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11–4  Meiosis

11–4  Meiosis. The making of sex cells!. Making Sex Cells. Meiosis is the process of making sex cells (gametes) Gametes are sex cells Ex: sperm, egg, pollen, etc…. Female Gametes - Eggs. Human Eggs. Male Gametes - Sperm. Human Sperm. Chromosomes.

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11–4  Meiosis

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  1. 11–4  Meiosis The making of sex cells!

  2. Making Sex Cells • Meiosis is the process of making sex cells (gametes) • Gametes are sex cells • Ex: sperm, egg, pollen, etc…

  3. Female Gametes - Eggs Human Eggs

  4. Male Gametes - Sperm Human Sperm

  5. Chromosomes • A body cell in an adult fruit fly has 8 chromosomes, as shown in the drawing on the right. • Four of the chromosomes came from the fruit fly's male parent, and 4 came from its female parent.

  6. Homologous Chromosomes • chromosomes that each have a corresponding chromosome from the opposite-sex parent • Example: Fruit-Fly (Drosophila) Chromosomes - 8 chromosomes (4 from mom and 4 from dad)

  7. Different Types of Cells • A cell that contains both sets of homologous chromosomes is said to be diploid (“two sets”) • a diploid cell is sometimes represented by the symbol 2N • for Drosophila, the diploid number is 8, which can be written 2N = 8 • Diploid cells = body cells (somatic cells)

  8. HaploidCells = a cell that contains only a single set of chromosomes Therefore only a single set of genes a haploid cell is sometimes represented by the symbol N for Drosophila, the haploid number is 4, which can be written N = 4 Haploid cells = sex cells (gametes)

  9. How are haploid (N) gamete cells produced from diploid (2N) cells? Meiosis

  10. Meiosis • a process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell.

  11. Phases of Meiosis • two distinct divisions, called meiosis I and meiosis II • By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells.

  12. What is the diploid number for these cells? 2 What is the haploid number for these cells? 1

  13. Meiosis I • Before meiosis I, each chromosome is replicated. • Division looks similar to mitosis • Prophase I, Metaphase I, Anaphase I, Telophase I • prophase of meiosis I - each chromosome pairs with its corresponding homologous chromosome to form a structure called a tetrad

  14. Tetrads

  15. Crossing Over • As homologous chromosomes pair up and form tetrads in meiosis I, they exchange portions of their chromatids in a process called crossing-over • results in the exchange of alleles between homologous chromosomes and produces new combinations of alleles • Alleles = alternative forms of the same gene (ex: blue eyes vs. brown eyes)

  16. After Crossing Over… • homologous chromosomes separate • two new cells are formed • New cells: • each pair of homologous chromosomes was separated • neither of the daughter cells has the two complete sets of chromosomes (they have been shuffled and sorted) • The new cells are DIFFERENT from each other

  17. Meiosis II  • two cells produced by meiosis I now enter a second meiotic division • NO DNA replication before Meiosis II • Prophase II, Metaphase II, Anaphase II, Telophase II • paired chromatids separate • Produces: haploid cells (N) • Haploid cells are DIFFERENT from each other

  18. Gamete Formation • Males gametes = sperm (pollen in plants) • All sperm the same size at the end of Meiosis • 4 sperm produced for each round of meiosis • Female gametes = eggs • One egg produced and 3 polar bodies (egg is MUCH larger in size) • The one egg receives the most cytoplasm • One egg produced for each round of meiosis (and 3 polar bodies which can’t be fertilized)

  19. Fertilization of a human egg by sperm. Notice the HUGE size difference.

  20. Comparing Mitosis and Meiosis • Mitosis and Meiosis sound alike but are VERY different! • Mitosis produces two genetically identicaldiploid somatic cells • Meiosis produces four genetically differenthaploid sex cells

  21. Mitosis • Makes body cells (somatic cells) • diploid cell two diploid (2N) daughter cells. • Daughter cells are identical to each other & the original parent cell • Mitosis allows an organism's body to grow and replace cells.

  22. Meiosis • Makes sex cells (gametes) • a diploid cell four haploid (N) cells • These cells are genetically different from the diploid cell and from one another. • Purpose: Sexual reproduction.

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