CELL CYCLE, MITOSIS & MEIOSIS SMG

A brief account of cell cycle, mitosis and its significance ; meiosis and its significance; comparison between mitosis and meiosis.

1. Dr. Saji Mariam George Associate Professor (Retired) Assumption College Autonomous Changanacherry CELL CYCLE, MITOSIS AND MEIOSIS

2. CELL CYCLE The sequence of events a cell undergoes from the time of its formation by division to its own division into daughter cells. Duration varies – depends on the type of cell and external factors – temperature, supply of food, O2 etc. A growing cell undergoes a cell cycle.

3. Divided into 4 periods – G1, S, G2 and M (Mitosis ) G1, S, G2 phases are collectively called Interphase . In continuously dividing cells, a cell passes through 2 main phases of cell cycle – Interphase & Mitotic Phase. https://teachmephysiology.com

4. INTERPHASE Resting phase or stage between 2 mitotic divisions. No division of chromosomes or cytoplasm Metabolically active Longest phase – sub phases - G1, S and G2 https://quizlet.com

5. a ) G1 Phase (Post mitotic gap phase or First Gap phase ) The time “gap” between the end of Mitosis and the start of DNA synthesis. Most variable in length – days, months or years. Include the synthesis and organization of the substrate and enzymes necessary for DNA synthesis.

6. It is marked by the transcription of rRNA , tRNA , mRNA and synthesis of different types of proteins. Regulation of the duration of the cell cycle occurs primarily by arresting it at a specific point of G1 and the cell in the arrested condition is said to be in G0 state – the cell is withdrawn from cell cycle.

7. b) ‘S’ Phase The synthesis of DNA occurs in the ‘S’ phase (Synthetic period) . DNA content of the nucleus is doubled. Additional histones are also synthesized.

8. c ) G2 phase (Post DNA synthesis phase or premitotic gap phase or Second Gap phase.) The gap between the end of DNA synthesis and the beginning of mitosis. Synthesis of RNA and protein continues, but DNA synthesis stops.

9. The time taken for S phase, G2 phase and Mitosis is approximately equal. The length of G1 is usually long. The nucleus of an interphase cell contains very long chromatin fibres , which is suited for transcription and replication.

10. Cell Cycle Image:https :// slideplayer.com

11. M phase or MITOSIS (Gr. Mitos = Thread) The entry of a cell into M phase is initiated by a protein called maturation – promoting factor. The term Mitosis was coined by the German Biologist Walther Flemming to describe the thread like chromosomes. The type of division occurring in somatic cells and results in growth.

12. A cell divides to form two identical daughter cells which are similar to the parent cell with regard to the number and kind of chromosomes - Equational division. Involves a series of changes in the nucleus as well as cytoplasm – indirect division. Involves two main events – Karyokinesis (Division of nucleus) and Cytokinesis (Division of cytoplasm). Karyokinesis - involves 4 stages – Prophase, Metaphase, Anaphase and Telophase .

13. 1. PROPHASE First phase of Mitosis The important events during Prophase : The cell becomes spheroid and viscous. The extended form of interphase chromatin is converted into much shorter, thicker structures by a process of chromosome compaction ( chromosome condensation) - DNA supercoiling play an important role in chromosome condensation.

14. Each chromosome contains two chromatids . As the prophase progresses , the chromatids become shorter and thicker and the primary constriction becomes clearly visible. The nucleolus gradually disappears.

15. Spindle is formed in the cytoplasm. Nuclear envelope gradually disintegrates. Image:https ://lh6.ggpht.com

16. Mitotic apparatus - Spindle Image:https :// www.bio.umass.edu

17. PROMETAPHASE The nuclear membrane completely disintegrates, releasing the nuclear contents into the cytoplasm. A clear zone known as equator appears between the mid – line of the spindle and the nucleus. The chromosome move towards the equator.

18. The chromosomes of a prometaphase cell are moved by a process called congression towards the centre of the mitotic spindle. The forces required for chromosome movements during prometaphase are generated by motor proteins associated with both the kinetochores and arms of the chromosomes. The chromosomes are maintained in the equatorial plane by a “ tug – of – war” between balanced pulling forces exerted by chromosomal spindle fibres from opposite poles.

