Meiosis

1. Welcome 1

2. MEIOSISE CELL DIVISION Presented by Hemal sarkar : ( 11-04365 ) Farzana Rahman Tanwi : ( 11-04366 ) Nusrat Hossain : ( 11-04370 ) Jannatul Ferdous Tanni : ( 11-04371 ) Alpona Arephin : ( 11-04373 ) Avijit Chowdhury : ( 11-04375 ) Salman Khan Taunoy : ( 11-04377 ) 2

3. OUTLINE Cell division Types of cell division What is meiosis cell division History of meiosis Where it occurs Facts about meiosis Phases of meiosis 1 Prophase-1 Leptotene Zygotene Pachytene Diplotene Diakinesis 3

4. OUTLINE Crossing over Importance of crossing over Metaphase-1 Anaphase-1 Telophase-1 Phases of meiosis -2 Prophase-2 Metaphase-2 Anaphase-2 Telophase-2 Importance of Meiosis Differences between Meiosis and mitosis 4

5. Cell Division Cell division is the process by which a parent cell divides into two or more daughter cells. All cells are derived from pre-existing cells It is necessary to replace worn out cells in multicellular organisms It is required for growth in multicellular organisms It is necessary for reproduction in unicellular or multicellular organisms 5

6. Continued Cells can divide, and in unicellular organisms, this makes more organisms. In multicellular organisms, cell division is used for growth, development, and repair of the organism. Cell division is controlled by DNA 6

7. Types of cell Division Mainly cell division is 3 types. 1. Amitosis 2. Mitosis 3. Meiosis 7

8. What is meiosis cell Division Meiosis is a special type of cell division necessary for sexual reproduction in eukaryotes Meiosis begins with one diploid cell containing two copies of each chromosome one from the organism's mother and one from its father produces four haploid cells containing one copy of each chromosome 8

9. History of meiosis Meiosis was discovered and described for the first time in sea urchin eggs in 1876 by the German biologist Oscar Hertwig It was described again in 1883, at the level of chromosomes, by the Belgian zoologist Edouard Van Beneden in Ascaris worms' eggs The term meiosis was coined by J.B Farmer and J.B Moore in 1905 9

10. Where it occurs? Meiosis occurs in eukaryotic life cycles involving sexual reproduction It always Occurs in reproductive cells ( meiocyte ) In lower plant meiosis occurs after fertilization in zygote In higher plant it occurs before fertilization in the time of gamete creation. 10

11. Facts about Meiosis Meiosis is the type of cell division by which germ cells are produced. Two meiotic divisions --- Meiosis I and Meiosis II Called Reduction- division Original cell is diploid (2n) Four daughter cells produced that are monoploid (1n) 11

12. Continued Daughter cells contain half the number of chromosomes as the original cell Produces gametes (eggs & sperm) Occurs in the testes in males (Spermatogenesis) Occurs in the ovaries in females (Oogenesis) During meiosis, DNA replicates once , but the nucleus divides twice 12

13. Phases of Meiosis I Meiosis I: the reduction division -- diploid mother cell produces 2 haploid cells. 1. Prophase I i. Leptotene ii. Zygotene iii. Pachytene iv. Diplotene v. Diakinesis 2. Metaphase I 3. Anaphase I 4. Telophase I 13

14. Prophase I During prophase I, DNA is exchanged between homologous chromosomes in a process called homologous recombination. This often results in chromosomal crossover. The paired and replicated chromosomes are called bivalents or tetrads. The process of pairing the homologous chromosomes is called synopsis. At this stage, non-sister chromatids may cross-over at points called chiasmata (plural; singular chiasma). 14

15. Leptotene The first stage of prophase I is the Leptotene stage Leptotene also known as Leptonema from Greek words meaning thin threads During this stage, individual chromosomes begin to condense into long strands within the nucleus However the two sister chromatids are still so tightly bound that they are indistinguishable from one another 15

16. Zygotene The zygotene stage , also known as zygonema, from Greek words meaning "paired threads” Zygotene, occurs as the chromosomes approximately line up with each other into homologous chromosomes. The combined homologous chromosomes are said to be bivalent 16

17. Pachytene In pachynema, the homologous chromosomes become much more closely associated. This process is known as synapses The synapsed homologous pair of chromosomes is called a tetrad, because it consists of four chromatids It can't be observed until the next stage, but the synapsed chromosomes may undergo crossing over in pachynema The chromosomes continue to condense 17

18. Diplotene During the diplotene stage, also known as diplonema, from Greek words meaning "two threads," the homologous chromosomes separate from one another a little The chromosomes themselves uncoil a bit, allowing some transcription of DNA 18

