Cell Division: Stages, Cytokinesis, and Mitosis

Cell Cycle and Division

The cell cycle includes the period from when a cell is formed until it divides, resulting in new cells. It has two main stages: Interphase (initial, long duration) and Division (final, short, resulting in two daughter cells).

1. Interphase: This stage includes DNA duplication and consists of three phases: G1, S, and G2. During interphase, the cell nucleus is called the interphase nucleus.

• G1 Phase: mRNA and protein synthesis occurs. The cell has one diplosome (two centrioles). At the end of G1, there is a restriction point (R point), after which the cell is committed to completing S, G2, and M phases.
• S Phase: DNA replication occurs once the cell has reached sufficient size, synthesized essential proteins, and has enough ATP. DNA, associated with proteins called histones, forms chromatin. It must be unwound for replication to occur.
• G2 Phase: Metabolic activity continues, with the synthesis of proteins essential for cell division. Structures involved in cell division are assembled, providing extra time for cell growth.

2. Division (M Phase) - Mitosis:

Mitosis distributes duplicated chromosomes so each new cell gets a full complement. This depends on chromosome condensation and the assembly of microtubules called the spindle.

Stages of Mitosis

a) Interphase: Chromatin is duplicated but not yet condensed. Two pairs of centrioles are located near the nuclear envelope.

b) Prophase: Centrioles move to opposite poles. Condensed chromosomes become visible. The nuclear envelope breaks down, and the mitotic spindle begins to form.

c) Early Metaphase: Spindle fibers pull each pair of chromatids to opposite sides.

d) Late Metaphase: Chromatid pairs align at the cell's equatorial plane.

e) Anaphase: Sister chromatids separate and are pulled to opposite poles.

f) Telophase: A nuclear envelope forms around each set of chromosomes. Chromosomes decondense, appearing fuzzy. Nucleoli reappear. The mitotic spindle disorganizes and the plasma membrane begins to divide.

Cytokinesis

Cytokinesis is the division of the cytoplasm, differing significantly in plant and animal cells.

Cytokinesis in Animal Cells

During early telophase, a membrane begins to constrict around the cell's circumference at the equatorial plane. This constriction is caused by a contractile ring of actin and myosin filaments attached to the cell membrane's cytoplasmic face, which pinches the cell into two daughter cells.

Cytokinesis in Plant Cells

Vesicles produced by the Golgi complex, containing polysaccharides, migrate to the equatorial plane, guided by remnants of the mitotic spindle microtubules. These vesicles merge to form a flat, membrane-bound structure called the cell plate. As more vesicles fuse, the cell plate grows, eventually fusing with the cell membrane and forming a layer of polysaccharides between the daughter cells, completing the separation. This layer is impregnated with pectin, and each new cell constructs its own cell wall, depositing cellulose and other polysaccharides on the outer surface of the cell membrane.

When cell division is complete, there are two daughter cells, smaller than the parent cell but otherwise indistinguishable.

Plane of Cell Division

The plane of cell division is established in late G2 phase when the microtubule cytoskeleton reorganizes into a circular structure called the preprophase band just inside the cell wall. This band disappears at prophase but determines the future location of the equatorial plane and cell plate. The band's microtubules reassemble into the spindle during prophase. In cytokinesis, the cell plate spreads until it reaches the region previously occupied by the preprophase band. Vesicles forming the cell plate are guided by spindle fibers between the daughter nuclei.