meiosis

cell division

1. CELL DIVISION-MEIOSIS PRESENTED BY, SREYA K. R Msc Biochemistry& Molecular Biology 17-MSVM-21

2. MEIOSIS HISTORY DIFFERENT STAGES IN MEIOSIS MEIOSIS I MEIOSIS II TYPES OF MEIOSIS SIGNIFICANCE OF MEIOSIS REFERENCE CONTENTS

3. MEIOSIS

4. MEIOSIS M eiosis is a type of cell division occur only in eukaryote They are also called as Reduction division Diploid(2n)  Haploid(n) Two stages in meiosis: meiosis I and meiosis II The two different sex cells are: sperm and egg Sex cells produce from germ cells

5. Gametes are haploid The fusion of gamete lead to production of zygote Zygote attaining maturity capable of dividing in to daughter cells In plants, meiosis is observed after spore production whereas in animals meiosis take place during gamete formation Meiosis is similar to mitosis but there is a significant difference Meiosis consist of two type division: Heterotypic division Homotypic division

6. HISTORY Meiosis was discovered and described for the first time in sea urchin eggs , German biologist Oscar Hertwig,1876 At the level of chromosomes, by the Belgian zoologist Edouard Van in  Ascaris roundworm eggs,1883 The significance of meiosis for reproduction and inheritance, by German biologist  August Weismann , 1890

7. Noted that two cell divisions were necessary to transform one diploid cell into four haploid cells if the number of chromosomes had to be maintained In 1911 the American geneticist Thomas Hunt Morgan detected crossovers in meiosis in the fruit fly  Drosophilla Melanogaster This helped to establish that genetic traits are transmitted on chromosomes.

8. The term meiosis (originally spelled "maiosis") was introduced to biology by J.B. Farmer and J.E.S Moore in 1905 It is derived from a Greek word , meaning 'lessening'

9. STAGES OF MEIOSIS

10. DIFFERENT STAGES IN MEIOSIS After chromosomes duplicate, two divisions follow Meiosis I (reduction division) Meiosis II (equational division)

11. INTERPHASE The parent cell or the dividing cell undergoes a preparatory phase, known as interphase The parent cell synthesizes more DNA and proteins, increasing the overall size and mass of the cell The dividing cell duplicates or doubles its chromosomes

12. In animal cells, Outside the nucleus there are two centrosomes each containing a pair of centrioles The two centrosomes are produced by the duplication of single centrosomes during premeiotic interphase . The centrosomes serve as a microtubule organizing centers (MTOCs) Microtubules readily extend from centrosomes forming an aster Plant cells do not have centrosomes

13. Meiosis I

14. Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Centrosome (with centriole pair ) Sister chromatids Chiasmata Spindle Homologous chromosomes Fragments of nuclear envelope Duplicated homologous chromosomes (red and blue) pair and exchange segments; 2 n  6 in this example. Centromere ( with kinetochores ) Metaphase plate Microtubule attached to kinetochore Chromosomes line up by homologous pairs. Sister chromatids remain attached Homologous chromosomes separate Each pair of homologous chromosomes separates. Cleavage furrow Two haploid cells form; each chromosome still consists of two sister chromatids.

15. (a)Prophase I: The chromosomes have already duplicated and they coil and become shorter and thicker. The duplicated homologous chromosomes pair, and crossing-over occurs

16. Crossing–over is the process that can give rise to genetic recombination The sites of crossing over are seen as crisscrossed non-sister chromatids and are called as chiasmata Prophase I is divided into five phases: Leptotene Zygotene Pachytene Diplotene Diakinesis

17. 1) LEPTOTENE[ THIN -THREADSTAGE ] Chromosomes starts to condense Homologous dyads find each other and align themselves from end to end with the aid of an axial element.

