Cell cycle and division
- 1. Class - 11
- 2. Why cell Divided ? • Every organism start their life from a single cell. • Growth and reproduction are characteristics of cells. • Every organism repair their body by cell division.
- 3. Amitosis
- 4. Mitosis
- 5. Meiosis
- 6. The sequence of events by which a cell duplicates its genome, synthesizes the other constituents of the cell and eventually divides into two daughter cells is termed cell cycle.
- 8. Phases of Cell Cycle Mitosis phase Interphase karyokinesis cytokinesis G1 phase G2 phase S phase Prophase Metaphase Anaphase Telophase
- 10. During the S or synthesis phase DNA synthesis or replication takes place. In this time the amount of DNA per cell doubles However, there is no increase in the chromosome number;
- 11. During the G2 phase, proteins are synthesised in preparation for mitosis while cell growthcontinues.
- 12. Karyokinesi s Prophase (1)Chromosomal material condenses to form compact mitotic chromosomes. Chromosomes are seen to be composed of two chromatids attached together at the centromere. (2) Chromosomes hold spindle fibre (3) Nucleolus membrane Become Disappear
- 13. Metaphase - (1) Spindle fibers attach to kinetochores of chromosomes. (2) Chromosomes are moved to spindle equator. (3) Chromatids arms are toward pole.
- 14. Anaphase - (1) Centromeres split and chromatids separate. (2) Chromatids move to opposite poles
- 15. Telophase - (1) Chromosomes cluster at opposite spindle poles and their identity is lost as discrete elements. (2) Nuclear membrane assembles around the chromosome clusters. (3) Nucleolus, Golgi complex and ER reform
- 16. Cytokinesis - (1) After the division of nucleolus cell divided into two daughter cell. (2) At the time of cytoplasmic division, organelles like mitochondria and plastids get distributed between the two daughter cells
- 18. The growth of multicellular organisms is due to mitosis. A very significant contribution of mitosis is cell repair. The cells of the upper layer of the epidermis, cells of the lining of the gut, and blood cells are being constantly replaced. Mitotic divisions in the meristematic tissues – the apical and the lateral cambium, result in a continuous growth of plants throughout their life
- 19. • Meiosis involves two sequential cycles of nuclear and cell division called meiosis I and meiosis II but only a single cycle of DNA replication. • Meiosis I is initiated after the parental chromosomes have replicated to produce identical sister chromatids at the S phase. • Meiosis involves pairing of homologous chromosomes and recombination between them. • Four haploid cells are formed at the end of meiosis II.
- 20. Meiosis-I Prophase I: Prophase of the first meiotic division is typically longer and more complex when compared to prophase of mitosis. It has been further subdivided into the following five phases based on chromosomal behavior. • Leptotene, • Zygotene, • Pachytene, • Diplotene • Diakinesis
- 21. Leptotene - During leptotene stage the chromosomes become thick and gradually visible under the light microscope.
- 22. Zygotene - During this stage chromosomes start pairing together and this process of association is called synapsis. Such paired chromosomes are called homologous chromosomes
- 23. Pachytene- During this stage bivalent chromosomes now clearly appears as tetrads. This stage is characterized by the appearance of recombination nodules, the sites at which crossing over occurs between non-sister chromatids of the homologous chromosomes. Crossing over is the exchange of genetic material between two homologous chromosomes
- 24. Diplotene - The beginning of diplotene is recognised by the dissolution of the synaptonemal complex and the tendency of the recombined homologous chromosomes of the bivalents to separate from each other except at the sites of crossovers. These X-shaped structures, are called chiasmata
- 25. Diakinesis - Repulsion process start in the homologous chromosomes. Chromosomes become seprate. By the end of diakinesis, the nucleolus disappears and the nuclear envelope also breaks down. Diakinesis represents transition to metaphase.
- 26. Metaphase I: The bivalent chromosomes align on the equatorial plate. The microtubules from the opposite poles of the spindle attach to the pair of homologous chromosomes
- 27. Anaphase I: The homologous chromosomes separate, while sister chromatids remain associated at their centromeres.
- 28. Telophase I - (1) Chromosomes cluster at opposite spindle poles and their identity is lost as discrete elements. (2) Nuclear membrane assembles around the chromosome clusters.
- 29. Meiosis II - Meiosis second division is same as mitosis division. It is also completed in the following stages, Prophase II, Metaphase II, Anaphase II, and Telophase II at the end of meiosis division four haploid daughter cell obtained.
- 30. 1. Meiosis form gametes that are required for sexual reproduction 2. Meiosis maintains the fixed number of chromosomes in sexually reproducing organisms by having the same during gametogenesis 3. In meiosis, paternal and maternal chromosomes assort independently. 4. Meiosis introduces a new combination of traits or variations. 5. Chromosomal and genomic mutations occur by irregularities of meiotic division. Some of these mutations are useful to the organism.
- 31. P.N. Patidar -