cell cycle

• The cell cycle is the sequence of events by which a cell duplicates its genome, synthesises the other constituents of the cell, and eventually divides into two daughter cells
• Eukaryotic cells divide once in approximately every 24 hours
• The cell cycle is divided into two main phases: Interphase and M Phase
• Interphase is the time during which the cell is preparing for division by undergoing cell growth and DNA replication in an orderly manner
• Interphase is further divided into three phases:

  • G1 phase
  • S phase
  • G2 phase
• M Phase represents the phase when the actual cell division or mitosis occurs
• Cell cycle stages:

  • G1 phase: Cell is metabolically active and continuously grows but does not replicate its DNA
  • S phase: DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm
  • G2 phase: Proteins are synthesized in preparation for mitosis while cell growth continues
  • Cells that do not divide further exit G1 phase to enter an inactive stage called quiescent stage (G0) of the cell cycle
• Mitosis involves four stages:

  • Prophase
  • Metaphase
  • Anaphase
  • Telophase
• In mitosis, the number of chromosomes in the parent and progeny cells is the same, leading to equational division
• Prophase is marked by the initiation of condensation of chromosomal material
• Characteristics events marking the completion of prophase:

  • Chromosomal material condenses to form compact mitotic chromosomes
  • Chromosomes are composed of two chromatids attached together at the centromere
  • Centrosome, which duplicated during interphase, moves towards opposite poles of the cell
  • Each centrosome radiates out microtubules called asters
  • The two asters, along with spindle fibers, form the mitotic apparatus
• At the end of prophase, cells do not show:

  • Golgi complexes
  • Endoplasmic reticulum
  • Nucleolus
  • Nuclear envelope
• The second phase of mitosis is marked by the complete disintegration of the nuclear envelope
• Metaphase features:

  • Spindle fibers attach to kinetochores of chromosomes
  • Chromosomes are moved to the spindle equator and aligned along the metaphase plate through spindle fibers to both poles
• Anaphase stage characteristics:

  • Centromeres split and chromatids separate
  • Chromatids move to opposite poles
• Telophase key events:

  • Chromosomes cluster at opposite spindle poles, losing their identity as discrete elements
  • Nuclear envelope develops around the chromosome clusters at each pole, forming two daughter nuclei
  • Nucleolus, golgi complex, and ER reform
• Cytokinesis is the process where the cell itself is divided into two daughter cells by the separation of cytoplasm
• In animal cells, cytokinesis is achieved by the appearance of a furrow in the plasma membrane
• The furrow in animal cells gradually deepens and ultimately joins in the centre, dividing the cell cytoplasm into two
• Plant cells undergo cytokinesis by a different mechanism due to being enclosed by a relatively inextensible cell wall
• In plant cells, wall formation starts in the centre of the cell and grows outward to meet the existing lateral walls
• The formation of the new cell wall in plant cells begins with the formation of a simple precursor called the cell-plate, representing the middle lamella between the walls of two adjacent cells
• During cytoplasmic division in cytokinesis, organelles like mitochondria and plastids get distributed between the two daughter cells
• In some organisms, karyokinesis is not followed by cytokinesis, leading to a multinucleate condition and the formation of syncytium
• Mitosis, or equational division, is usually restricted to diploid cells
• In some lower plants and social insects, haploid cells also divide by mitosis
• Understanding the significance of mitosis in an organism's life is essential
• Examples of haploid and diploid insects where you may have studied about mitosis
• Mitosis results in the production of diploid daughter cells with identical genetic complement
• The growth of multicellular organisms is due to mitosis
• Cell growth can disturb the ratio between the nucleus and the cytoplasm, requiring cell division to restore the nucleo-cytoplasmic ratio
• Mitosis plays a significant role in cell repair
• Cells of the upper layer of the epidermis, cells of the lining of the gut, and blood cells are constantly replaced through mitotic divisions
• Mitotic divisions in meristematic tissues like the apical and lateral cambium result in continuous growth of plants throughout their life
• The production of offspring by sexual reproduction involves the fusion of two gametes, each with a complete haploid set of chromosomes
• Gametes are formed from specialised diploid cells
• Specialised cell division that reduces the chromosome number by half and produces haploid daughter cells is called meiosis
• Meiosis ensures the production of the haploid phase in the life cycle of sexually reproducing organisms
• Fertilisation restores the diploid phase in sexually reproducing organisms
• Meiosis occurs during gametogenesis in plants and animals, leading to the formation of haploid gametes