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A typical bio lab ……

A typical bio lab ……. // www.youtube.com/watch?v=Vc1UqeHhjeo. Meiosis. Unlike mitosis, which occurs in somatic cells, meiosis has evolved to make sexual reproduction possible. Meiosis produces sex cells. Meiosis – the big picture .

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A typical bio lab ……

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  1. A typical bio lab …… • //www.youtube.com/watch?v=Vc1UqeHhjeo

  2. Meiosis • Unlike mitosis, which occurs in somatic cells, meiosis has evolved to make sexual reproduction possible. Meiosis produces sex cells.

  3. Meiosis – the big picture • THIS PROCESS GENERATES THE DIVERSITY OF ALL SEXUALLY REPRODUCING ORGANISMS. Including, naturally, us.

  4. Asexual reproduction – no meiosis here

  5. Asexual reproduction – NOT meiosis

  6. Chromosome reduction … • How a father with 46 chromosomes in each cell and a mother with 46 chromosomes per cell can have a child with 46 chromosomes per cell.

  7. Diploid and Haploid • Diploid cell – contains two of each kind of chromosome, one from the male parent, one from the female parent ..... It is 2n. (2n = 46 in human) • Haploid cell – contains one of each kind of chromosome, from either of the parents ..... It is n. (n = 23 in human)

  8. Meiosis defined • Meiosis- process where the number of chromosomes per cell is cut in half through the separation of chromosomesin a diploid cell.

  9. Homologous chromosomes are … • A pair of chromosomes having the same gene sequences, each derived from one parent. • So, the chromosomes have the same genes at the same loci but possibly different alleles.

  10. Gene – DNA sequence coding for protein. Basic unit of heredity.

  11. Allele – One member of a pair of genes occupying a specific spot on a chromosome (locus) that controls the same trait.

  12. Human Karyotype

  13. Haploid or Diploid? • Mitosis produces _________ cells. • Sex cells (eggs, sperm) are _________ cells.

  14. Phases (Stages) of Meiosis • Interphase I -- • cell replicates its DNA, like in mitosis • each chromosome now consists of 2 sister chromatids held together by a centromere

  15. Interphase

  16. Interphase

  17. Meiotic stages (cont’d.) • Prophase I-- • Chromosomes coil up • Spindle forms • Homologous chromosomes pair up as tetrads • Crossing over occurs – genetic material exchanges between homologues

  18. Homologues in a tetrad … how romantic!

  19. Crossing over – very important

  20. Stirring the pot … • Crossing over -The chromosomes you receive from your mom’s egg and your dad’s sperm are NOT the same chromosomes that your mom and dad have. They are “shuffled” versions.

  21. How do we know this is .....

  22. Meiosis I stages (cont’d.) • Metaphase I – • the centromere of each chromosome becomes attached to a spindle fiber • the spindle fibers pull the tetrads to the equator of the spindle • homologous chromosomes are lined up side by side along the equator

  23. More shuffling … • Independent assortment – homologues line up or “shuffle” randomly on the metaphase plate in Meiosis I. 223 or 8 million possible assortments of chromosomes for every sperm cell. (Random fertilization – the ovum also has 8 million possible chromosome combos, so … 8 mill. X 8 mill. = 64 trillion possible diploid combinations in each zygote!!!)

  24. How do we know this is …..

  25. Metaphase I

  26. Metaphase I -- after crossing over

  27. Meiotic stages (cont’d.) • Anaphase 1: • homologous chromosomes separate and move to opposite ends of the cell • centromeres do not split • this ensures that each new cell will receive only one chromosome from each homologous pair

  28. How do we know this is …..

  29. Meiotic stages (cont’d.) • Telophase 1: • the spindle breaks down and the chromosomes uncoil • the cytoplasm divides to yield two new cells • each cell has half the genetic information of the original cell because it has only one homologous chromosome from each pair

  30. How do we know this is …..

  31. Cell plate forms …..

  32. Meiosis II – mitosis of the products of Meiosis I • Prophase II- • a spindle forms in each of the two new cells and the fibers attach to the chromosomes • Metaphase II- • the chromosomes are pulled to the center of the cell and line up randomly at the equator

  33. How do we know this is …..

  34. How do we know this is …..

  35. Meiosis II – mitosis of the products of Meiosis I (cont’d.) • Anaphase II- • the centromere of each chromosome splits • the sister chromatids separate and move to opposite poles • Telophase II- • -nuclei re-form • -the spindles break down • -the cytoplasm divides

  36. How do we know this is …..

  37. The spindle

  38. How do we know this is …..

  39. What Meiosis produces: • four haploid sex cells from one original diploid cell • -each haploid cell contains one chromosome from each homologous pair • -haploid cells will become gametes transmitting genes to offspring

  40. A different type of tetrad ….

  41. Put it together …

  42. Don’t blink …

  43. Meiosis overview

  44. Spermatogenesis • Spermatogenesis -- in testes. 2 months from start to finish. Every day, several hundred million sperm are made by meiosis! Spermatogonia – 2n, undergo meiosis Primary spermatocytes– 2n, meiosis I. 23 pairs of homologues incl. X,Y Spermatids – n, meiosis II, 23 chromosomes, incl. X or Y Spermatozoa - n, pumped and ready to roll

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