Meiosis reductiodivision
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  • 1. Reduction-DivisionGenetic Recombination 1
  • 2.  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). II 2
  • 3.  Sex cells divide to produce GAMETES (sperm or egg). egg) Gametes have HALF the # of chromosomes. Occurs only in GONADS (testes or ovaries). Male: SPERMATOGENESIS -sperm Female: OOGENESIS - egg or ova 3
  • 4. n=23 human sex cell sperm n=23 n=23 2n=46 n=23diploid (2n) n=23 haploid (n) n=23 Meiosis I Meiosis II 4
  • 5. Haploid n=23 (1n) human egg sex cell n=23 2n=46 Polar Bodies (die)diploid (2n) n=23 Meiosis I Meiosis II 5
  • 6.  Similar to mitosis interphase. CHROMOSOMES (DNA) replicate in the S phase Each duplicated chromosome consist of two identical SISTER CHROMATIDS attached at their CENTROMERES. CENTROMERES CENTRIOLE pairs also replicate. 6
  • 7.  Nucleus and nucleolus visible. Nucleus chromatin cellmembrane nucleolus 7
  • 8.  Cell division that reduces the chromosome number by one-half. Four phases: phases a. Prophase I b. Metaphase I c. Anaphase I d. Telophase I Prophase I 8
  • 9.  Longest and most complex phase (90%). Chromosomes condense. Synapsis occurs - Homologous chromosomes come together to form a tetrad. tetrad Tetrad is two chromosomes or four chromatids (sister and non-sister chromatids). 9
  • 10. •Homologs contain DNAthat codes for the samegenes , but differentversions of those genes•Genes occur at the sameloci 10
  • 11. Homologous chromosomes Tetradsister chromatids sister chromatids 11
  • 12.  Pair of chromosomes (maternal and paternal) paternal that are similar in shape and size. Homologous pairs (tetrads) carry GENES controlling the SAME inherited traits. traits Each locus (position of a gene) is in the LOCI same position on homologues. Humans have 23 pairs of homologous chromosomes: a. First 22 pairs of autosomes b. Last pair of sex chromosomes 12
  • 13. eye color eye color locus locushair color hair color locus locus Paternal Maternal 13
  • 14.  Crossing over may occur between non- sister chromatids at sites called chiasmata. chiasmata Crossing over: segments of nonsister over chromatids break and reattach to the other chromatid. chromatid Chiasmata (chiasma) are where chromosomes touch each other and exchange genes (crossing over.) over Causes Genetic Recombination 14
  • 15. nonsister chromatids Tetrad chiasmata: site variation of crossing over 15
  • 16. 16
  • 17. XX chromosome - female XY chromosome - male 17
  • 18. Meiosis I Homologs separate 18
  • 19. • Nucleus & Nucleolus disappear• Spindle forms• Chromosomes coil & Synapsis (pairing) occurs• Tetrads form & Crossing over Occursspindle fiber centrioles aster fibers TETRAD 19
  • 20.  Shortest phase Tetrads align on the equator. Independent assortment occurs – chromosomes separate randomly causing GENETIC RECOMBINATION 20
  • 21. ORHomologs line up at equator or metaphase plate 21
  • 22.  Formula: 2n Example: 2n = 4 then 1n = 2 thus 22 = 4 combinations 22
  • 23.  In terms of Independent Assortment -how many different combinations of sperm could a human male produce? 23
  • 24.  Formula: 2n Human chromosomes: 2n = 46 n = 23 223 = ~8 million combinations 24
  • 25.  Homologous chromosomes separate and move towards the poles. Sister chromatids remain attached at their centromeres. centromeres 25
  • 26. Homologs separate 26
  • 27.  Each pole now has haploid (1n) set of chromosomes. chromosomes Cytokinesis occurs and two haploid daughter cells are formed. 27
  • 28. cytokinesis 28
  • 29. SisterMeiosis II Chromatids Separate 29
  • 30.  No Interphase II or very short No DNA Replication Remember: Meiosis II is similar to mitosis 30
  • 31.  Same as Prophase in mitosis  Nucleus & nucleolus disappear  Chromosomes condense  Spindle forms 31
  • 32.  Same as Metaphase in mitosisChromosomes (not homologs) line up at equator 32
  • 33.  Same as Anaphase in mitosis SISTER CHROMATIDS separate 33
  • 34.  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 34
  • 35. 35
  • 36. 36
  • 37.  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 37
  • 38.  What are the 3 sources of genetic recombination or variation? 38
  • 39. 1. CROSSING OVER (prophase I)2. INDEPENDENT ASSORTMENT (metaphase I)3. RANDOM FERTILIZATION 39
  • 40.  A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? chromosomes 40
  • 41.  10 chromosomes (haploid or 1n) 41
  • 42.  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 Male - XY CHROMOSOMES 42
  • 43. Female - XX 43
  • 44. Down Syndrome – Trisomy 21 Female - XX 44
  • 45.  The fusion of a sperm and egg to form a zygote. zygote A zygote is a FERTILIZED EGG sperm n=23 n=23 egg 2n=46 zygote 45
  • 46.  A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? chromosomes 46
  • 47.  10 chromosomes 47
  • 48. 48