19. 2. METAPHASE The chromosomes are aligned at the equatorial portion of the spindle. Usually the arms of the chromosomes lie on the equator of spindle. Occasionally , only the centromeres lie on the equatorial plane and arms are directed towards the poles. Smaller chromosomes are usually central in position while larger ones are peripheral.

20. Metaphase Image:https ://home.cc.umanitoba.ca

21. Some spindle fibres are attached to the centromere of each chromosome – Chromosomal fibres . Some spindle fibres extend from one pole of the cell to another pole – Continuous fibres . Some spindle fibres occur in between the chromosomes – interzonal fibres or interchromosomal fibres .

22. 3. ANAPHASE The centromere of each chromosome divides into two. The chromatids of each chromosome are separated and form two daughter chromosomes. The daughter chromosomes migrate towards the opposite poles of the cell – achieved by the contraction of chromosomal fibres and the stretching of interchromosomal or interzonal fibres - Spindle fibres shorten to one third to one fifth of the original length.

23. Anaphase Image:https :// search.library.wisc.edu

24. 4. TELOPHASE ( Telos =end) Final stage of Mitosis The chromosomes start to unfold and become less condensed. Nucleolus reappears Nuclear envelope is re-formed from the Endoplasmic reticulum(ER). Spindle fibres gradually disappear by the depolymerisation of tubulin sub units of microtubules. Two daughter nuclei are formed.

25. Telophase Image:https :// search.library.wisc.edu

26. CYTOKINESIS Process of separation of cytoplasm. In plant cells, cytokinesis starts with the formation of the phragmoplast , which comprises interzonal microtubules and Golgi vesicles. This structure is transformed into the cell plate between the group of chromosomes. Cell plate grows from the middle towards the periphery and finally joins the cell wall. In animal cells, there is a constriction at the equator that finally results in the separation of the daughter cells.

27. Cytokinesis Cytokinesis in a Plant cell Cytokinesis in an animal cell Image:https ://celldivisionandreproduction.weebly.com Image:https :// slideplayer.com

28. MITOSIS

29. Image:https :// slideplayer.com

30. SIGNIFICANCE OF MITOSIS Maintainance of chromosomal continuity and diploid chromosome number. The two daughter cells receive exactly the same number and kind of chromosomes as that of parent cell. Helps the cell in maintaining its proper size . Provides opportunity for the growth and development of the body of the organisms. Provides new cells for repair. Helps the organisms in asexual reproduction.

31. MEIOSIS The term Meiosis was coined by Farmer & Moore (1905) Special type of cell division present in the germ cells of all sexually reproducing organisms. Consists of two divisions which takes place one after another. The chromosome number is reduced to half – helps to maintain a constant chromosome number of a species. Helps to maintain the regularity of reproductive cycle in sexually reproducing organisms.

32. TYPES OF MEIOSIS 1. Gametic or terminal meiosis - In animals and lower plants meiosis occurs before the formation of gametes. Image: https://quizlet.com

33. 2. Zygotic meiosis – This occurs in lower plants. Fertilization is immediately followed by meiosis. Image: https://quizlet.com

34. 3.Sporogenic or Indeterminate meiosis – Found in higher plants – occur at the time of spore formation. Image: https://quizlet.com

35. Two divisions in rapid succession, with the chromosomes replicating only once – Meiosis I & II and each division involves 4 stages – Prophase, Metaphase, Anaphase , Telophase . A cell produces 4 daughter nuclei, each having half the number of chromosomes – reduction division.

36. MEIOSIS I (FIRST MEIOTIC DIVISION) Heterotypic division Stages – Prophase I, Metaphase I, Anaphase I and Telophase I Prophase I Most important stage Longest stage – sub stages Preleptotene ( preleptonema ), Leptotene ( Leptonema ), Zygotene ( Zygonema ), Pachytene ( Pachynema ), Diplotene ( Diplonema ) and Diakinesis .

37. PRELEPTOTENE (PRELEPTONEMA) Early prophase Increase in nuclear volume Chromosomes are thin and difficult to observe.

38. LEPTOTENE (LEPTONEMA ; Greek. Leptos = Thin , nema = thread) Chromosomes become more clear – appear as thin threads. Looks single even though DNA duplications have already occurred and have two chromatids .

39. In some cases , granular chromomeres are visible – Number, shape, size and location of chromomere is characteristic of individual chromosomes and therefore helps in their identification. Chromosomes take up a specific orientation inside the nucleus . The ends of the chromosomes converge towards one side of the nucleus where the centrosome lies – Bouquet stage .

