Cell Division and Cytokinesis

• Generation of New Cells by Cell Division

  Cell division is a fundamental process in all living organisms, allowing for growth, repair, and reproduction
• Parent Cell to Daughter Cells


  In all living organisms, a parent cell (also called a mother cell) divides to produce two daughter cells. This process is essential for:

  1. Growth of multicellular organisms
  2. Repair of damaged tissues
  3. Asexual reproduction in single-celled organisms
• The term "parent cell" doesn't imply gender; it's simply the cell that gives rise to new cells.
• The main types of cell division are:

  • Mitosis: Produces two genetically identical daughter cells
  • Meiosis: Produces four genetically diverse daughter cells with half the chromosome number
• Cytokinesis: 


  Splitting of Cytoplasm
• Splitting of Cytoplasm


  Cytokinesis is the final stage of cell division where the cytoplasm of the parent cell is divided between the daughter cells. 
• Animal Cell Cytokinesis:


  In animal cells, cytokinesis occurs through a process called contractile ring formation:

  1. A ring of contractile proteins (actin and myosin) forms beneath the cell membrane at the cell's equator.
  2. This ring contracts, pinching the cell membrane inward.
  3. The contraction continues until the cell is completely divided into two.
• Plant Cell Cytokinesis:


  Plant cells, due to their rigid cell walls, undergo cytokinesis differently:

  1. Vesicles containing cell wall and membrane materials move to the center of the cell.
  2. These vesicles fuse to form a structure called the cell plate.
  3. The cell plate expands outward until it reaches the existing cell wall, effectively dividing the cell in two.
• Key Differences:

  1. Mechanism: Animal cells use contraction, plant cells use construction.
  2. Structures involved: Animal cells use actin and myosin, plant cells use vesicles and the cell plate.
  3. Direction of division: Animal cells divide from the outside in, plant cells from the inside out.
• Importance of Cytokinesis:


  Cytokinesis ensures that:

  • Each daughter cell receives a complete set of organelles
  • The cytoplasmic contents are fairly distributed
  • The daughter cells are physically separated and can function independently
• Cytokinesis, the division of cytoplasm following nuclear division, can occur in two main ways: equal and unequal
• Equal Cytokinesis


  In most cases, cytokinesis results in two daughter cells of approximately equal size and cytoplasmic content.

  Key points:

  • Both daughter cells receive similar amounts of cytoplasm and organelles
  • This is the most common form of cytokinesis in somatic cells
  • Ensures each daughter cell has the resources needed for survival and function
• Budding in yeast:


  • A small bud forms on the parent cell and grows
  • The bud receives some cytoplasm and a nucleus
  • Results in a smaller daughter cell (bud) and larger parent cell
• Oogenesis in humans:


  • Results in a large ovum (egg cell) and smaller polar bodies
  • The ovum retains most of the cytoplasm and nutrients
  • Polar bodies eventually degenerate
• Unequal Cytokinesis


  In some cases, cytokinesis results in daughter cells of unequal size or cytoplasmic content.

  Key points:

  • One daughter cell may be significantly larger than the other
  • The distribution of cytoplasm and organelles is asymmetrical
  • Often serves a specific biological purpose
• Even in "equal" cytokinesis, the division may not be perfectly 50/50 due to the random distribution of some cellular components.
• Important Consideration

  Regardless of whether cytokinesis is equal or unequal, both daughter cells must receive:

  • At least one mitochondrion
  • Any other organelles that can only be made by dividing pre-existing structures (e.g., chloroplasts in plant cells)
• Nuclear Division Precedes Cell Division


  It's important to emphasize that nuclear division (karyokinesis) occurs before cell division (cytokinesis). This sequence is critical because:

  • It ensures each daughter cell receives a nucleus
  • It prevents the formation of anucleate cells (cells without a nucleus)
• Role of mitosis:


  1. Growth and development of multicellular organisms
  2. Tissue repair and regeneration
  3. Asexual reproduction in some organisms
• Mitosis: 


  Maintaining Chromosome Number and Genome
• Key features of Mitosis

  • Produces two genetically identical daughter cells
  • Maintains chromosome number and genome integrity
  • Results in diploid cells (2n) from a diploid parent cell
• Meiosis: 


  Halving Chromosome Number and Generating Genetic Diversity
• Role of meiosis:


  1. Production of gametes (sperm and egg cells in animals, spores in plants)
  2. Generation of genetic diversity
• Key features of Meiosis

  • Produces four genetically diverse daughter cells
  • Halves the chromosome number
  • Results in haploid cells (n) from a diploid parent cell
  • Involves crossing over and independent assortment, increasing genetic variation
• DNA Replication as a Prerequisite for Mitosis and Meiosis


  DNA replication is a crucial step that occurs before both mitosis and meiosis. This process ensures that each daughter cell receives a complete set of genetic information.
• Key points of DNA Replication as a Prerequisite for Mitosis and Meiosis:

  1. DNA replication occurs during the S (synthesis) phase of the cell cycle, which is part of interphase.
  2. Each chromosome is duplicated, resulting in two identical sister chromatids.
  3. Sister chromatids are held together by a protein structure called the centromere.
• Chromosome Structure After Replication:


  • Each chromosome consists of two elongated DNA molecules (chromatids)
  • These chromatids are genetically identical
  • They remain attached at the centromere until anaphase
• Importance of DNA Replication:


  • Ensures each daughter cell receives a complete set of genetic information
  • Allows for the equal distribution of genetic material during cell division
  • Maintains genetic stability across generations of cells