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Unit 3: Cell Division

Unit 3: Cell Division. Meiosis. Unit 3: Cell Division Chapter 11-4. Learning Goals. 1. Describe the overall process of meiosis. 2. Define homologous chromosomes and explain diploid and haploid. 3. Explain crossing over, its importance, and during which step it occurs.

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Unit 3: Cell Division

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  1. Unit 3: Cell Division

  2. Meiosis Unit 3: Cell Division Chapter 11-4

  3. Learning Goals • 1. Describe the overall process of meiosis. • 2. Define homologous chromosomes and explain diploid and haploid. • 3. Explain crossing over, its importance, and during which step it occurs. • 4. Describe the two types of gamete formation and how fertilization occurs.

  4. Meiosis • A process of cell division that creates gamete cells, or non-identical cells with half the number of chromosomes • The gametes from a male and a female combine in sexual reproduction to make a new individual

  5. Homologous Chromosomes • Each cell has 2 sets of chromosomes • Homologous Chromosomes = a pair of chromosomes that have the same genes at the same locations • 1 came from mom • 1 came from dad

  6. Homologous chromosomes From mom From dad

  7. Chromosome Number • Diploid (2n): cells that have 2 sets of homologous chromosomes (2n) • Human body cells are diploid (46 chromosomes or 23 pairs) • Haploid (n): cells that have 1 set of chromosomes (n) • sex cells (gametes) are haploid • human haploid cells (egg & sperm) have 23 chromosomes total

  8. Diploid meiosis meiosis Haploid gamete Haploid gamete • 1) A bug has a haploid number n=5. What is the diploid number (2n)? • 2) A crocodile has a diploid number 2n=50. What is the haploid number (n)? 2n n n

  9. Human Female Karyotype

  10. Human Male Karyotype

  11. Down Syndrome Female Trisomy

  12. Meiosis Overview • A type of cell division specific to sexual reproduction (two parent cells) • Cells (diploid) divide twice resulting in 4 daughter cells (haploid) • Each cell has half the number of chromosomes as the original cell • Each new cell is genetically different

  13. Steps of Meiosis

  14. Interphase • DNA is copied before the start of Meiosis I. • This makes 2 identical sister chromatids • They have the same genes at the same locations.

  15. Homologous chromosomes vs. sister chromatids From mom From dad

  16. Meiosis 1 • Four steps: • Prophase 1 • Metaphase 1 • Anaphase 1 • Telophase 1

  17. Prophase 1 • The homologous pairs of chromosomes come together and line up side by side. (tetrad = 4)

  18. Crossing-over • Homologous chromosomes can Cross-over and exchange a portion of the chromosome (DNA) at random

  19. Crossing-over is very important in meiosis because it createsgenetic variation (new combinations of genes), which makes us all different. • This is why you do not look exactly like your siblings.

  20. Red hair Freckles • Crossing over happens randomly. • However, some genes are linked, or close together on the chromosome, meaning they are more likely to be inherited together. Chromosome 16

  21. After crossing-over, the homologous chromosomes separate & the cells divide

  22. Meiosis II • Four steps: • Prophase 2 • Metaphase 2 • Anaphase 2 • Telophase 2

  23. Sister chromatids separate & the cells divide. • Result is 4 genetically different haploid daughter cells

  24. Principal of Independent Assortment • Genes for different traits can segregate (separate) independently during the formation of gametes. • Independent assortment and crossing-over account for the many genetic variations observed in plants, animals, and other organisms.

  25. Assume there was only one cross-over in a man’s sperm and one cross-over in a woman’s egg, they have the possibility of producing 4,951,760,200,000,000,000,000,000,000 genetically different children

  26. 23 23 23 23 Gamete Formation • Gametes = Sex Cells (eggs & sperm) • Spermatogenesis: • In males meiosis occurs in the testes • produces 4 sperm

  27. Spermatogenesis

  28. Oogenisis: • In females meiosis occurs in the ovaries • produces only 1 egg and 3 polar bodies Egg 23 Polar Bodies

  29. Oogenesis

  30. 23 23 Zygote 46 First cell of a new organism • Fertilization: the fusion of haploid sex cells

  31. Only one sperm and one egg come together • The first sperm to reach the egg will be the one to fertilize it.

  32. Comparing Mitosis & Meiosis • Mitosis results in the production of 2 genetically identical diploid cells. • Meiosis produces 4 genetically different haploid cells.

  33. Learning Goals • 1. Describe the overall process of meiosis. • 2. Define homologous chromosomes and explain diploid and haploid. • 3. Explain crossing over, its importance, and during which step it occurs. • 4. Describe the two types of gamete formation and how fertilization occurs.

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