18. 2) ZYGOTENE[ YOKE-THREAD STAGE] Homologues chromosomes become closely associated to form pairs of chromosome It is known as synapsis Paired homologous chromosome join with thick protein containing synaptonemal Complex (SC)

19. 3) PACHYTENE[ THICK-THREAD STAGE ] Crossing over take place between pairs of homologous chromosome to form chiasmata There represent point where DNA recombination is occurring The steps in recombining DNA continue to the end of pachytene

20. 4) DIPLOTENE[ DOUBLE - THREAD STAGE ] Homologous chromosome start to separate but remain attached to chiasmata DNA recombination is complete The SC begins to breakdown At first chiasmata are located at the sites of the recombination nodules, But later they migrate towards the end of the chromatids

21. 5) DIAKINESIS Homologous chromosome continue to separate, and chiasmata move to the end of chromosomes In some organisms, the chromosomes decondense and begin to be transcribed for a time

22. Fig. Showing sub division of prophase I

23. (b)Metaphase I The pairs of homologous chromosomes, as tightly coiled and condensed It arranged on a plane equidistant from the poles called the metaphase plate . Spindle fibers from one pole of the cell attach to one chromosome of each pair S pindle fibers from the opposite pole attach to homologous chromosome.

24. (C)Anaphase I In anaphase I, pairs of homologous chromosomes separate One chromosome moves toward each pole, guided by the spindle apparatus Sister chromatids remain attached at the Centromere and move towards the pole

25. (d) Telophase I and Cytokinesis In the beginning of telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids Nuclear envelope reforms around each chromosome set, the spindle disappears, and cytokinesis follows. Cytokinesis usually occurs simultaneously, forming two haploid daughter cells

26. In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated

27. Meiosis II

28. Division in meiosis II also occurs as four phases Prophase II Metaphase II Anaphase II Telophase II and cytokinesis Meiosis II is very similar to mitosis

29. Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Sister chromatids separate Haploid daughter cells forming During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes.

30. (a)Prophase II The nuclear envelope breaks down and the spindle apparatus forms. Here the centrioles duplicate occurs by the separation of two members of the pair, then the formation of a daughter centrioles perpendicular to each original centriole The two pairs of centrioles separate in to two centrosomes

31. (b) Metaphase II In metaphase II, the sister chromatids are arranged at the metaphase plate Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical The kinetochores of sister chromatids attach to microtubules extending from opposite poles

32. (c)Anaphase I: The centromeres separate The two chromatids of each chromosome move to opposite poles on the spindle The separated chromatids are now called as chromosomes

33. (d)Telophase II A nuclear envelope forms around each set of chromosomes Cytokinesis take place, producing four daughter cells, each with a haploid set of chromosomes because of crossing-over Some chromosomes have recombined segments of original parental chromosomes.

34. TYPES OF MEIOSIS

35. Gametic/Terminal meiosis Zygotic/Initial meiosis Sporic or Intermediate meiosis

36. GAMETIC/TERMINAL MEIOSIS In male vertebrates,

37. In female vertebrates,

38. SPORIC / INTERMEDIATE MEIOSIS

39. Zygotic/Initial meiosis

40. SIGNIFICANCE OF MEIOSIS

41. Meiosis facilitates stable sexual reproduction. If there is no meiosis, the chromosome number is doubled. This would result in the formation of abnormal forms. Constant number of chromosome in a given species is maintained by meiosis During crossing over, the hereditary factors from female and male parent get mixed. This can lead to the genetical variation among the species

42. REFERENCES Cooper , G. M., & Hausman , R. E. 2009.  The cell: a molecular approach . Washington, D.C., ASM Press. Lodish,Berk,Kaiser,Kreiger,Scott,Bretscher,Ploegh,Matsudaura,6 th edition,Molecular Cell Biology. Albert,Johnson,Lewis,Raff,Roberts,Walton,5 th edition,The Molecular Biology Of The Cell Gerald Karp, Cell Biology, 6 th edition, John Wiley & Sons, Inc.