Cell cycle, phases of cell cycle.

 

Cell cycles

Cell undergoes series of changes involving phases of cell growth, replication of DNA, and cell division, these series of events are known as cell cycles.

Phases of cell cycle

 In eukaryote Cells the cell cycles comprises of two phases:

1) The period of cell growth and development, interphase (which includes G¹ phase, S phase and G° phase)

2) The period of cell division , the mitotic phase

The cell spend most of its time in above of the three phases.

In prokaryotes cells the cell cycles comprise of three phases.

• The growth phase, in which the prokaryotic cell grows and increased its mass and size.
• The chromosomal duplication phase, where DNA replication takes place.
• The duplicated chromosome now get separated and splitting of cell into two daughter cell.

 Interphase

It is the period of cell cycle between two consecutive divisions. It is the longest phase of cell cycle, as number of events occur during interphase. Interphase includes: G¹ phase, S phase, and G² phase. G° phase is the extension of interphase and sometime consider the part of it.

G¹ phase

G¹ phase is referred as the gap 1 phase, it is the phase of extensive metabolic activities that takes place in eukaryotes. G¹ phase is the intermediate, growth phase that occurred between the end of previous mitotic phase and the beginning of S phase. In this phase cell normally grows in size and volume, accumulate variety of necessary proteins, cell organelles, mRNAs, and prepared itself to enter into the next synthetic phase for DNA replications. The duration of all phase varies from cell to cell, in human’s somatic cell it last for 10 to 11 hours. Before entering into the next succeeding phase (S phase), there is proper checkpoint signaling mechanism that ensures the correct order of events in the cell and proper check of cellular environment to enter into the next phase. Checkpoints are the molecular complexes, G¹ checkpoint is called as restriction points (R), which ensure whether the cellular environment is ready and capable to enter into the next Synthetic phase for DNA replication, if not checkpoint restrict the cell until it get fully prepared or it may destined to  enter into the G° phase

However, in some cases the post-mitotic cell instead of entering into the S phase restrict to non-proliferating phase called G° phase.

G° phase

The post-mitotic cell exist in sort of dormant or resting state called G° phase. G° phase is the resting phase of cell where cell remain non-proliferated for days, weeks or even years in some cases, or for lifetime of cell as in the case of neurons cell. In G° phase either the cell remain in quiescent state or inactive state or it may undergo senescence mechanism (stopping the cellular division or replicating machinery as in neuron cell) or apoptosis (programmed cell death). In quiescent inactive state the cells are metabolically active, perform its function and cell maintenance but inactive to prepare the cell for division or replication. However, quiescent is reversible state and enabling the cell to reenter into the cell cycle after gap¹ (G¹) phase arrest. Hence G° phase is the extension of interphase and sometimes included the part of interphase. 

S phase

S phase is referred as the syntactic phase, during S phase the most crucial mechanism occur in the cell which is DNA replication, both the strand of DNA must replicated and new complimentary DNA strands are synthesized. Hence new copies of DNA is synthesized and chromosome duplication occur. Cell grow in size, because of extra copy of chromosome and number of proteins and enzymes are synthesized that continuing the process. Additionally the checkpoint at the end of the S phase detect the problem with the DNA replication, if there is any delay in the replication process, S phase checkpoint will triggered and prevent the cell to enter into the next gap 2 phase   

 

G² phase

G² phase is the premitotic phase, cell grows in size and prepare itself for the division. Many necessary events occur at G² phase, the cell store energy for chromosome movement, grow in size and many contractile protein, RNA, organelles, and microtubules subunit are synthesized. The final event in the G² phase is the formation of centrioles pair and their movement to the opposite pole (preprophase), microtubules appears radiating at the area of centrioles. The checkpoints at the G² phase ensure the quality control of entire DNA with in the chromosome, cell competence, and let the cell to enter into the next mitotic phase.

Mitotic phase

Mitotic phase in the cell cycle is the dividing phase, parent cell divided into two daughter cell with exact copy of chromosome, mitotic phase includes the further four phases i-e Prophase, Metaphase, Anaphase and telophase. Mitotic phase is also regulated by checkpoints signaling mechanism at various stages, the checkpoint at metaphase ensure the proper assembly of spindle fibers and their attachments to the kinetochores.

Cytokinesis

It is the last phase of the cell cycles, where cytoplasm of two dividing cells divides and split into two daughter cell. In animal cell it is divided by inward cell membrane cleavage formation while in plant cell it is divided by cell plate formation.

Figure 1: represents of phases of cell cycle; G¹ phase ,S-phase, G² phase, and meiotic phase.