Meiosis
- 1. Lesson: MEIOSIS By Dasari Sreekanth. M.Sc.,NET, TS/AP-SET,GATE, (Ph.D.) Research Scholar Department of botany, O.U. Email: [email protected]
- 2. Meiosis • Makes 4 cells genetically different from parent cell & from each other • Production of gametes = sperm & egg • Contain half the number of chromosomes = haploid (1N) • Called the “reduction” division = – Diploid Haploid – 2N 1N • Used in sexual reproduction 4 6 2 3
- 3. Meiosis allows sex cells to join to form a complete set of chromosomes/instructions to make a new organism. Copyright Pearson Prentice Hall Haploid gametes MEIOSIS Diploid zygote MITOSIS Diploid adult
- 4. If egg and sperm had same number of chromosomes as other body cells . . . baby would have too many chromosomes! No Good! The Problem!
- 5. The Solution! Much Better! If egg and sperm had half the number of chromosomes than other body cells . . . baby would have just the right amount!
- 6. You tell me! • Drosophila (fruit flies) – Diploid: 2N = 8 – Haploid: 1N = ? • Lettuce – Diploid: 2N = 8 – Haploid: 1N = ? • Goldfish: – Diploid: 2N = ? – Haploid: 1N = 48
- 8. MeiosisMeiosis • Sex cells divide to produce gametes (sperm or egg). • Gametes have half the # of chromosomes. • In animals Occurs only in gonads (testes or ovaries). Male: spermatogenesis Female: oogenesis In plants meiosis takes place in : Microscporangium and megasporangium.
- 9. During Meiosis gamete (sex) cells undergo a “double division”, maintaining the DNA, but reducing the chromosomal count to 23 + = Sperm (23) + Egg (23) = Fertilized Cell (46)
- 10. Meiosis ensures that all living organisms will maintain both Genetic Diversity and Genetic Integrity
- 11. SpermatogenesisSpermatogenesis 2n=46 human sex cell diploid (2n) n=23 n=23 meiosis I n=23 n=23 n=23 n=23 sperm haploid (n) meiosis II
- 12. FertilizationFertilization • The fusion of a spermsperm and eggegg to form a zygotezygote. • A zygote is a fertilized egg n=23 egg sperm n=23 2n=46 zygote
- 13. Interphase IInterphase I • DNA replicates forming duplicate homologous chromosomes. (S phase). • Centriole pairs also replicate. • Nucleus & nucleolus visible.
- 14. MeiosisMeiosis • Meiosis can be devided into 2 phases; • Meiosis-I • Meiosis-II
- 15. Meiosis IMeiosis I • Cell division that reduces the chromosome number by one-half. • four phases: a. prophase I b. metaphase I c. anaphase I d. telophase I & cytokinesis Prophase I Metaphase I Anaphase I Telophase I & Cytokinesis
- 16. Prophase IProphase I • Longest and most complex phase (90%). • Homologous chromosomes come together to form a tetrad = Synapsis • Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids).
- 17. MEIOSIS I • Prophase I • TheThe longestlongest phase.phase. This phase can beThis phase can be divided into 5divided into 5 stages;stages; a) Leptotene b) Zygotene c) Pachytene d) Diplotene e) Diakinesis
- 18. 1.Leptotene 2.Zygotene 3.Pachytene 4.Diplotene 5.Diakinesis Prophase I
- 19. LEPTOTENE(THIN- THREADLEPTOTENE(THIN- THREAD STAGE)STAGE) Leptonema. Chromosomes condense into long threads. Two sister chromatids are tightly bound. Flower bouquet phase Centriole duplicate and earch daughter centriole migrates towards opposite poles of the cell.
- 20. Zygotene (yoked- threadZygotene (yoked- thread stage)stage) Zygonema Chromosomes become shorter and thicker. Pairing of homologous chromosomes = synapsis or syndesis A pair of homologous chromosomes lying together is called as Bivalent. Synoptonemal complex formed in between two homologous chromosomes. Pairing may occur in 3 ways 1. proterminal pairing 2.procentric pairing 3. random pairing.
- 21. Zygotene.Zygotene. Homologous chromosomes sister chromatids paternal sister chromatids maternalTetrad
- 22. 22
- 23. Pachytene (thick-threadPachytene (thick-thread stage)stage) Pachynema Genetic recombination stage Two visible chromatids of each chromosomes are called = Dyad. A group of 4 homologous chromatids are called = Tetrad (2 Dyads). Crossing over (Genetic recombination ) takes place between non non sister chromatids of homologous chromosomes. Breakage and reunion of chromatid segments mediated by endonuclease and ligase. Pachytene responsible for the origin of new species and thus leads to evolution.
- 24. Crossing Over - variationCrossing Over - variation nonsister chromatids chiasmata: site of crossing over variation Tetrad
- 25. Diplotene (double stage)Diplotene (double stage) Homologous chromosomes start separating called disjunction. Separation will not takes place at chiasmata. Chiasmata are clearly visible as ‘X’ shape. Chiasmata are the sites where the crossing over occurred during pachytene. Chiasmata helps in holding homologous chromosomes.