40. Bouquet stage This is an arrangement of chromosomes in such a way that the telomeres bunch together in a confined area of the nuclear periphery with centromeres at a polar position. This arrangement resembles a bouquet. This help for the pairing of homolgous chromosomes within the cell.   Image:http ://scienceyoucandigest.blogspot.com

41. Leptotene ( Leptonema ) Images:https ://biology4isc.weebly.com

42. ZYGOTENE ( ZYGONEMA, Gr. Zygon = Adjoining) Pairing of homologous chromosomes, Synapsis takes place. A pair of homologous chromosomes lying together is a bivalent. The two homologues do not fuse during pairing , but remain separated by a space – 0.15 to 0.2 µm which is occupied by the synaptonemal complex. Pairing starts at random . Pairing is specific, point for point and chromomere for chromomere in each homologue.

43. Zygotene ( Zygonema ) Images:https ://biology4isc.weebly.com

44. Zygotene …. Synaptonemal Complex Formed during pairing of homologous chromosomes. It is mainly composed of proteins It is tripartite, ribbon like structure. It consists of a dense central element with a dense lateral element on either side. Each lateral element is attached on the inner side of a homologous chromosome. The space between the central and lateral elements is traversed by series of transverse units or LC fibres .

45. Synaptonemal Complex Image:https :// bio.libretexts.org

46. Synaptonemal Complex….. Image: http://cc.scu.edu.cn

47. Synaptonemal Complex… Images:http ://bio3400.nicerweb.com

48. PACHYTENE (PACHYNEMA , Gr. Pachus = thick) Chromosomes become shorter and thicker In the middle of the pachytene , chromosome split length wise to form chromatids . Two chromatids of the same chromosome are called sister chromatids and those of two homologous chromosomes are called non-sister chromatids . Each chromatid pair is united by a centromere .

49. Pachytene chromosome thus consists of 4 chromatids closely joined together in one complex unit known as a bivalent. The most important event, crossing over occur during pachytene . Crossing over involves the exchange of chromosomal segments between non – sister chromatids of the homologous chromosomes.

50. Crossing over takes place by breakage and reunion of chromatid segments. Points of interchange of crossing over are known as chiasmata (Sing. Chiasma ). Crossing over result in the shuffling of genes which leads to genetic recombinations .

51. Pachytene ( Pachynema ) Images:https ://biology4isc.weebly.com

52. Image:https :// www.tes.com

53. Pachytene – Crossing over Image:http :// fig.cox.miami.edu

54. DIPLOTENE (DIPLONEMA) Paired chromosomes begin to separate, but they are held together at the chiasmata . A chiasma formed at the ends chromosomes – terminal chiasma . Chiasmata formed along the lengths of chromosomes – interstitial chiasmata . The number and position of chiasmata varies with the length of the chromosomes - there is at least one chiasma per bivalent chromosome.

55. Terminalisation of chiasma takes place – the chiasma begin to be displaced along the length of the chromosomes. The terminal chiasma slips off the ends of the chromosomes and its position is taken up by an interstitial chiasma , which is now called the terminal chiasma . When terminalisation is complete, the homologues remain in contact through the terminal chiasma .

56. Diplotene ( Diplonema ) Images:https ://biology4isc.weebly.com

57. Diplotene ( Diplonema )

58. DIAKINESIS (Gr. Dia = Across, Kinesis = movement) The chromosomes become more condensed. The bivalents are more evenly distributed in the nucleus and migrate towards the periphery. Terminalisation is complete and the homologues remain in contact with each other by their terminal chiasma . Nucleolus disappear.

59. Diakinesis Images:https ://biology4isc.weebly.com

60. PROMETAPHASE I Condensation of chromosomes reaches its maximum. Nuclear envelope disappears. Spindle formation begins.

61. METAPHASE I The chromosomes (bivalents) are arranged at the equator . Centromeres of homologous chromosomes point towards opposite poles and lie on either side of the equatorial plate.

62. M etaphase I Images:https ://biology4isc.weebly.com

63. ANAPHASE I The homologous chromosomes of each pair (bivalents) with its two chromatids and undivided centromere moves towards the opposite poles of the cell. This results in the reduction of the chromosome number into half. Movement of chromosomes towards the pole is at random. The chromosomes with single or a few terminal chiasmata usually separate more quickly than longer chromosomes with many interstitial chiasma .

