Phases of Cell Cycle

There are four distinct phases in the cell cycle - G₁ phase, S phase (synthesis), G₂ phase and M phase. The G₁, S phase and G₂ phase together are known as interphase. The M phase or the mitotic phase is of two processes, one where the chromosomes of the cell is divided between two sister cells and the other is cytokinesis where the cell's cytoplasm divides into half forming two distinct cells.

 

The cells that have stopped dividing temporarily or reversibly are said to be in the state of quiescence called G₀ phase. Progression from phase to another depends on the proper completion of the previous one. After the process of cell division, the daughter cells begin the interphase of the new cycle. The stages of interphase are not morphologically distinguishable, yet each phase has a distinct biochemical process that prepares the cell for initiation of cell division. 

G₀ Phase:

Sometimes the cells in the quiescent and senescent cells are referred to as post mitotic. The cells which are indivisible in  multicellular eukaryotes generally enter the quiescent G₀ state from G_1. They may remain in the quiescent state for long periods of time. This state can be for indefinite like in neurons and is very common in cells that are fully differentiated. Death of the cells in response to damage of DNA or degradation would make the progeny of the cells nonviable. Some cells like the cells of liver and kidney enter the G₀ phase semi-permanently. 

Interphase:

Earlier to to the cell division process, the cell needs to accumulate nutrients. During the interphase all the preparations are done. In interphase of a newly formed cell, a series of changes takes place in the cell and the nucleus, before it is capable of division. This phase is also known as intermitosis. Earlier this stage was known as resting stage because no remarkable activiyt realated to cell division takes place here. Interphase proceeds in a series of three stages, G₁,S, and G₂. Division of cell operates in a cycle, hence the interphase of the cycle is preceded by the previous cycle of M phase and cytokinesis. Interphase is also called the preparatory phase. In the interphase stage the division of nucleus and cytosol does not occur. The cell prepares for division. This is a stage between the end of mitosis and start of the next phase. Many events occur in this stage and most significant event that occurs is the replication of genetic material.   

G₁ Phase:

This is the first phase in the interphase. From the end of the previous M phase till the beginning of the DNA synthesis in the next cycle is called the G₁ phase, here G indicates gap. This phase is also called growth phase. In this phase the biosynthetic activities of the cell, which shows a considerable slow down during the M phase of resumes it activities at a high rate. In this phase there is a marked production of proteins by the use of 20 amino acids. Also enzymes that are required in S phase needed during DNA replication. The duration of the G₁ phase is highly variable, also among different cells of the same species. The G₁ phase is under the control of the p53 gene. 

S  phase:

The start of the S phase is when the DNA replication commences. When the phase completes all the chromosomes have been replicated. Each chromosome has two sister chromatids. During this phase, the amount of DNA in the cell is doubled but the ploidy of the cell remains unchanged. In this phase the synthesis is completed as soon as possible as the exposed base pairs are sensitive to external factors like drugs or mutagens. 

G₂ phase: 

It is again the gap phase which happens during the gap between the DNA synthesis and mitosis. During this phase the cell will continue to grow. The G₂ checkpoint mechanism controls to ensure that the cell is ready to enter the M (mitosis) phase and divides. 

Mitosis or M phase:

The M phase consists of karyokinesis - nuclear division. The M phase is of several distinct phases, known as 

Prophase,

Metaphase,

Anaphase,

Telophase,

Cytokinesis.

The process of mitosis takes place only in eukaryotes, the chromosomes in the nucleus of the cell into two identical nucleus. This stage is followed by cytokinesis. In cytokinesis the cell, nuclei, cytoplasm, organelles and cell membrane is divided into two equal shares. Mitosis and cytokinesis together make the mitotic (M) phase of the cell cycle. The mother cell divides into two daughter cells that are genetically identical to each other. Mitosis is seen only in eukaryotic cells, but it occurs in different ways in different species. The process of mitosis is a sequence of events divided into three stages - prophase, metaphase, anaphase and telophase. During this process of mitosis the chromosome pairs condense and they attach to fibres that pull sister chromatids to opposite sides of the cell. The cell with the process of cytokinesis produces two identical daughter cells.