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MEIOSIS

MEIOSIS. Meiosis. The form of cell division by which gametes , with half the number of chromosomes, are produced. Diploid (2n)  haploid (n) Meiosis is sexual reproduction. Two divisions ( meiosis I and meiosis II ). Meiosis.

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MEIOSIS

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  1. MEIOSIS

  2. Meiosis • The form of cell division by which gametes, with half the number of chromosomes,are produced. • Diploid (2n)  haploid (n) • Meiosis is sexual reproduction. • Two divisions (meiosis Iand meiosis II).

  3. Meiosis • Sex cells divide to produce gametes(sperm or egg). • Gametes have half the # of chromosomes. • Occurs only in gonads (testes or ovaries).

  4. Three sexual life cycles differing in the timing of meiosis and fertilization

  5. Moss with Sporophytes Gametophytes

  6. n=2 sex cell sperm n=2 n=2 2n=4 haploid (n) n=2 n=2 diploid (2n) n=2 meiosis I meiosis II Meiosis

  7. n=23 Spermato- cyte sperm n=23 n=23 2n=46 haploid(n) n=23 n=23 diploid (2n) n=23 meiosis I meiosis II Spermatogenesis

  8. n=23 Oocyte ovum n=23 2n=46 haploid(n) n=23 diploid (2n) meiosis I meiosis II Oogenesis polar bodies

  9. Interphase • Similar to mitosis interphase. • Chromosomes replicate (S phase). • Each duplicated chromosome consists of two identical sister chromatids attached at their centromeres. • Centriole pairs also replicate.

  10. Interphase • Nucleus and nucleolus visible. chromatin nuclear membrane cell membrane nucleolus

  11. Meiosis I (four phases) • Cell division that reduces the chromosome number by one-half. Four Phases: a. prophase I b. metaphase I c. anaphase I d. telophase I

  12. Prophase I • Longest and most complex phase (90%). • Chromosomes condense. • Synapsis occurs: homologous chromosomescome together to form a tetrad.

  13. Homologous Chromosomes • Pair of chromosomes (maternal and paternal) that are similar in shape and size. • Homologous pairs (tetrads)carry genes controlling the same inherited traits. • Each locus(position of a gene)is in the same position on homologues. a. 22 pairs of autosomes b. 01 pair of sex chromosomes

  14. eye color locus eye color locus hair color locus hair color locus Paternal Maternal Homologous Chromosomes

  15. spindle fiber centrioles aster fibers Prophase I

  16. Homologous chromosomes sister chromatids sister chromatids Tetrad Prophase I - Synapsis

  17. Crossing Over • Crossing over (variation) may occur between nonsister chromatids. • Crossing over: segments of nonsister chromatids break and reattach to the other chromatid.

  18. Tetrad nonsister chromatids site of crossing over Crossing Over - variation variation

  19. OR equator equator Metaphase I

  20. Metaphase I • Shortest phase • Tetrads align on the equator • INDEPENDENT ASSORTMENT OCCURS: genes for different traits sort independently of one another into gametes

  21. Anaphase I

  22. Anaphase I • Homologous chromosomesseparate and move towards the poles. • Sister chromatidsremain attached at their centromeres.

  23. Telophase I

  24. Telophase I • Each pole now has haploid set of chromosomes. • Cytokinesis occurs and two haploid daughter cells are formed.

  25. Meiosis II • No interphase (or very short - no more DNA replication) • Remember: Meiosis II is similar to mitosis but the cells are different from each other

  26. Prophase II • same as prophase in mitosis

  27. equator equator Metaphase II • same as metaphase in mitosis

  28. Anaphase II • same asanaphaseinmitosis • sister chromatids separate

  29. Telophase II

  30. Telophase II • Same as telophase in mitosis. • Nuclei form. Cytokinesis occurs. • Remember: four haploid daughter cells produced; 3 polar bodies and an egg in females, 4 sperm in males gametes = sperm or egg

  31. sperm n=23 n=23 egg 2n=46 zygote Fertilization • The fusion of a sperm and egg to form a zygote - a fertilized egg

  32. Variation • Important to population as the raw material for natural selection. • Question: What are the three sexual sources of genetic variation?

  33. Answer: 1. crossing over (prophase I) 2. independent assortment (metaphase I) 3. random fertilization Remember:variation is good !

  34. Karyotype • A method of organizing the chromosomes of a cell in relation to number, size, and type.

  35. Question: • A cell containing 20 chromosomes(diploid)at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

  36. Answer: • 10 chromosomes (haploid)

  37. Question: • A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

  38. Answer: • 10 chromosomes

  39. Question: • In terms of Independent Assortment -how many different combinations of sperm could a humanmale produce?

  40. Answer • Formula: 2n • Human chromosomes: 2n = 46 n = 23 • 223 = ~8 million combinations

  41. 1) Three major differences between meiosis and the mitosis are:

  42. 2) Compare the number of chromatids present in human cells before and after DNA replication occurs in interphase. 3) Compare and contrast oogenesis with spermatogenesis.

  43. Determine the number of chromo- somes in a cell whose diploid (2n) number is 10 after: a. meiosis I b. meiosis II c. mitosis

  44. What is the primary difference between sexual life cycles in animals when compared to fungi and plants?

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