64. Anaphase I Images:https ://biology4isc.weebly.com

65. TELOPHASE I Begins when the group of chromosomes arrive at their respective poles. The chromosomes undergo despiralization and become elongated. The nuclear membrane is reformed. Nucleolus also reappear. Two daughter nuclei with haploid number of chromosomes are formed at each pole.

66. Telophase I Images:https ://biology4isc.weebly.com

67. CYTOKINESIS After karyokinesis , cytokinesis takes place. In most plant cells, a cell plate is formed in between the two nuclei resulting in two daughter cells. In animal cells, the cell membrane constricts in the middle and two daughter cells are formed

68. In some plant cells, cytokinesis does not takes place until both meiotic divisions are completed. Following Telophase is a short interphase . At interphase between the two meiotic divisions, there is no duplication of DNA.

69. MEIOSIS II (SECOND MEIOTIC DIVISION) STAGES : Prophase II Metaphase II Anaphase II Telophase II

70. PROPHASE II Short duration Nuclear membrane and nucleolus disappear The chromosomes with two chromatids become short and thick Spindle is formed

71. Prophase II Images:https ://biology4isc.weebly.com

72. METAPHASE II Chromosomes get arranged on the equatorial plate. The microtubules of the spindle are attached with the centromere of the chromosomes. The centromere divide , thus separating the two chromatids of each chromosome.

73. Metaphase II Images:https ://biology4isc.weebly.com

74. ANAPHASE II Daughter chromosomes move towards the opposite poles. Images:https ://biology4isc.weebly.com

75. TELOPHASE II Nuclear membrane and nucleoli reappear. The chromosomes despiralise and elongate . Images:https ://biology4isc.weebly.com

76. After the karyokinesis , in each haploid meiotic cell, cytokinesis occurs and thus four haploid cells are formed. Images:https ://biology4isc.weebly.com

77. SIGNIFICANCE OF MEIOSIS Maintainance of Chromosome number – Meiosis halves the chromosome number in the gametes so that fertilization may restore the original diploid number in the zygote. By crossing over, the meiosis provides an opportunity for the exchange for the genes and this causes the genetic variation among the species.

78. COMPARISON BETWEEN MITOSIS & MEIOSIS MITOSIS Occur in somatic cells Involves a single division , resulting in 2 daughter nuclei which are identical to that of the parent cell . The chromosome number remains constant at the end of mitosis. The genetic constitution of the daughter cells is identical to that of parent cells. Occurs in both sexually as well as asexually reproducing organisms . MEIOSIS In germ cells (Reproductive cells ) at the time of gametogenesis . Involves 2 successive divisions , resulting in 4 daughter nuclei with chromosome number reduced to half . Chromosome number is reduced from diploid to haploid . The genetic constitution of the daughter cells differ from that of the parent cell. Occurs only in sexually reproducing organisms.

79. MITOSIS A short process DNA replication takes place during interphase Prophase is simple and of short duration. MEIOSIS A long process Takes place during Interphase I , but not in Interphase II Prophase is very long and complex. It comprises five substages viz. Leptotene , Zygotene , Pachytene , Diplotene and Diakinesis .

80. MITOSIS Prophase chromosome appear double from the very beginning. No pairing or synapsis takes place . No formation of synaptonemal complex Each chromosome consists of two chromatids united by centromere . MEIOSIS Prophase I chromosome appear as single in the beginning. Pairing or synapsis takes place between homologous chromosomes. Synaptonemal complex is formed between synapsed homologous chromosomes. The two homologous chromosomes form bivalents or tetrads. Each bivalent has four chromatids and two centromeres .

81. MITOSIS No crossing over and genetic recombination. Metaphase -The centromere of the chromosome remain directed towards the equator and the arms of the chromosome remains directed towards the poles. MEIOSIS Crossing over takes place which leads to genetic recombination. Metaphase- The centromeres of the chromosomes remain directed towards the poles and the chromosomal arms remain directed towards the equator.

82. MITOSIS Anaphase involves the separation of chromatids of each chromosome. The chromosomes separate simultaneously during Anaphase. Telophase occurs in all cases. MEIOSIS Anaphase I involves separation of homologous chromosomes. The chromatids move apart in Anaphase II. Short chromosomes separate early. Separation of long chromosome is delayed. Telophase I is eliminated in some cases.

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