Understanding Cell Division: Mitosis and Cytokinesis
Cell Division Stages
This process involves two stages: the division of the nucleus (mitosis or karyokinesis) and the division of the cytoplasm (cytokinesis or cytodieresis).
Nuclear Division: Mitosis or Karyokinesis (Equational Division)
Prophase
Most animal cells have a centrosome surrounded by dense pericentriolar material. At the beginning of prophase, a second centrosome forms next to the first one. The pericentriolar material generates microtubules, forming the aster. The two asters move to opposite poles of the cell. As cells prepare for division, they become more rounded as the cytoskeleton is disrupted. Proteins form a structure of microtubules called the spindle. These microtubules cluster, forming polar fibers, which elongate and separate the two asters. In plants, without centrioles, a spindle forms without asters, called an anastral spindle. Inside the nucleus, chromatin condenses, becoming more pronounced in certain regions (chromomeres). Two sister chromatids become visible, joined by the centromere. The nucleolus disappears, and kinetochores form.
Prometaphase (Premetaphase, Late Prophase)
The nucleus swells as water enters, and the nuclear envelope disintegrates. The nucleoplasm disperses into the cytoplasm. Chromosomes continue to condense and kinetochores form, acting as microtubule organizing centers. Kinetochore microtubules form and cluster into kinetochore fibers.
Biological Significance of Mitosis
Genetic Level: Mitosis ensures the equal distribution of genetic information.
Cellular Level: Mitosis allows for cell line perpetuation and colony formation.
Organismal Level: Mitosis enables tissue and organ growth, development, repair, and regeneration in multicellular organisms.
Metaphase
Due to the elongation of kinetochore microtubules, chromosomes reach their maximum condensation and align at the spindle equator, forming the equatorial plate. Each chromatid faces a spindle pole.
Anaphase
Polar microtubules elongate, causing the spindle to lengthen. Kinetochore microtubules shorten, separating chromatids, which migrate to opposite spindle poles. Each chromatid becomes a chromosome (anaphase chromosome). Anaphase ends when the anaphase chromosomes reach the poles and kinetochores disappear.
Telophase
Chromosomes are surrounded by fragments of the endoplasmic reticulum and the nuclear envelope, which fuse to form a new nuclear envelope. The nuclear lamina reforms. Chromosomes decondense, and genes are transcribed. The nucleolus reappears.
Cytoplasmic Division or Cytokinesis
Cytokinesis in Animal Cells
Cytokinesis begins with the plasma membrane invaginating at the spindle equator. A contractile ring of actin forms, causing the cell to constrict and divide.
Cytokinesis in Plant Cells
Vesicles from the Golgi apparatus, rich in pectic substances, gather at the equatorial plane, forming a cell plate (fragmoplasto). The cell plate grows from the center to the periphery, dividing the cell. Cytoplasmic division is incomplete, leaving plasmodesmata (cytoplasmic connections) between the daughter cells.
Note: In some fungi, mitosis is not followed by cytokinesis, resulting in plasmodia (multinucleated cells).