Here is your short essay on cell cycle

The most remarkable feature of cells and entire organism is their ability reproduces. The cells of an organism can grow only up to a limited extent and beyond and therefore, growth can be accomplished only by cell division.

Every organism starts its life from a single cell which divides and re divides to form tissues and org the larger the organisms the more are the number of cells they have.

There is no such thing that the bigger animals or plants shall have large cells and the smaller ones s cells. The size of the cell is genetic character that seldom changes.

The cell after attaining a certain size can duplicate itself. The simplest type of cell reproduction entails the division of a “parent” cell into two “daughter” cells. This occurs as part of the cell cycle, a series of events that prepares a cell to divide followed by the actual division process, called mitosis.

The eukaryotic cell cycle has at least four stages. The chromosomes and the DNA they carry are copied during S (Synthesis) phase. The M Phase is marked by changes at the chromosome level, which is visible under microscope. Both the M and S phases are preceded by two gap stages, the G₁ phase and the G₂ phase respectively.

During these gap stages, m- RNA and proteins are made in sufficient quantity to be used to produce two equal halves with complete chromosomal set-up in each.

MITOSIS (M Phase)

While changes during interphone are biochemical and not visible, the M-Phase is clearly visible under a microscope. This phase is divisible into Prophase, Metaphase, Anaphase and Telophase, each phase being characterized by the morphological changes associated with the chromosomes.

Prophase

It is the longest phase of division. In the early part, the diffused chromatin of inter phase slowly condenses to form well-defined chromosomes. Later, the chromosomes are spitted longitudinally to form two chromatids each. Both the chromatids are, however, associated at the centromere region.

Cytoplasm microtubules are organ to form mitotic spindle near the poles. In the final stage the nuclear membrane a nucleolus disappear and chromosomes are found in the cytoplasm. In plants due to t complete absence of centriole, no astral arrangement is seen and hence the spindlet called anastral spindle.

In animals spindle formation is dependent on centrioles, where each ‘centriole pair’ at the pole acts as mitotic center. Short microtubules (astral rays) radiate from the centrioles and so the spindle is called astral spindle (aster means star).

Metaphase

This phase begins with the complete disappearance of the nuclear membrane. The chromosomes get themselves arranged at the central part of the cell or the equatorial portion of the spindle called as the equatorial plate or metaphasic plate.

The orientation of the chromosomes is such that their centromeres lie in the equatorial plate while the arms are projected freely in the cytoplasm.

Anaphase

In this phase, the centromere holding the chromatid pairs gets divided simultaneously. Now the chromatids are called daughter chromosomes.

Soon the daughter chromosomes repel from each other and start moving in opposite direction towards the respective poles. The spindle fibers help the movement of the chromosomes

The spindle fibers are of three types viz. pole to pole, pole to centromere and centromere to centromere. Together by contraction and expansion of spindle fibers of pole to centromere and centromere to centromere respectively, the chromosomal movement is facilitated. During the movement, the chromosome may assume that shape of V, J or I depending upon the chromosome structure.

Telophase

This phase starts after the full set of chromosomes reach the opposite poles. The spindle fibers disappear and nuclear membranes are formed around the chromosome sets. The nucleolus reappears in each nucleus forming two daughter nuclei in the cell,

At the end of M-phase, cytoplasm of the parent cell is divided into two halves by the formation of a cell-plate in the middle in case of plant cells and simply by inward constrictions on either side in case of animal cells. Some authors also call this division of cytoplasm as D-phase but the process is called this cytokinesis.