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Slideshare presantation

  2. 2. Cell Division MEIOSIS
  3. 3. Meiosis: Important Vocabulary  Diploid- contains 2 sets of chromosomes    (2n = 46) Example: human somatic cells Haploid- contains 1 set of chromosomes  (n = 23)  Example: human gametes
  4. 4. Meiosis: Important Vocabulary  Fertilization- fusion of two haploid gametes forms a diploid zygote  Zygote- fertilized egg cell   The first cell of a new individual What would happen if gametes were diploid?
  5. 5. Meiosis  Meiosis occurs in sexual reproduction when a diploid germ cell produces four haploid daughter cells that can mature to become gametes (sperm or egg).
  6. 6. Meiosis Goal: reduce genetic material by half Why? n (mom) + n (dad) = 2n (offspring) from mom from dad child too much! meiosis reduces genetic content Just right!
  7. 7. Meiosis: cell division in two parts Sister chromatids separate Chromatids separate Meiosis I (reduction division) Meiosis II (equational division) Diploid 2n Diploid 2n Haploid n Result: one copy of each chromosome in a gamete.
  8. 8. Meiosis I : the reduction division Spindle fibers Nucleus Prophase I (early) (diploid) Prophase I (late) (diploid) Metaphase I (diploid) Nuclear envelope Anaphase I (diploid) Telophase I (diploid)
  9. 9. Prophase I Late prophase Early prophase Chromatids pair. Crossing over occurs. Chromosomes condense. Spindle forms. Nuclear envelope
  10. 10. Metaphase I Chromatid pairs align along the equator of the cell.
  11. 11. Anaphase I Chromosomes separate and move to opposite poles. Sister chromatids remain Attached at their centromeres.
  12. 12. Telophase I Nuclear envelopes reassemble. Spindle disappears. Cytokinesis divides cell into two.
  13. 13. Meiosis II Gene X Sister chromatids carry identical genetic information . Meiosis II produces gametes with one copy of each chromosome and thus one copy of each gene.
  14. 14. Meiosis II : the equational division Prophase II (diploid) Metaphase II Anaphase II Telophase II Four Non-identical haploid daughter cells
  15. 15. Prophase II Nuclear envelope fragments. Spindle forms.
  16. 16. Metaphase II Chromosomes align along equator of cell.
  17. 17. Anaphase II Sister chromatids separate and move to opposite poles.
  18. 18. Telophase II Nuclear envelope assembles. Chromosomes unravel. Spindle disappears. Cytokinesis divides cell into two.
  19. 19. Results of meiosis Four haploid cells • One copy of each chromosome •
  20. 20. What Meiosis is About Meiosis allows the creation of unique individuals through sexual reproduction.
  21. 21. Mitosis Meiosis Number of divisions 1 2 Number of daughter cells 2 4 Yes No Same as parent Half of parent Where Somatic cells Sex cells When Throughout life At sexual maturity Role Growth and repair Sexual reproduction Genetically identical? Chromosome #
  22. 22. MITOSIS MEIOSIS Occurs in somatic (body) cells Occurs only in gonads (sex organs: ovary/testes) Produces cells for repair, maintenance, growth, asexual reproduction Only produces gametes (sex cells: egg/sperm) Results in identical diploid (2n) daughter cells Reduction division results in 4 haploid (n) cells
  23. 23. KARYOTYPE 1. Photograph a cell in metaphase. 2. Cut out the chromosomes from the picture. 3. Paste homologous pairs together according to size, banding pattern and centromere.
  24. 24. Karyotype is used to determine 1. If there is an abnormality in number or structure of the chromosome s (eg. Down’s syndrome)
  25. 25. •Karyotypes of embryos are obtained from: CVS (chorionic villus sampling) and amniocentesis. •Karyotypes of adults can be obtained from any growing cells. •Karyotypes can not tell if there are mutations of a gene.
  26. 26. Products of MITOSIS: 2 x 2n cells. 2n 2n 2n 2 diploid cells Products of MEIOSIS: 4 x n cells 2n n(4 n n monoploid cells) n
  27. 27. MEIOSIS is two mitotic divisions in a row, (meiosis I and II) except you will see the homologous chromosomes coming together . -    -- in prophase I the double homologous chromosomes come together to form a tetrad - Crossing-over (synapsis ) occurs in the tetrad in prophase to increase the variability in gametes. - There is no dna replication in the interphase
  28. 28. Homologous Chromosomes  Pair of chromosomes ( maternal and paternal) that paternal 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. b. 22 pairs of autosomes 1 pair of sex chromosomes
  29. 29. Homologous Chromosomes eye color locus eye color locus hair color locus hair color locus Paternal Maternal
  30. 30. Crossing Over (SYNAPSIS)  Crossing over (variation) may occur between nonsister chromatids at the chiasmata. chiasmata  Crossing over: over segments of nonsister chromatids break and reattach to the other chromatid. chromatid  Chiasmata (chiasma) are crossing over. over the sites of Synapsis increases genetic variability – and that’s a good thing…
  31. 31. Crossing Over - variation nonsister chromatids Tetrad chiasmata: site of crossing over variation
  32. 32. Sex Chromosomes XX chromosome - female XY chromosome - male
  33. 33. Cytokinesis: cytoplasm divides. Total product is 4 cells, each with only a mom or Dad’s chromosome, in other words, half the normal chromosome number, or 4 monoploid (n) cells, or 4 gametes. Uneven cytokinesis in females: In oogenesis, get one big egg and 3 nonfunctional polar bodies: ( who die off) and 1 big functional gamete
  34. 34. Spermatogene sis There are equal divisions producing four equal sized sperm.
  35. 35. Non-disjunction = Nonseparatio n of chromoso mes resulting in diploid or empty gametes.
  36. 36. Conditions caused by non-disjunction Downs syndrome = Trisomy 21 - 3 copies of chromosomes 21 equalling a total of 47 chromosomes. = 2n + 1(chance of occurring in oogenesis increases with maternal age) Turners syndrome = Monosomy X - has only one X chromosomes totalling only 45 chromosomes in her body cells. (monosomy X) = 2n-1 Klinefelter’s syndrome = XXY - male which has an extra X chromosome = 2n + 1
  37. 37. Down’s syndrome – trisomy 21 A simple check for the presence of a bone in the nose could more accurately test unborn babies for Down's syndrome, scientists say. Researchers say combining the nose test with existing screening methods (amniocentesis, CVS) could lead to a five-fold reduction in the number of miscarriages linked to an invasive procedure used to confirm Down's syndrome.
  38. 38. Klinefelter’s syndrome XXY Normal boy who develops some female secondary sex character-istics at puberty. -slightly lower IQ, infertile, delayed motor, speech, maturation -treated with testosterone.
  39. 39. REFERENCES Farrellw (2012) MEIO. ACCESS ON 7 MARCH 2014 Docsawyer (2010) MEIOSIS. ACCESS ON 07 MARCH 2014