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Meiosis: The Process of Genetic Recombination and Reduction Division

Learn about the process of meiosis, which involves the reduction division of chromosomes and genetic recombination. Understand how it differs from mitosis and its significance in sexual reproduction.

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Meiosis: The Process of Genetic Recombination and Reduction Division

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  1. MEIOSIS Reduction-Division Genetic Recombination

  2. Mitosis REVIew!!! • Mitosis replicates an EXACT COPY of a cell with a complete set of 46 chromosomes!!! (23 pairs) • Mitosis starts with ONE parents cell and divides to create 2 IDENTICAL daughter cells • One DIPLOID cell creates 2 DIPLOID cells

  3. MEIOSIS • In Meiosis, we will be taking ONE parent cell and creating 4 daughter cells, each with different DNA! • This is SEXUAL REPRODUCTION • One DIPLOID cell creates 4 HAPLOID cells

  4. DIPLOID VS HAPLOID • Diploid cell- Cell that has 46 chromosomes • Haploid cell – Cell that has 23 chromosomes • Diploid – 2n • Haploid- n

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

  6. Meiosis • 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

  7. 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

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

  9. 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.

  10. Interphase I • Nucleus and nucleolus visible. Nucleus chromatin 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 Prophase I

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

  13. Homologous Chromosomes “1 set” of chromosomes (2 X’s) Chromosomes that carry genes to code for a specific trait 2 chromosomes carry same gene to code for trait (hair color, eye color….etc..)? They are homologous chromosomes!

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

  15. Homologous chromosomes sister chromatids sister chromatids Tetrad Prophase I

  16. Homologous Chromosomes • Pair of chromosomes 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. First 22 pairs of autosomes b. Last pair of sex chromosomes

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

  18. Crossing Over • Crossing over may occur between non-sister chromatids at sites called chiasmata. • Crossing over: segments of nonsister chromatids break and reattach to the other chromatid. • Chiasmata (chiasma) are where chromosomes touch each other and exchange genes (crossing over.) • Causes Genetic Recombination

  19. Crossing over • Crossing over is why all egg and sperm have slightly different genetic code!! • This is why you DO NOT look identical to your older/younger brother or sister!

  20. Tetrad nonsister chromatids chiasmata: site of crossing over Genetic Recombination variation

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

  22. MEIOSIS I Meiosis I Homologs separate

  23. spindle fiber centrioles aster fibers Prophase I • Nucleus & Nucleolus disappear • Spindle forms • Chromosomes coil & Synapsis (pairing) occurs • Tetrads form & Crossing over Occurs TETRAD

  24. Metaphase I • Shortest phase • Homologou chromosomes align on the equator.

  25. OR Homologous chromosomes line up at equator Metaphase I

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

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

  28. Anaphase I Homologs separate

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

  30. Telophase I cytokinesis

  31. MEIOSIS II Sister Chromatids Separate Meiosis II

  32. Meiosis II • No Interphase II or very short • No DNA Replication • Remember: Meiosis II is SAM AS mitosis

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

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

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

  36. 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

  37. Telophase II

  38. 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

  39. Question: • What are the 2 sources of genetic recombination or variation?

  40. Answer: • CROSSING OVER (prophase I) • RANDOM FERTILIZATION

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

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

  43. Karyotype • An organized picture of the chromosomes of a human arranged in pairs by size from largest to smallest. • Pairs 1-22 called AUTOSOMES • Last pair are SEX CHROMOSOMES Male - XY

  44. Karyotype Female - XX

  45. Karyotype Down Syndrome – Trisomy 21 Female - XX

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

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

  48. Answer: • 10 chromosomes

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