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MEIOSIS

MEIOSIS. The Great Divide. Meiosis. 1. 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 ). sperm n=23. n=23. egg.

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MEIOSIS

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  1. MEIOSIS The Great Divide

  2. Meiosis 1. The form of cell divisionby which GAMETES, with HALFthe number of CHROMOSOMES, are produced. • DIPLOID (2n)  HAPLOID (n) • Meiosis is SEXUAL reproduction. • TWO divisions (MEIOSIS Iand MEIOSIS II).

  3. sperm n=23 n=23 egg 2n=46 zygote 2. Fertilization • The fusion of aspermand egg to form a zygote. • A zygote is a FERTILIZED EGG

  4. Meiosis 3. Sex cells divide to produce GAMETES(sperm or egg). • Gametes have HALF the # of chromosomes. • Occurs only in GONADS (testes or ovaries). • Male: SPERMATOGENESIS -sperm • Female: OOGENESIS - egg or ova

  5. n=23 human sex cell sperm n=23 n=23 2n=46 n=23 n=23 diploid (2n) haploid (n) n=23 Meiosis I Meiosis II Spermatogenesis

  6. Haploid (1n) n=23 human sex cell egg n=23 2n=46 n=23 diploid (2n) Meiosis I Meiosis II Oogenesis Polar Bodies (die)

  7. 4. Interphase I • Similar to mitosis interphase. • CHROMOSOMES(DNA)replicate in the S phase • Each duplicated chromosome consist of twoidenticalSISTER CHROMATIDS attached at their CENTROMERES. • CENTRIOLE pairs also replicate.

  8. Interphase I • Nucleus and nucleolus visible. Nucleus chromatin cell membrane nucleolus

  9. Meiosis I (four phases) 5. Cell division that reduces the chromosome number byone-half. • Four phases: a. Prophase I b. Metaphase I c. Anaphase I d. Telophase I Prophase I

  10. 6. Prophase I • Longest and most complex phase (90%). • Chromosomescondense. • Synapsis occurs - Homologous chromosomescome togetherto form a tetrad. • Tetrad is two chromosomesor four chromatids (sister and non-sister chromatids).

  11. Non-Sister Chromatids-HOMOLOGS • Homologs contain DNA that codes for the same genes , but different versions of those genes • Genes occur at the same loci

  12. spindle fiber centrioles aster fibers Prophase I • Nucleus & Nucleolus disappear Spindle forms • Chromosomescoil& Synapsis (pairing) occurs • Homologous chromosomes come together to form a tetrad. • A Tetrad is two chromosomes or four chromatids. Sister and non-sister chromatids TETRAD

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

  14. Homologous Chromosomes 7. 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. • Humans have 23 pairs of homologous chromosomes: a. First22 pairs of autosomes b. Last pair -sex chromosomes LOCI

  15. A Gene is a unit of heredity that is transferred from parent to offspring and is held to determine characteristics of the offspring. • A trait is a feature of characteristic of a living thing.

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

  17. 8. Karyotype • Is a method for organizing chromosomes of a cell in relation to number, size and type. AUTOSOMES are numbered 1-22 and SEX CHROMOSOMES are number 23 Male - XY

  18. Sex Chromosomes XX chromosome - female XY chromosome - male

  19. Karyotype Female - XX

  20. 9. sex chromosomes code for the sex of the offspring. If the offspring has two X chromosome it will be a female. If the offspring has one X and one Y chromosome it will be a male.

  21. 10. Crossing Over • Crossing over may occur between non-sister chromatids at sites called chiasmata. • Crossing over: segments of nonsister chromatidsbreak and reattach to the other chromatid. • Chiasmata (chiasma) are the sites of crossing over . • Causes Genetic Recombination • Crossing over multiplies the already huge number of different gamete types produced by meiosis.

  22. 11. Metaphase I • Shortest phase 11. Tetrads align on the metaphase plate (equator).Independent assortment occurs – chromosomes separate randomly creates (2)GENETIC RECOMBINATION

  23. OR Homologs line up at equator or metaphase plate Metaphase I

  24. Formula: 2n • Example: 2n = 4 then 1n = 2 thus 22 = 4 combinations

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

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

  27. 12. Anaphase I • Homologous chromosomes separate and move towards the poles. • Sister chromatids remain attached at their centromeres.

  28. Anaphase I Homologs separate

  29. Non- disjunction • Non-disjunction occurs when chromosomes fail to separate properly. • Some cells end up with more or less sets of chromosomes.

  30. Karyotype Down Syndrome – Trisomy 21 Female - XX

  31. 13. Telophase I • Each pole now has haploid (1n) set of chromosomes. • Cytokinesis occurs and two haploid daughter cells are formed.

  32. Telophase I cytokinesis

  33. MEIOSIS II Sister Chromatids Separate Meiosis II

  34. 14. Meiosis II • No Interphase II or very short • No DNA Replication • Remember: Meiosis II is similar to mitosis

  35. Prophase II • Same as Prophase in mitosis • Nucleus & nucleolus disappear • Chromosomes condense • Spindle forms

  36. Chromosomes (not homologs) line up at equator Metaphase II • Same as Metaphase in mitosis

  37. Anaphase II • Same as Anaphase in mitosis • SISTER CHROMATIDS separate

  38. Telophase II • Same as Telophase in mitosis. • Nuclei and Nucleoli reform, spindle disappears • CYTOKINESIS occurs. • Remember: FOUR HAPLOID DAUGHTER cells are produced. • Called GAMETES (eggs and sperm) 1n Sperm cell fertilizes 1n egg to form 2n zygote

  39. Telophase II

  40. Question: • What are the 3 sources of genetic recombination or variation?

  41. Answer: • CROSSING OVER (prophase I) • INDEPENDENT ASSORTMENT (metaphase I) • RANDOM FERTILIZATION

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

  43. Answer: • 10 chromosomes (haploid or 1n)

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

  45. Answer: • 10 chromosomes

  46. Variation • Also known as GENETIC RECOMBINATION • Important to population as the raw material for NATURAL SELECTION. • All organisms are NOT alike • Strongest “most fit” survive to reproduce & pass on traits

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