Cell Cycle And Cell Division

• All organisms, regardless of size, start their life from a single cell
• Growth and reproduction are characteristics of cells and all living organisms
• Cells reproduce by dividing into two, with each parental cell giving rise to two daughter cells each time they divide
• Cell division, DNA replication, and cell growth must occur in a coordinated way to ensure correct division and formation of progeny cells containing intact genomes
• The sequence of events by which a cell duplicates its genome, synthesises other cell constituents, and eventually divides into two daughter cells is termed the cell cycle
• Cell cycle is divided into two basic phases: Interphase and M Phase (Mitosis phase)
• Interphase is the phase between two successive M phases and lasts more than 95% of the duration of the cell cycle
• M Phase represents the phase when actual cell division or mitosis occurs
• Interphase is divided into three phases: G1 phase (Gap 1), S phase (Synthesis), G2 phase (Gap 2)
• G1 phase: interval between mitosis and initiation of DNA replication, cell grows but does not replicate DNA
• S phase: DNA synthesis takes place, amount of DNA per cell doubles
• G2 phase: proteins are synthesised in preparation for mitosis while cell growth continues
• Some cells in adult animals do not exhibit division (e.g., heart cells) and many cells divide only occasionally, as needed for cell replacement
• Cells that do not divide further exit G1 phase to enter an inactive stage called quiescent stage (G0) of the cell cycle
• Mitotic cell division is seen in diploid somatic cells in animals, with few exceptions like haploid cells in male honey bees
• Plants can show mitotic divisions in both haploid and diploid cells
• Mitosis involves four stages of nuclear division: Prophase, Metaphase, Anaphase, Telophase
• Prophase: marked by condensation of chromosomal material and movement of centrosomes towards opposite poles of the cell
• Metaphase: chromosomes align at the equator with spindle fibres attaching to kinetochores
• Anaphase: chromatids separate and move to opposite poles
• Telophase: chromosomes decondense, nuclear envelope forms around chromosome clusters at each pole
• Cytokinesis completes cell division by separating cytoplasm into two daughter cells
• In plant cells, cytokinesis occurs by the formation of a cell plate that grows outward to meet existing lateral walls
• Mitosis is usually restricted to diploid cells, but in some lower plants and social insects, haploid cells also divide by mitosis
• Mitosis results in the production of diploid daughter cells with identical genetic complement
• The growth of multicellular organisms is due to mitosis
• Cell division in mitosis restores the nucleo-cytoplasmic ratio disturbed by cell growth
• Mitosis plays a significant role in cell repair, constantly replacing cells in the upper layer of the epidermis, lining of the gut, and blood cells
• Mitotic divisions in meristematic tissues like the apical and lateral cambium result in continuous growth of plants throughout their life
• Meiosis is the mechanism by which specific chromosome number is conserved across generations in sexually reproducing organisms
• Meiosis increases genetic variability in populations of organisms from one generation to the next, important for evolution
• Meiosis ensures the production of haploid phase in the life cycle of sexually reproducing organisms, while fertilization restores the diploid phase
• Meiosis involves two sequential cycles of nuclear and cell division called meiosis I and meiosis II, with only a single cycle of DNA replication
• Meiosis involves pairing of homologous chromosomes and recombination between non-sister chromatids of homologous chromosomes
• Four haploid cells are formed at the end of meiosis II
• Meiosis I involves prophase I, metaphase I, anaphase I, and telophase I
• Prophase I of meiosis I is longer and more complex than prophase of mitosis, with stages like Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis
• During prophase I, chromosomes pair together, forming bivalents or tetrads, and crossing over occurs between non-sister chromatids of homologous chromosomes
• Metaphase I involves alignment of bivalent chromosomes at the equatorial plate
• Anaphase I sees homologous chromosomes separate, while sister chromatids remain associated at their centromeres