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Meiosis

Learn about meiosis, the process of cell division that creates haploid cells and genetic variation. Understand the phases of meiosis and how it differs from mitosis.

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Meiosis

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  1. Meiosis

  2. Remember these key points… • How many sets of genes are in each organism? 2 sets - one from the mom, one from the dad A cell with 2 sets of genes is said to be DIPLOID

  3. If the fertilized egg has 2 copies of the genes, how many copies are in 1 sperm cell or one egg cell? • One copy of the genes in the egg cell. One copy of the genes in the sperm cell. • These cells are called HAPLOID • They have only one set of genes

  4. How does an organism create haploid cells? • Glad you asked! • This happens through a process called MEIOSIS • Meiosis - the process of cell division in which the number of chromosomes per cell is cut in half

  5. The Phases of Meiosis • Before Meiosis I begins the chromosomes are replicated • In Prophase I the nuclear envelope breaks down and HOMOLOGOUS chromosomes (chromosomes that look the same) pair up to form TETRADS.

  6. The Phases of Meiosis • Prophase I continued – • Portions of the tetrads get exchanged in a process called CROSSING OVER. This makes new combinations of alleles so the new cells will be genetically different from the original cell.

  7. The Phases of Meiosis • Metaphase I – • Microtubules grow from the centrioles and connect to the centromeres • Tetrads line up along the center of the cell

  8. The Phases of Meiosis • Anaphase I – • The chromosome pairs are split apart (the sister chromatids are still together) • Each cell now has the haploid number of replicated chromosomes (2N)

  9. The Phases of Meiosis • Telophase I – • The chromosomes decondense in some species • Nuclear membrane reforms

  10. The Phases of Meiosis • Cytokinesis I – • Two new cells are formed • Each cell has only one replicated copy of each gene and each is genetically different from the mother cell

  11. The Phases of Meiosis • Prophase II – • Nuclear membrane dissolves • Centrioles replicate

  12. The Phases of Meiosis • Metaphase II – • Sister chromatids line up in the center of the cell • Spindle fibers attach to the centromeres

  13. The Phases of Meiosis • Anaphase II – • Sister chromatids are pulled apart

  14. The Phases of Meiosis • Telophase II and cytokinesis II – • Four new haploid daughter cells are formed

  15. Meiosis • Why do it? • creates cells that are haploid (N) which allows reproduction to create a baby with the correct number of genes • Creates cells that are genetically unique which gives the baby a new set of traits, that might make it better able to survive.

  16. The four cells produced by Meiosis II are called GAMETES • In a male, the gametes produced are called sperm • In a female, only one of the four cells created becomes the egg. The other three cells are called polar bodies.

  17. Let’s think… • What are some ways meiosis is different from mitosis? (Other than their names!)

  18. Comparing Mitosis and Meiosis • Place each of the following words or phrases in one section of your Venn Diagram: Makes 2 cells Makes 4 cells Genetically different cells Genetically identical cells Makes haploid cells Makes diploid cells A process of cell division Produces gametes Produces normal body cells Cells produced are used for growth Cells produced are used for reproduction

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