MEIOSIS

• MEIOSIS

  Another process of cell division that takes place in sexually mature organisms.
• MEIOSIS

  Process that results in the reduction of the chromosome number from diploid to haploid in the germ cells (egg and sperm cells)
• MEIOSIS

  Gametes have one set of chromosomes
• MEIOSIS

  Successful completion of meiosis ensures the genetic continuity of species.
• MEIOSIS

  Sexual reproduction also results in genetic variety of the members of a species.
• MEIOSIS

  Two divisions: Meiosis I (reduction division) and Meiosis II (Interkinesis)
• MEIOSIS I

  Meiosis I has the following phases: prophase I, metaphase I, anaphase I, and telophase I
• Prophase I

  Nuclear membrane and the nucleolus start to disappear.
• Prophase I

  Spindle fibers and replicated chromosomes during interphase are visible.
• Prophase I


  Dyad (chromosome with two sister chromatids held together by a centromere)
• Prophase I

  Pairing of homologous chromosomes
• Homologous Chromosomes


  Chromosomes with the same length, staining pattern and position of the centromere.
• Homologous Chromosomes

  Carry the same genes but different alleles or type (one from mother, one from father).
• Prophase I

  Crossing over takes place between two nonsister chromatids along a point called chiasma.
• Prophase I

  During crossing over, the other two chromatids remain uncrossed.
• Prophase I

  Pair of homologous chromosomes – composed of four sister chromatids called tetrad.
• Metaphase I

  Paired homologous chromosomes (tetrads) are moved by the spindle fibers to the equator of the cell.
• Metaphase I


  Spindle fibers increase in number and attach to the kinetochore of each chromatid.
• Metaphase I

  Kinetochore- the protein complex assembled at each centromere, serves as the attachment site for spindle microtubules.
• Anaphase I

  Homologous pairs of chromosomes (dyads) separate and migrate toward their respective poles.
• Anaphase I

  Sister chromatids are still attached at their centromere, move toward the poles.
• Anaphase I

  46 double-stranded chromosomes equivalent to 92 chromatids
• Telophase I

  Two daughter cells are completely divided during telophase I.
• Telophase I

  Each daughter cell has 23 chromosomes
• Interkinesis


  A short pause between meiosis I and meiosis II.
• Interkinesis


  it is similar to the interphase of mitosis except that, here there is no replication of chromosome.
• MEIOSIS II


  Meiosis II is composed of prophase II, metaphase II, anaphase II, and telophase II.
• Prophase II

  Nuclear envelope and nucleoli disappear.
• Prophase II

  Chromatids start to shorten and thicken.
• Prophase II

  Centrioles move to opposite poles, spindle fibers arrange in preparation for metaphase.
• Prophase II

  23 chromosomes (46 chromatids)
• Metaphase II

  Characterized by the movement of chromosomes to the cell equator.
• Metaphase II

  Each chromosome has two sister chromatids joined together by a centromere and are attached to the spindle fiber
• Anaphase II

  Centromeres joining the chromatids divide, allowing microtubules to attach to the kinetochores to pull the sister chromatids apart.
• Anaphase II 


  Sister chromatids are now called sister chromosomes
• Telophase II

  Nuclear membrane around the chromosomes of each daughter cells form.
• Telophase II

  Chromosomes uncoil and lengthen.
• Telophase II

  Cleavage furrow
• Telophase II

  Four new cells with haploid chromosomes (23 chromosomes) are produced
• answer this

  A) Produces genetically identical cells

  B) Results in diploid chromosome number

  C) Results in diploid cells

  D) Produces two new cells

  E) Takes place throughout the organism’s life

  F) Involved in asexual reproduction

  G) Produces genetically unique cells

  H) Results in haploid chromosome number

  I) Results in haploid cells

  J) Produces four new cells

  K) Involved in sexual reproduction

  L) during the reproductive years of an organism