline cells. To appreciate meiosis and gamete formation it is important to first understand two ideas, chromosome sets and homologous chromosomes. Chromosome sets: The 46 chromosomes you have consist of two sets. You are a diploid organism ('di' means two and 'ploid' means sets). One set of chromosomes came from each parent when their gametes fused. Therefore, human gametes are haploid (one set). Homologous chromosomes: The 46 chromosomes in a somatic cell can be arranged into 23 homologous or similar pairs. One chromosome from each pair came from the male parent, the other from the female. Homologous chromosomes have the same genes although in heterozygous people the genes would be different alleles (A,a). The exception to this would be the sex (X and Y) chromosomes. Passing on a complete set of human genes requires one chromosome from each pair to end up in each gamete. There are several key differences between meiosis and mitosis that are summarized in the following table: Mitosis Meiosis Chromosome number stays the same Chromosome number is halved One division occurs to make two cells. Four stages of this division. Two divisions occur to make four cells. Eight stages in these divisions. Similar or homologous chromosomes do not pair. Homologous chromosomes pair during prophase l. Pairing is called synapsis. Crossover exchanges between homologous chromosomes is rare. Synapsis allows crossing over between homologous chromosomes. Two cells made are genetically identical. Four cells made are genetically different. The key events that happen in each of the stages of meiosis are summarized. Comments Be the first to write a comment... Add Comment

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This project was supported in part by the National Research Initiative Competitive Grants CAP project 2011-68002-30029 from the USDA National Institute of Food and Agriculture, adminstered by the University of California-Davis and by the National Science Foundation (NSF), Division of Undergraduate Education, National SMETE Digital Library Program, Award #0938034, administered by the University of Nebraska. Any opinions, findings, conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the USDA or NSF.
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