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Chapter 9 Meiosis

Chapter 9 Meiosis. Asexual vs. sexual reproduction. asexual: one parent sexual: two parents What is MITOSIS? ASEXUAL or SEXUAL?. asexual: one parent sexual: two parents What is MITOSIS? ASEXUAL or SEXUAL?. Somatic (body) vs. Reproductive (sex).

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Chapter 9 Meiosis

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  1. Chapter 9Meiosis

  2. Asexual vs. sexual reproduction • asexual: one parent • sexual: two parents • What is MITOSIS? ASEXUALor SEXUAL? • asexual: one parent • sexual: two parents • What is MITOSIS? ASEXUAL or SEXUAL?

  3. Somatic (body) vs. Reproductive (sex) • somatic (body) cells: all the cells in your body (muscle, liver, heart, tissue, etc.) EXCEPT for sex cells • reproductive (sex) cells: sperm (male) and egg (female) cells

  4. Chromosome Number • Pairs of matching chromosomes are homologous chromosomes • homologous: a chromosome that has a corresponding chromosome from the opposite sex parent (one from the dad, one from the mom) • Homologs have the same genes, in the same order along the chromosome

  5. Karyotype • Homologous chromosomes can be arranged into a karyotype • Humans have 23 pairs of chromosomes • Is this a male or female?

  6. Chromosome Number • diploid: “two sets”; both sets of homologous chromosomes - human body cells have 46 chromosomes (diploid = 46) - fruit fly has 8 chromosomes (diploid = 8) • haploid: “one set”; a single set of chromosomes (or half the needed DNA) - human sex cells have 23 chromosomes (haploid = 23) - fruit fly sex cells have 4 chromosomes (haploid = 4)

  7. Human Chromosomes

  8. What if? • What would happen if two diploid cells (with 2 complete sets of 23 chromosomes or 46 total) came together during reproduction? • How many chromosomes would that new cell have? 92 chromosomes! • Would it be human?

  9. Mitosis • You have already learned how a cell makes a complete copy of its chromosomes in Mitosis

  10. But … • How do sex cells end up with half the chromosomes that body cells have???? Meiosis

  11. Life Cycle • Germ (or sex) cells fuse during fertilization to make a zygote

  12. Purpose of Meiosis • To produce eggs and sperm with only 23 chromosomes, so fertilization can produce a fertilized egg (zygote) with 46 chromosomes.

  13. Meiosis • Specialized type of cell division that produces haploid cells from diploid parent cell

  14. Stages of Meiosis • 2 cell divisions after replication: • Meiosis I • Meiosis II • Each division has 4 stages: • Prophase • Metaphase • Anaphase • Telophase

  15. Meiosis I: Interphase I Interphase I Prophase I Metaphase I Anaphase I Cells undergo a round of DNA replication, forming duplicate Chromosomes. Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. Spindle fibers attach to the chromosomes. The fibers pull the homologous chromosomes toward the opposite ends of the cell.

  16. Meiosis I: Prophase I Interphase I Prophase I Metaphase I Anaphase I Cells undergo a round of DNA replication, forming duplicate Chromosomes. Each chromosome pairs in synapsis with its corresponding homologous chromosome to form a tetrad. Crossing over may occur. Spindle fibers attach to the chromosomes. The fibers pull the homologous chromosomes toward the opposite ends of the cell.

  17. “Crossing Over” • In meiosis, chromosomes get together in homologous (similar) pairs & crossing over occurs. • Pieces from one chromosome break off and rejoin the other!

  18. Genetic Diversity • Independent assortment and crossing over create new combinations of genes in gametes

  19. Prophase I Pause • Prophase I is longest phase of meiosis • Production of proteins and structures in egg • In humans, meiosis starts before birth, pauses in prophase I until puberty, then 1 egg/month continues for up to 40 years

  20. Meiosis I: Metaphase I Interphase I Prophase I Metaphase I Anaphase I Cells undergo a round of DNA replication, forming duplicate Chromosomes. Each chromosome pairs in synapsis with its corresponding homologous chromosome to form a tetrad. Crossing over at chiasma. Spindle fibers attach to the chromosomes. The fibers pull the homologous chromosomes toward the opposite ends of the cell.

  21. Meiosis I: Anaphase I Interphase I Prophase I Metaphase I Anaphase I Cells undergo a round of DNA replication, forming duplicate Chromosomes. Each chromosome pairs in synapsis with its corresponding homologous chromosome to form a tetrad. Crossing over at chiasma. Spindle fibers attach to the chromosomes. The fibers pull the homologous chromosomes toward the opposite ends of the cell.

  22. At the end of Meiosis I … • 2 new cells are formed; although each new cell now has 4 chromatids (as it would after mitosis), something is different • neither of the daughter cells has the two complete sets of chromosomes that it would have in a diploid cell • the two cells produced by Meiosis I have sets of chromosomes and alleles that are different from each other and different from the diploid cell that entered Meiosis I

  23. Now on to Meiosis II … • There is NO Interphase II • so there is no DNA replication

  24. Separation of Chromatids

  25. Meiosis II: Prophase II Metaphase II Anaphase II Telophase II Prophase II The chromosomes line up in a similar way to the metaphase stage of mitosis. The sister chromatids separate and move toward opposite ends of the cell. Meiosis II results in four haploid (N) daughter cells. Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.

  26. Meiosis II: Metaphase II Metaphase II Anaphase II Telophase II Prophase II The chromosomes line up in a similar way to the metaphase stage of mitosis. The sister chromatids separate and move toward opposite ends of the cell. Meiosis II results in four haploid (N) daughter cells. Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.

  27. Meiosis II: Anaphase II Metaphase II Anaphase II Telophase II Prophase II The chromosomes line up in a similar way to the metaphase stage of mitosis. The sister chromatids separate and move toward opposite ends of the cell. Meiosis II results in four haploid (N) daughter cells. Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.

  28. Meiosis II: Telophase II Metaphase II Anaphase II Telophase II Prophase II The chromosomes line up in a similar way to the metaphase stage of mitosis. The sister chromatids separate and move toward opposite ends of the cell. Meiosis II results in four haploid (N) daughter cells. Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.

  29. At the end of Meiosis II … • You have FOUR (4) daughter cells with the haploid number (N) of chromosomes!

  30. … a male: the new haploid cells will grow flagella and become sperm … a female: only 1 of the 4 haploid cells will survive and become an egg If this took place in …

  31. Male + female = A zygote with46chromosomes!

  32. Problems in Mitosis and Meiosis • What happens if something goes wrong during cell division? • In Mitosis, incorrect cell division or uncontrolled cell division leads to cancer. • Cells divide incorrectly or too fast. This can lead to the growth of tumors or abnormal cells that do not do their job correctly. • In Meiosis, incorrect cell division leads to birth defects from non-disjunction.

  33. Nondisjunction • Occurs when chromosomes do not separate in either anaphase I or anaphase II • Gamete is missing 1 chromosome or has 1 extra chromosome • Fertilization will produce a zygote with 45 or 47 chromosomes

  34. Down’s Syndrome • Trisomy 21 results from nondisjunction of chromosome 21 • These kids have 3 chromosomes at pair 21.

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