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Meiosis

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  1. Meiosis Gamete Production

  2. Similar in many ways to mitosis • Several differences however • Involves 2 cell divisions • Results in 4 cells with 1/2 the normal genetic information. • Vocabulary: • Diploid (2N) - Normal amount of genetic material • Haploid (N) - 1/2 the genetic material.

  3. Meiosis results in the formation of haploid cells. • In Humans, these are the egg (ova) and sperm. • Eggs are produced in the ovaries in females • Process is called oogenesis • Sperm are produced in the testes of males. • Process is called spermatogenesis • Meiosis occurs in 2 phases: Meiosis I & Meiosis II

  4. Spermatogenesis & Oogenesis Sperm formation Egg formation

  5. Chromosome Numbers: • Humans somatic or body cells have 23 pairs of chromosomes or 46 chromosomes (diploid or 2n number) • The 2 chromatids of a chromosome pair are called homologues or homologous chromosomes (have genes for the same trait at the same location) Homologs

  6. Human reproductive cells or gametes (sperms & eggs) have one set or 23 chromosomes (haploid or n number) • Every organism has a specific chromosome number

  7. Fertilization, joining of the egg & sperm, restores the diploid chromosome number in the zygote (fertilized egg cell) • Sex chromosomes, either X or Y, determine the sex of the organism • There are 2 sex chromosomes X and Y if you are , XX, will be female and XY will be male • All other chromosomes, except X & Y, are called autosomes

  8. Meiosis & Sexual Reproduction • Reduces the number of chromosomes in new cells to half the number in the original cell • New cells have a single copy of chromosomes (23 total) but are not identical to each other or the original parent cell • Used for making gametes ( sperm and eggs) with the haploid or n number

  9. In meiosis, cells divide twice after a single DNA duplication • Meiosis I separates homologous chromosomes & the Meiosis II separates sister chromatids • Meiosis I stages are Prophase I, Metaphase I, Anaphase I, & Telophase I • Meiosis II stages are Prophase II, Metaphase II, Anaphase II, & Telophase II

  10. Produces 4 haploid cells or gametes • When a sperm fertilizes an egg to form a zygote, the diploid number of chromosomes is restored (23 + 23 = 46) • When meiosis occurs in females, it produces 1 egg and 3 polar bodies; eggs are larger and are produced in the ovaries  • Sperm contain less cytoplasm so they're smaller & have a flagellum to swim to the egg • When meiosis occurs in males, it produces 4 sperm cells & occurs in the testes

  11. Stages of Meiosis: Meiosis I • Prophase I: • Chromosomes coil tightly & are visible • Nuclear membrane & nucleolus disintegrate • Spindle forms • Synapsis (joining) of homologous chromosomes occurs making tetrads • Chromosomes in tetrad exchange fragments by a process called crossing over

  12. Segments of homologous chromosomes break and reattach at similar locations. • Results in new genetic combinations of offspring. • Crossing over is amain advantage of sexual reproduction

  13. Metaphase I: • Tetrads become aligned in the center of the cell attached to spindle fibers • Anaphase I: • Homologous chromosomes separate • Telophase I: • Cytokinesis also occurs now producing 2 cells • In females,  2nd cell in females is called the first Polar Body • First Polar Body dies due to uneven splitting of the cytoplasm

  14. Meiosis II • Prophase II: • DNA is not copied before cell divides • Chromatids attach to spindle fiber • Metaphase II: • Chromosomes become aligned in the center of the cell attached to spindle fibers • Anaphase II: • Sister chromatids separate randomly • Called independent assortment

  15. Telophase II: • Cytokinesis also occurs producing 4 cells total in males which mature & form flagellum to become sperm • Cytokinesis in females produces a 2nd Polar Body for each cell that dies and an ovum (mature egg); the 1st polar body also divides and forming two polar bodies; there is a total of one mature egg and 3 polar bodies produced 

  16. Genetic Diversity • Because of the independent assortment of chromosome there are 223 possible combinations of chromosomes in humans • This is also a main advantage of sexual reproduction • Crossing Over and Independent Assortment create genetic diversity in organisms when they undergo sexual reproduction • Mutations add to this diversity

  17. Overview of Meiosis