19. Diakinesis Chromosomes condense further during the diakinesis stage, from Greek words meaning "moving through." This is the first point in meiosis where the four parts of the tetrads are actually visible. In this stage, the homologous chromosomes separate further, and the chiasmata terminalize . Making chiasmata clearly visible. 19

20. Different stages of meiotic prophase-1 20

21. Crossing over During prophase I of meiosis, a part of chromatid of one chromosome goes over to a chromatid of the other chromosome and vice versa. This interchange of the parts of the chromatids of a pair of homologous chromosomes is known as crossing over crossing over is a process by which two chromosomes of a homologous pair exchange equal segments with each other Crossing over occurs in the first division of meiosis 21

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23. Importance of crossing over Crossing over takes place between the nonsister chromatids of homologous chromosomes Crossing over is one of the most important events in meiosis because it allows variation in the produced offspring a. produces new individuals having new combinations of traits. b. Helped in establishing the concept of linear arrangement of genes. c. Helps in the mapping of chromosomes. d. Selection of useful recombination. 23

24. Metaphase I Metaphase 1 is the second phase of Meiosis The tetrads from prophase I line up in the middle of the dividing cell randomly Spindle fibers attach to the tetrads from both ends of the cell 24

25. Anaphase I Anaphase I begins when the two chromosomes of each bivalent separate and start moving toward opposite poles of the. In anaphase I the sister chromatids remain attached at their centromeres and move together toward the poles. Fig: Anaphase-1 25

26. Telophase I The homologous chromosome pairs reach the poles of the cell. The homologous chromosome pairs complete their migration to the two poles A nuclear envelope reforms around each chromosome set, the spindle disappears, and cytokinesis follows. 26

27. Figure of meiosis 27

28. Continued 28

29. Phases of Meiosis II Meiosis II: mitotic division of 2 haploid cells to produce 4 haploid daughter cells. 1.Prophase -2 2.Metaphase -2 3.Anaphase 2 4.Telophase-2 29

30. Prophase II Meiosis II begins without any further replication of the chromosomes. In prophase II, the nuclear envelope breaks down and the spindle apparatus forms While chromosome duplication took place prior to meiosis I, no new chromosome replication occurs before meiosis II The centrioles duplicate. This occurs by separation of the two members of the pair, and then the formation of a daughter centriole perpendicular to each original centriole. The two pairs of centrioles separate into two centrosomes The nuclear envelope breaks down, and the spindle apparatus forms 30

31. Metaphase II The chromosomes become arranged on the metaphase plate. Centromeres are arranged in a line called equatorial plate of invisible spindle apparatus. previously occurring nuclear membrane is not present. 31

32. Anaphase II The centromeres separate and the sister chromatids now individual chromosomes move toward the opposite poles of the cell. The separated chromatids are now called chromosomes in their own right 32

33. Telophase II Nuclear envelope forms around each set of chromosomes Nucleolus appears in each nucleus Chromosomes lengthen and become indistinct 33

34. Figure of Meiosis- 2 34

35. Different stages in Meiosis- I & Meiosis II 35

36. Importance of meiosis Produces haploid gametes so that the diploid number of the species remains constant generation after generation. 2. Source of genetic variation because crossing over brings together new gene combinations on chromosomes 36

37. Differences between meiosis and mitosis Mitosis Meiosis Occurs in body cells Occurs in reproductive cells Number of chromosomes remains the same in the daughter cells Number of chromosomes is halved in the daughter cells Daughter cells are identical to parent cells and each other Daughter cells are genetically different to the parent cells and each other Two daughter cells are formed Four daughter cells are formed Homologous chromosomes do not come together Homologous chromosomes come together There is no exchange of genetic material between Chromosomes There is exchange of genetic material between chromosomes 37

38. Summery Lets see the video. Link. Animation: How meiosis work 38

39. Reference Cytology: introduction to cellular structure and function Edmund J. Messina The Principles of Clinical Cytogenetics   By Martha B. Keagle, Steven L. Gerse Cytology and Cytogenetics Carl P. Swanson Links http://en.wikipedia.org/wiki/Meiosis Animation: How Meiosis Works http://highered.mcgraw http://www.daviddarling.info/encyclopedia/M/meiosis.html http://phasesofmeiosis.blogspot.es/ http://www.youtube.com/watch?v=35ncSrJOwME&feature=related http://www.youtube.com/watch?v=D1_mQS_FZ0&feature=related 39

40. Thanks for Your patience hearing 40