Lesson 6a The Stages Of Meiosis

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Lesson 6a The Stages Of Meiosis

  1. 1. 2B.4 Sexual reproduction <ul><li>Aims </li></ul><ul><li>to recall that gamete formation involves a reduction division (meiosis) and understand its significance as the division of a diploid nucleus to give haploid nuclei; </li></ul><ul><li>to understand the behaviour of chromosomes during the first and second divisions of meiosis, including chiasmata formation </li></ul><ul><li>to understand that haploid and diploid phases occur in the lifecycles of organisms. </li></ul>
  2. 2. Cell division: Involves the division of the nucleus then the division of the cytoplasm. The two types are mitosis and meiosis :
  3. 3. Sexual reproduction involves: <ul><li>Gamete formation </li></ul><ul><li>Meiosis – halves the number of chromosomes to form haploid gametes – takes place in the reproductive organs </li></ul><ul><li>Fertilisation – haploid male gamete fuses with haploid female gamete to form a diploid zygote. </li></ul><ul><li>Growth of the zygotes due to cells dividing by mitosis. </li></ul>
  4. 4. What are homologous pairs? <ul><li>A diploid cell will contain one set of chromosomes from the male parent and one set from the female parent. </li></ul><ul><li>Before cell division there will be two copies of each chromosome (called homologous pairs) in the nucleus. </li></ul><ul><li>Two homologous chromosomes will share the following characteristics: </li></ul><ul><li>Same length </li></ul><ul><li>Centromere in same position </li></ul><ul><li>Same number of genes </li></ul><ul><li>Genes arrange in the same order </li></ul>
  5. 5. Gametogenesis <ul><li>Gametogenesis – the production of sperm and ova in animals </li></ul><ul><li>The number of chromosomes reduces from the diploid number (2n) to the haploid number (n) </li></ul><ul><li>Replication of the DNA takes place during interphase – but then there are two cycles of nuclear divisions: </li></ul><ul><li>First meiotic division : separates the homologous pairs </li></ul><ul><li>Second meiotic division : separates the chromatids. </li></ul><ul><li>These two divisions result in the formation of 4 haploid cells from 1 diploid cell. The gametes will not be identical and will contain a single set of chromosomes. </li></ul>
  6. 7. Summary diagram of meiosis
  7. 8. The stages of meiosis (see hand out) <ul><li>Meiosis I: the homologous chromosomes separate: </li></ul><ul><li>Prophase I </li></ul><ul><li>Metaphase I </li></ul><ul><li>Anaphase I </li></ul><ul><li>Telophase I </li></ul><ul><li>Meiosis II: the chromatids separate: </li></ul><ul><li>Prophase II </li></ul><ul><li>Metaphase II </li></ul><ul><li>Anaphase II </li></ul><ul><li>Telophase II </li></ul>
  8. 9. Meiosis animation
  9. 10. How does meiosis lead to genetic variation? <ul><li>1. Crossing over – during prophase I of meiosis the homologous chromosomes come together in pairs. Each chromosome is divided into 2 chromatids. The homologous chromosomes twist around each other – causing tension and possible breaks across the chromatids. During crossing over, fragments of chromatids often get swapped over – so this cutting and sticking of chromosomes causes genetic material to be exchanged </li></ul>
  10. 11. How does meiosis lead to genetic variation? <ul><li>2. At metaphase I – when the chromosomes arrive at the equator they arrange themselves randomly . </li></ul>
  11. 12. Comparing mitosis and meiosis
  12. 13. Comparing mitosis and meiosis
  13. 14. Difference in DNA quantity between mitosis and meiosis
  14. 15. Difference in DNA quantity between mitosis and meiosis
  15. 16. IPMAT <ul><li>The stages of the </li></ul><ul><li>Cell Cycle can be </li></ul><ul><li>remembered </li></ul><ul><li>by IPMAT : </li></ul><ul><li>I nterphase </li></ul><ul><li>P rophase </li></ul><ul><li>M etaphase </li></ul><ul><li>A naphase </li></ul><ul><li>T elophase </li></ul>

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