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Meiosis

Learn about the process of meiosis, which creates haploid gametes with half the number of chromosomes, leading to genetic continuity and variability. Discover the stages of meiosis and its importance in creating genetic diversity through segregation, independent assortment, and crossing over.

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Meiosis

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  1. Meiosis • A type of cell division that results in gametes(sperm and egg) being created with half the number of chromosomes(haploid-n) as the parent.

  2. Purpose of Meiosis • genetic continuity • Genetic variability

  3. Where does meiosis occur? • It occurs in the gonads( testes and Ovaries)

  4. Two Types of Meiosis • Oogenesis • spermatogenesis

  5. Meiosis

  6. Haploid cells Diploid cells Somatic cells Gametes Role of Meiosis To produce haploid gametes from diploid germ cells. Occurs only in the gonads Plays an important role in creating genetic variability through the processes of: Segregation Independent assortment Crossing over Meiosis

  7. Interphase S phase—replication Cell is now diploid with all chromosomes being mitotic Meiosis I—reductive division Creates 2 haploid cells with mitotic chromsomes Consists of several stages Prophase I—Like prophase in mitosis. Synapsis—homologous chromosomes attach to form tetrads. Crossing over occurs. Reshuffling of genes. Genetic recombination. Important for creating variability. Meiosis

  8. The stages of meiosis • Meiosis 1 • Four stages • Prophase 1 • Metaphase 1 • Anaphase 1 • Telophase 1 • Meiosis 2 • Four stages • Prophase 2 • Metaphase 2 • Anaphase 2 • Telophase 2

  9. Stages of meiosis 1 • Meiosis one is called the reduction division • The process of reducing the number of chromosomes in a cell by half (1/2).In other words, going from diploid (2n) to haploid(n) • Ex. 46 chromosomes to 23 chromosomes

  10. Prophase 1 • During prophase 1 of meiosis, homologous chromosomes(similar chromosomes) line up next to each other as pairs • Pairs of homologous chromosomes are called Tetrads. • Crossing over of non-sister chromatids occurs. • Crossing over : the overlapping of non-sister chromatids resulting in an exchange of genes.

  11. Prophase 1

  12. Metaphase 1 • During this phase homologous chromosomes line up at the middle of the cell. • tetrads are aligned in such a way that the homologous chromosomes are situated across from each other • The spindle from one end of the cell attaches to one pair of sister chromatids while a spindle fiber from the other end attaches to the other pair of sister chromatids. • Independent Assortment—maternal and paternal chromosomes are line up on the opposite sides of the metaphase plate in a random fashion. When these homologues split during anaphase I. The new nuclei consist of a mixture of maternal and paternal

  13. Metaphase 1

  14. Anaphase 1 • Spindle fibers pull homologous chromosomes to separate ends of the cell

  15. Anaphase 1

  16. Telophase 1 • Nuclear membrane reappears around each set of separated chromosomes • Spindle disappears • Cytokinesis (division of the cytoplasm) occurs • Two new cells are created with ½ (haploid) the chromosomes number of the original cell • 46 DS chromosomes – 23 DS chromosomes per cell or 96 chromatids- 46 chromatids per cell

  17. Telophase 1

  18. Almost the same as Mitosis. 2 cells are formed from each of the cells from Meiosis I. These cells are haploid and have chromosomes formed by only one chromatid. These cells develop into gametes Prophase II Metaphase II Anaphase II Telophase II Cytokinesis Results in 4 haploid cells that will eventually form gametes Meiosis II

  19. Stages of Meiosis II • Stages of meiosis 2 are identical to the stages of Mitosis • Prophase II • MetaphaseII • Anaphase II • Telophase II

  20. Prophase II • Nuclear membrane disappears • Spindle fibers form • Chromosomes become visible

  21. Prophase II

  22. Metaphase II • Chromosomes line up at the middle of the cell. • Spindle attaches to centromere of each chromosome

  23. Metaphase II

  24. Anaphase II • Spindle pulls one chromatid from each chromosome to opposite ends of cell

  25. Anaphase II

  26. Telophase II • Nuclear membrane reappears around each group of chromatids (now called SS chromosomes) • Spindle disappears • Cytokinesis occurs • two new cells are created each having the haploid number of chromosomes

  27. Telophase

  28. In males: spermatogenesis 1 diploid germ cell goes through meiosis to form 4 haploid cells. These develop in the testes to form sperm. In females: Oogenesis 1 diploid germ cell split unequally into 4 haploid cells. The large cell develops into an egg. The small cells are know as polar bodies and usually die. Why does this unequal division occur? Gametogenesis

  29. Oogenesis • This is the meiosis that occurs in the ovaries of females • Resulr in the creation of An egg with ½ the normal number of chromosomes

  30. Explanation of Oogensis • Within the ovaries of females, a diploid 2n cell called an oogonium undergoes meiosis • Before meiosis begins the oogonium replicates(doubles) its chromosomes so that it has 46 DS chromosomes or 92 sister chromatids • During the first meiotic division the Homologous (similar chromosomes are separated. During this stage cytokinesis occurs unevenly and as a result one large and one samll cell are created. The large cell is called the primary Oocyte and the small one is a polar body. • Each of these chromosomes will have 23 chromosomes or 46 SS chromatids

  31. During the second metiotic division the cells divide again creating 4 cells, but this time the new cells created have 23 SS chromosomes, Again cytokinesis has resulted in an unequal distribution of the cytoplasm • Note: The three polar bodies will die because they do not have enough cytoplasm to keep them running

  32. Spermatogensis • This is meiosis that occurs in the testes of males • Results in the creation of 4 sperm each with ½(haploid) the number of chromosomes as the original cell

  33. Explanation of Spermatogenesis • Within the testicles of a male, a spermatogonium replicates its chromosomes to contain 46 Ds chromosomes or 92 chromatids. • During the first meiotic division, the cell divides and the homologous chromosomes separate. This results in 2 new cells each with 23 DS chromosomes or 46 chromatids each • During the second division two new cells are created, each with 23 SS chromosomes. These cells are called Primary spermatids will mature into sperm • Each spermatid matures into sperm in the Epididymis

  34. Definitions • Diploid • Haploid/ monoploid • Tetrad • Synapsis • Crossing Over • Nondisjunction • Reduction Division • Spermatids • Primary body • Oognium

  35. End Result of Meiosis • 4 new cells are created • Each cell has ½ or haploid (n) the number of chromosomes as the parent /original cell • Crossing over has resulted in a new combination of genes on chromosomes leading to variety in orgasnisms

  36. Meiosis / Mitosis Compared

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