Cell Cycle


 
 
Concept Explanation
 

Cell Cycle

Cell division is an essential process in all living organisms. The mode of cell division is fundamentally similar in all organisms, During the process of division a cell, the processes like DNA replication and cell growth must take place in a sequential and coordinated manner to ensure the correct division and formation of progeny cells with intact genomes.

Cell cycle is an orderly sequence of events or a set of stages by which a cell duplicates its genome, synthesize the other constituents (important for the cell) of the cell and eventually divides into two daughter cells.

In terms of cytoplasmic increase, cell growth is a continuous process, DNA synthesis occurs during a specific stage of the cell cycle only. The DNA (chromosomes) are further distributed to daughter nuclei by a series of events. All these events are well-coordinated and are under genetic control.

Phases of Cell Cycle : 

A typical eukaryotic cell (human cell) divides once in every 24 hr. This duration of cell cycle can vary from an organism to other organism and also from one cell type to another. e.g., yeast cell progresses through the cell cycle and divides in only about 90 min.

i) Interphase: It is the period between the end of one cell division to the beginning of the next cell division, i.e., (between two successive M-phase).

During this phase, cell prepares itself for both cell growth and DNA replication in an orderly manner. So, it is also known as preparation phase. It lasts for about 90-96%, i.e., more than 95% of the total duration of cell cycle. In a cell cycle of human cell, only about one hour is taken by the dividing phase out of 24 hours duration of one cell cycle.

Interphase is further divided into following three sub stages on the basis of various synthetic activities.

(a) G_1 (Gap - 1) - Phase                     (b) S(Synthesis) - phase

(c) G_2 (Gap - 2) - Phase                      G_Q - phase (Quiescent Stage)

ii) M - Phase : - Following the interphase, the cell enters the  M - Phase or mitotic  phase.

MITOSIS : - In this type of division, the chromosomes replicate themselves and get equally distributed into daughter nuclei, i.e., the chromosome number in the parental and progeny cell (diploid) becomes the same. Therefore, it is also known as equational division.

Mitosis is also known as somatic cell division because it always occur in somatic cells. Mitotic cell division is seen in the diploid somatic cells in animals, whereas, in plants, mitotic division is seen in both haploid and diploid cells. It is known to be the phase of actual cell division, which starts with the division of nucleus, followed by the separation of daughter chromosomes, i.e., karyokinesis and terminates with the cytoplasmic division, i.e., cytokinesis.

                        

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