- 26. DiakinesisDiakinesis Chromosomes are highly condensed. Chiasmata begin to move towards the chromosomes end. Displacement of chiasmata towards the terminal position is called Terminalization. Nucleolus and nuclear membrane disappear. Chromosomes are released into cytoplasm.
- 27. Metaphase IMetaphase I • Shortest phase • Spindle fibres attached to the centromeres of chromosomes. • Tetrads align on the metaphase plate. • INDEPENDENT ASSORTMENT OCCURS: – Orientation of homologous pair to poles is random. – Leads to variation. – Congression .
- 28. Metaphase IMetaphase I metaphase plate OR metaphase plate
- 29. Anaphase IAnaphase I Separation of homologous chromosomes Towards opposite pole with the help of spindle fibers. Actual reduction of chromosomes takes place. Paternal and maternal chromosomes of each homologous Pair segregate during anaphase I. This phase corresponds to mendel’s law of independent Assortment. Anaphase I introduces genetic variability.
- 31. Telophase ITelophase I Chromosomes arrived at the poles. Nuclear membrane and nucleolus reappear. Two daughter nuclei are organized each possessing a haploid number of chromosomes. Spindle fibers will disappear. The cytoplasm divides as its middle by cleavage (constriction) in animal cell. In plant cell by cell plate formation. 2 daughter cells are produced.
- 33. Metaphase I Anaphase I Telophase I Prophase II Prophase I
- 34. Meiosis IIMeiosis II • No interphase II • four phases: a. prophase II b. metaphase II c. anaphase II d. telophase II & cytokinesis • Similar to mitosis
- 35. Prophase IIProphase II • Centrioles separate and moved to polar regions. • Nucleoli and Nuclear envelope disappears. • Prophase II is inversely proportional time compared to telophase I.
- 36. Metaphase IIMetaphase II • Sister chromatids are oriented on equitorial plate. • Spindle fibers attach at centromere
- 37. Metaphase IIMetaphase II metaphase platemetaphase plate
- 38. Anaphase IIAnaphase II Centromere is cleaved. Sister chromatids are separated Sister chromatids are pulled towards opposite poles.
- 39. Anaphase II
- 40. Telophase II • Nuclei and nuclear membrane reform. • Cytokinesis occurs. • Four haploid daughter cells produced. – gametes = sperm or egg. – Spindle fibers lost.
- 42. Meiosis 2Meiosis 2ndnd division:division: like mitosislike mitosis with haploid setwith haploid set
- 43. 4 Haploid cells Gametes4 Haploid cells Gametes
- 45. VariationVariation Also known asAlso known as GENETICGENETIC RECOMBINATIONRECOMBINATION Important to population as the rawImportant to population as the raw material formaterial for NATURAL SELECTION.NATURAL SELECTION. All organisms areAll organisms are NOTNOT alikealike StrongestStrongest ““most fit”most fit” survive tosurvive to reproduce & pass on traitsreproduce & pass on traits 45
- 46. Question:Question: What are theWhat are the 33 sources ofsources of genetic recombinationgenetic recombination oror variation?variation? 46
- 47. Answer:Answer: 1.1. CROSSING OVERCROSSING OVER (prophase I)(prophase I) 2.2. INDEPENDENT ASSORTMENTINDEPENDENT ASSORTMENT (metaphase I)(metaphase I) 3.3. RANDOM FERTILIZATIONRANDOM FERTILIZATION 47
- 48. Question:Question: A cell containing 20 chromosomes20 chromosomes (diploid)(diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomeschromosomes? 48
- 49. Answer:Answer: 1010 chromosomes (haploid orchromosomes (haploid or 1n)1n) 49
- 50. • Genetic Diversity through Crossing Over: • Prophase I • Genetic Diversity through Independent Assortment: • During Metaphase I
- 53. The Key Difference Between Mitosis and Meiosis is the Way Chromosomes Uniquely Pair and Align in Meiosis Mitosis The first (and distinguishing) division of meiosis
- 55. END OF INTERPHASE PROPHASE I METAPHASE I ANAPHASE I MEIOSIS I Genetic recombination results from crossing over during prophase I of meiosis
- 56. TELOPHASE II ANAPHASE II METAPHASE IIPROPHASE IITELOPHASE I MEIOSIS
- 57. Mitosis vs MeiosisMitosis vs Meiosis MitosisMitosis MeiosisMeiosis Results inResults in 2 Diploid Cells2 Diploid Cells (2N)(2N) 4 Haploid Cells4 Haploid Cells (N)(N) Cells areCells are GeneticallyGenetically IdenticalIdentical GeneticallyGenetically DifferentDifferent Occurs inOccurs in Somatic (Body)Somatic (Body) CellsCells Sex CellsSex Cells
- 58. Mitosis vs. Meiosis • Mitosis • Meiosis Diploid (2n) Parent CellsDiploid (2n) Parent Cells Replication ofReplication of Chromosomes – doubleChromosomes – double the #the # End of Mitosis & Meiosis IEnd of Mitosis & Meiosis I Results in 2 diploid (2n)Results in 2 diploid (2n) daughter cellsdaughter cells End of Meiosis IIEnd of Meiosis II Results in 4 haploid (n)Results in 4 haploid (n) daughter cellsdaughter cells