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Chapter 11.4

Chapter 11.4. Meiosis. Mendel’s Principles. What are Mendel’s Four Main Principles of Genetics? Biological characteristics, traits , are determined by units known as genes . For a single trait, different forms of the gene, alleles, may be dominant or recessive.

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Chapter 11.4

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  1. Chapter 11.4 Meiosis

  2. Mendel’s Principles What are Mendel’s Four Main Principles of Genetics? • Biological characteristics, traits, are determined by units known as genes. • For a single trait, different forms of the gene, alleles, may be dominant or recessive. • In sexually reproducing organisms, each organism has two copies of each gene. These genes segregate when gametes form. • The alleles for different genes segregateindependently from one another.

  3. Chromosome • Genes • Determine traits • Located in specific positions on chromosomes Gene A Gene B Gene C Gene D Gene E

  4. Chromosomes • Each organism must inherit a single copy of every gene from each of its “parents.” • When gametes are produced, the two sets of genes must be separated so that each gamete ends up with just one set.

  5. Chromosome Number • All organisms have different numbers of chromosomes. • A body cell in an adult fruit fly has 8 chromosomes: 4 from the fruit fly's male parent, and 4 from its female parent. • Human Chromosome Number?

  6. Chromosome Number • These two sets of chromosomes are homologous. • Each chromosome from the male parent has a corresponding chromosome from the female parent. • There are four homologous pairs of chromosomes! • Humans?

  7. Chromosome Number • A cell that contains both sets of homologous chromosomes is said to be diploid. • The number of chromosomes in a diploid cell is sometimes represented by the symbol 2N. • For Drosophila fruit flies, the diploid number is 8, which can be written as 2N=8. • Humans Diploid Number?

  8. Chromosome Number • Gametes contain only a single set of chromosomes, and therefore only a single set of genes. • These cells are haploid. Haploid cells are represented by the symbol N. • For Drosophila fruit flies, the haploid number is 4, which can be written as N=4. • Human haploid number?

  9. Meiosis • Process of division in which the number of chromosomes per cell is cut in half • Creation of haploid cells • Separates homologous chromosomes • Makes gametes

  10. Meiosis • Meiosis involves two divisions • Meiosis I and Meiosis II. • By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells.

  11. Telophase I and Cytokinesis Prophase I Interphase I Metaphase I Anaphase I Meiosis I

  12. Interphase I • Cells undergo a round of DNA replication, forming duplicate chromosomes.

  13. MEIOSIS I ProphaseI • Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. • There are 4 chromatids in a tetrad.

  14. MEIOSIS I Metaphase I • Spindle fibers attach to the chromosomes.

  15. MEIOSIS I Anaphase I • The fibers pull the homologous chromosomes toward opposite ends of the cell.

  16. MEIOSIS I Telophase I and Cytokinesis • Nuclear membranes form. • The cell separates into two cells. • The two cells produced by meiosis I have chromosomes and alleles that are different from each other and from the diploid cell that entered meiosis I.

  17. Meiosis II • The two cells produced by meiosis I now enter a second meiotic division. • Unlike meiosis I, neither cell goes through chromosome replication. • Each of the cell’s chromosomes has 2 chromatids.

  18. Meiosis II Meiosis II Telophase I and Cytokinesis I Metaphase II Anaphase II Telophase II and Cytokinesis Prophase II

  19. MEIOSIS II Prophase II • Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell.

  20. MEIOSIS II Metaphase II • The chromosomes line up in the center of cell.

  21. MEIOSIS II Anaphase II • The sister chromatids separate and move toward opposite ends of the cell.

  22. MEIOSIS II Telophase II and Cytokinesis • Meiosis II results in four haploid (N) daughter cells.

  23. Gamete Formation • In male animals, meiosis results in four equal-sized gametes called sperm.

  24. Gamete Formation • In many female animals, only one egg results from meiosis. The other three cells, called polar bodies, are usually not involved in reproduction.

  25. Comparing Mitosis and Meiosis • Mitosis results in the production of two genetically identical diploid cells. Meiosis produces four genetically different haploid cells.

  26. Comparing Mitosis and Meiosis

  27. Comparing Mitosis and Meiosis • Mitosis • Cells produced by mitosis have the same number of chromosomes and alleles as the original cell. • Mitosis allows an organism to grow and replace cells. • Some organisms reproduce asexually by mitosis.

  28. Comparing Mitosis and Meiosis • Meiosis • Cells produced by meiosis have half the number of chromosomes as the parent cell. • These cells are genetically different from the diploid cell and from each other. • Meiosis is how sexually-reproducing organisms produce gametes.

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