Cells increase in number by cell division. The parent cell divides and passes on genetic material to the daughter cells. This genetic material (DNA) is found inside the nucleus. The most important part of cell division concerns events inside the nucleus. Cell Cycle The orderly sequence of events by which the cell duplicates its contents and divides into two is termed as cell cycle. The cell cycle comprises fundamentally two periods: (i) Interphase, and (ii) Mitosis.
Interphase is called ‘resting stage’, but it is in fact a period of great activity. Three important processes, which are preparatory to cell division, take place during interphase. These processes are: (i) Replication of DNA along with the synthesis of nuclear proteins such as the histones. (ii) In animal cells, duplication of a centriole takes place by the outgrowth of daughter centrioles from the parent centrioles, which are at right angle to each other. (iii) Synthesis of energy-rich compounds, which provide energy for mitosis, and synthesis of proteins at the end of interphase. The interphase can be divided into three periods:
1. G1 phase. This post mitotic gap phase takes place at the end of one cell division. RNA and protein are synthesised during this period, but there is no synthesis of DNA.
2. S phase. This period marks the synthetic activity of the cell before M-phase starts. During this phase, DNA is formed from nucleotides and the DNA content of the nucleus is doubled. The proteins associated with DNA in eukaryotic chromosomes are also synthesized during this stage.
3. G2 phase. During the pre-mitotic gap phase, synthesis of RNA and protein continues, but DNA synthesis stops. The centrioles replicate and microtubules start to construct the spindle.
The durations of the S phase, the G2 phase and mitosis is generally constant in most cell types. The length of G1 phase is generally variable. Cells that do not divide frequently have a longer G1 phase, whereas frequently dividing cells have a shorter phase.
In G1 phase, a cell may follow one of the three alternatives: (a) cell may continue on the cycle and divide, (b) the cell can permanently stop division and enter GO or quiescent stage, and (c) the cell cycle may be arrested at a definite point of G1 phase. The cell in the arrested condition is said to be in the GO state. Various phases of cell cycle are controlled by proteins cyclins and cyclin dependent kinases (CDKs).
When a eukaryotic cell entered the S phase and has begun DNA replication, it has generally committed itself to division. During interphase, replication of chromosomes takes place so that each chromosome now consists of two chromatids. Subsequently the cell enters into the mitosis (M) phase.
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