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Lesson 9.3: Meiosis: The Life Cycle of Sex Cells

Lesson 9.3: Meiosis: The Life Cycle of Sex Cells. Goals: Identify male and female gametes Compare chromosome numbers between somatic cells and gametes Describe the stages of meiosis Compare meiosis and mitosis. Meiosis.

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Lesson 9.3: Meiosis: The Life Cycle of Sex Cells

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  1. Lesson 9.3: Meiosis: The Life Cycle of Sex Cells • Goals: • Identify male and female gametes • Compare chromosome numbers between somatic cells and gametes • Describe the stages of meiosis • Compare meiosis and mitosis

  2. Meiosis • Somatic cell = body cell which is DIPLOID (2n), - it has 2copies of each chromosome (one from mom and one from dad) these are called HOMOLOGOUS CHROMOSOMES • In humans, somatic cells have 46 chromosomes (skin cells, liver cells etc.)

  3. Homologous Chromosomes • Homologous chromosome – one of a matching pair of chromosomes that comes from each parent Dad Mom

  4. Gametes, or sex cells do not perform mitosis • Human sex cells have 23 chromosomes… HOW DOES THIS HAPPEN? • Gametes are involved in sexual reproduction and are HAPLOID (n) (1 set of chromosomes) • Male gamete = sperm • Female gamete = egg • In humans the sperm and egg each have 23 chromosomes

  5. The Phases of Meiosis • Consists of 2 separate cell divisions named Meiosis I and Meiosis II • Starts with one diploid cell and ends with FOURhaploid cells called gametes (sex cells) • During Meiosis I the number of chromosomes is reduced by ½ therefore meiosis I is known as reduction • Meiosis II is the same as mitosis (division of the nucleus)

  6. Preceded by Interphase (therefore chromosomes have replicated  sister chromatids) Spindle fibers appear and attach to centromeres In meiosis I HOMOLOGOUS chromosomes pair up in a process called SYNAPSISwhich results in TETRADS While chromosomes are paired, they trade DNA pieces with each other CROSSING OVER Meiosis I – PROPHASE I

  7. Tetrads line up randomly along the metaphase plate (tetrads meet in the middle) Meiosis I – Metaphase I

  8. Homologous chromosome separate – move Away/Apart from one another towards opposite ends of the cell *Sister chromatids are still together! Meiosis I – Anaphase I

  9. Each end of the cell now has a haploid nucleus (nucleus only has one set of chromosomes) Nuclei and cytoplasm divide in half 2 haploid cells form Each contains 2 copies of the same set of chromosomes Meiosis I – Telophase I and Cytokinesis

  10. Does not begin with DNA replication Prophase II: Spindle fibers attach to sister chromatids Metaphase II: spindle fibers move sister chromatids to the middle/center Anaphase II: Centromeres divide, sister chromatids separate and move apart to the poles Telophase II: Nuclear envelope reforms, cytoplasm divides Result: 4 haploid (1/2 the number of chromosomes as the original parent cell), genetically different cells Meiosis II

  11. Lesson 3 Review • Please read text pages 256-258 and answer the following questions on page 259 • 1. __________ • 2. __________ • 3. __________ • 4. __________ • 5. __________ • 6. __________

  12. State the Phase(for meiosis - indicate meiosis I or II) • Mitosis • Nucleolus reappears • Sister chromatids line up at equator • Cleavage furrow forms • Cell elongates • Chromosomes move AWAY from each other • Spindle fibers stretch across the cell • Nucleolus and Nuclear membrane disappear • Meiosis • Tetrads form • Crossing over occurs • Chromatin condenses • Tetrads line up at equator • Sister chromatids separate and chromosomes move AWAY… • Centromere divides • 2 cells are formed • 4 haploid cells are formed • Centrioles move to opposite ends of the cell

  13. Review: A comparison of mitosis and meiosis comparison animation

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