Meiosis in eukaryotic cells

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Meiosis in eukaryotic cells

  1. 1. Meiosis ineukaryoticcellsStages of meiosis
  2. 2. DEFINING MEIOSIS Meiosis is a reduction divisionTakes place in the Gametes(sperm and egg) of an organismReducing the chromosomenumber from 46 in somatic cellsto 23 in sex cells (sperm and egg)
  3. 3. Alternating stageschromosomenumber must bereduceddiploid46haploid232n nhumans: 46 23meiosis reduceschromosome numbermakes gametesfertilization restoreschromosomenumberhaploid diploidn 2n
  4. 4. meiosis A diploid(2n) cell will contain one setof chromosomes from the male parentand one set from the female parent. Before cell division there will be twocopies of each chromosome (calledhomologous pairs) in the nucleus. Two homologous chromosomes willshare the following characteristics: Same length Centromere in same position Same number of genes Genes arrange in the same order
  5. 5. FIGURE 8.11 A KARYOTYPE SHOWS THAT THE CHROMOSOMES OCCURAS PAIRS8-5
  6. 6. THE OVERVIEWOF MEIOSISinterphase 1 prophase 1 metaphase 1 anaphase 1prophase 2 metaphase 2 anaphase 2 telophase 2telophase 1
  7. 7. STAGES OF MEIOSIS Requires two divisions Meiosis I - the chromosomes of eachhomologous pair separate Meiosis II - the sister chromatids of eachduplicated chromosome separate
  8. 8. MEIOSIS 1:EARLY PROPHASE1 . Chromosomes condense. Spindle forms. Nuclear envelope fragments.
  9. 9. MEIOSIS 1: Late prophase Homologs pair. Crossing over occurs.
  10. 10. PROPHASE 1: CROSSING OVER Homologous chromosomes in a tetradcross over each other Pieces of chromosomes or genes areexchanged Produces Genetic recombination in theoffspring
  11. 11. PROPHASE1: CROSSING OVER
  12. 12. MEIOSIS1: METAPHASE 1Homologous pairs of chromosomesalign along the equator of the cell
  13. 13. MEIOSIS 1:ANAPHASE 1 Homologs separate and move toopposite poles. Sister chromatids remain attached at their centromeres.
  14. 14. MEIOSIS 1: TELOPHASE 1Nuclear envelopes reassemble.Spindle disappears.Cytokinesis divides cell into two.
  15. 15. MEIOSIS 2: Meiosis II produces gametes with one copy of each chromosome and thusone copy of each gene.
  16. 16. 16PROPHASE2Nuclear envelopefragments.Spindle forms.
  17. 17. 17Metaphase 2Chromosomes alignalong equator of cell.
  18. 18. 18Anaphase 2Sister chromatidsseparate andmove to oppositepoles.EquatorPole
  19. 19. 19TELOPHASE 2Nuclear envelopeassembles.Chromosomesdecondense.Spindle disappears.Cytokinesis dividescell into two.
  20. 20. 20Results of MeiosisGametes (egg & sperm)formFour haploid cells withone copy of eachchromosomeOne allele of each geneDifferent combinations ofalleles for different genesalong the chromosome
  21. 21. Chromosomal AbnormalitiesCan Be Inherited8-21
  22. 22. AN ABNORMAL CHROMOSOME NUMBER IS SOMETIMESTRACEABLETO NONDISJUNCTION Polyploidy - a eukaryote has three or more complete setsof chromosomes Aneuploidy - an organism has more or less than thenormal number of chromosomes Monosomy only one of a type of chromosome Trisomy three of a type of chromosome Nondisjunction - in meiosis I homologues do not separateand both go into the same daughter cell, or in meiosis IIsister chromatids fail to separate and both daughterchromosomes go to the same gamete8-22
  23. 23. FIGURE 8.17B NONDISJUNCTION OF CHROMOSOMES DURING MEIOSIS II OFOOGENESIS, FOLLOWED BY FERTILIZATION WITH NORMAL SPERM8-23
  24. 24. 8.18 ABNORMAL CHROMOSOMENUMBERS CAUSE SYNDROMES Trisomy 21 (Down Syndrome) Over 90% of individuals with Down syndrome have threecopies of chromosome 21 Abnormal Sex Chromosome Inheritance Turner syndrome females are born with only a single Xchromosome A male with Klinefelter syndrome has two or more Xchromosomes in addition to a Y chromosome8-24
  25. 25. 8.19 ABNORMAL CHROMOSOME STRUCTURE ALSOCAUSES SYNDROMES Various agents in the environment, such asradiation, certain organic chemicals, or viruses, cancause chromosomes to break Deletion - an end of a chromosome breaks off or twosimultaneous breaks lead to the loss of an internalsegment Duplication - the presence of a particularchromosome segment more than once in the samechromosome Inversion - a segment of a chromosome is turned 180degrees Translocation - the movement of a chromosomesegment from one chromosome to another non-homologous chromosome8-25
  26. 26. REFERENCE LISTMcCammon, W. (2008). Chapter 8 Cell Division and Reproduction.http://www.slideshare.net/pisgahscience/08-lecture-ppt. Accessed : 18/03/2013.Patterson, C. (2008). Cell Division Mitosis and Meiosis.http://www.slideshare.net/catherinepatterson/cell-division-mitosis-and-meiosis-presentation.Accessed: 18/03/2013.Scuffruff. (2008). Lesson 6a The Stages Of Meiosis. http://www.slideshare.net/scuffruff/lesson-6a-the-stages-of-meiosis. Accessed: 18/03/2013Sherif, S. (2011). http://www.slideshare.net/shimaasherif/cell-cycle-cell Cell cycle & cell divisionby Shimaa Sherif on Feb 13, 2011-division. Accessed: 18/03/2013.Swan, J. (2011). Chapter 13 (part 1) – Meiosis. http://www.slideshare.net/jayswan/chapter-13-part-1-meiosis . Accessed : 18/03/2013Wraithxjmin. (2008). Lecture 6 Cell Division [Meiosis].http://www.slideshare.net/wraithxjmin/lecture-6-cell-division-meiosis. Accessed: 18/03/12

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