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Understanding Meiosis: The Great Divide

Meiosis is a form of cell division that produces gametes with half the chromosomes. This process is crucial for sexual reproduction and involves two divisions (Meiosis I and Meiosis II) resulting in the formation of sperm and eggs. Learn about the phases of meiosis, including Prophase I, Metaphase I, Anaphase I, Telophase I, and more. Discover the significance of homologous chromosomes, genetic recombination through crossing over, independent assortment, karyotyping, and the importance of Meiosis for creating genetic diversity. Dive into the fascinating world of cell division and reproduction with this comprehensive guide to Meiosis.

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Understanding Meiosis: The Great Divide

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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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