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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: Prophase I This stage starts meiosis and is the same as prophase of mitosis with one important change. As the chromosomes condense they form groups of four chromatids called tetrads or bivalents. Close inspection reveals that each chromosome is replicated and consists of two sister chromatids. The two chromosomes in each cell that are homologous and have the same genes (but perhaps different alleles if the organism is heterozygous) will pair closely. This close association, or synapsis, allows the homologous chromosomes to crossover and exchange identical parts. The impact of crossing over is that genes that are on the same chromosome (ie. D and B in Fig.7) can be recombined so that they are not always inherited together. The tetrad or bilvalent formed during synapsis remains assembled as prophase progresses. The tetrad therefore moves as a unit to the center of the cell. Fig. 7 and 8. Prophase l and metaphase l of meiosis 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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