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Chromosome Structure

Chromosome Structure. Sexually Reproducing Organisms: Chromosomes occurs in pairs (One from the mother and 1 from the father) These pairs are called homologous chromosomes Carry directions/alleles for traits but they do not always carry same version for the same trait.

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Chromosome Structure

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  1. Chromosome Structure • Sexually Reproducing Organisms: • Chromosomes occurs in pairs (One from the mother and 1 from the father) • These pairs are called homologous chromosomes • Carry directions/alleles for traits but they do not always carry same version for the same trait.

  2. Cell Types and their Chromosomes Number

  3. Chromosome Number • A cell that contains both sets of homologous chromosomes are Diploid • “Two Sets” • Diploid=2N • Cells containing One set of chromosome are Haploid • “One Set”’ • Haploid=1N • Sex Cells ONLY!

  4. Sex Cell Reproduction • All living things can reproduce. • Reproduction occurs in one of two ways: • Sexually • The Union of two cells, each from a different parent and containing one set of chromosomes • To produce the first cell of a new organism, having TWO full sets of chromosomes • Asexually • Process by which a single parent reproduces by itself (Ex. Budding)

  5. Meiosis Takes place in the Gametes of an organism People have a Chromosome count of 46 When an egg joins a sperm the count must stay at 46 to remain human So, the egg can only have 23 chromosomes, and the sperm can only have 23 chromosomes But, the integrity of the organism must be maintained. How does this happen?

  6. During Meiosis gamete (sex) cells undergo a “double division”, maintaining the DNA, but reducing the chromosomal count to 23 + = Sperm (23) + Egg (23) = Fertilized Cell (46)

  7. Original Gamete Metaphase Anaphase Telophase Cytokinesis 2 Daughter Cells Metaphase 2 Anaphase 2 Telophase 2 Cytokinesis – 4 Gametes

  8. Phases of Meiosis • Meiosis involves two divisions, meiosis 1 and meiosis II • By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells

  9. Phases of Meiosis I

  10. Phases of Meiosis • Cells undergo a round of DNA replication, forming duplicate chromosomes. Interphase 1

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

  12. Phases of Meiosis • When homologous chromosomes form tetrads in meiosis I, they exchange portions of their chromatids in a process called crossing over. • Crossing-over produces new combinations of alleles.

  13. Phases of Meiosis MEIOSIS I Metaphase I • Spindle fibers attach to the chromosomes.

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

  15. Phases of Meiosis 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.

  16. Phases of Meiosis • 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.

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

  18. Phases of Meiosis MEIOSIS II Metaphase II • The chromosomes line up in the center of cell.

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

  20. MEIOSIS II Telophase II and Cytokinesis Phases of Meiosis • Meiosis II results in four haploid (N) daughter cells. • Cytokinesis: results in formation of: • 4 sperm cells in males • 1 Egg and 3 polar bodies in females

  21. How is Meiosis different from Mitosis?

  22. Comparing Mitosis and Meiosis • Mitosis results in the production of two genetically identical diploid cells. • Meiosis produces 4 genetically different haploid cells.

  23. 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.

  24. 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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