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Cell Cycle

All cells arise from pre-existing cells by division. Every living cell today is said to be descended from a single ancestral cells that lived 3-4 billion years ago. In the vast time period, evolution of cells and organisms was seen, thus continuing the success of life on Earth. The genetic information has been preserved through cell division.

Reproduction of cells is fundamental to the development and function of all life. In single-celled organisms, cell division creates are a entire new organism. In multicellular organisms cell division transform a single founder cell into different communities of cells that constitute the tissues and organs. In adult organisms cell division replaces cells those die from natural causes or those that are lost to environmental changes.

 

What is Cell Cycle

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The cell cycle is the sequence of events that takes place in cells. It leads to cell division and replication (duplication). In prokaryotes, the cell cycle is through the process of binary fission. In eukaryotes, the cell cycle can be divided into two phases - interphase and mitotic phase. Interphase is the stage during which the cell prepares, grows and accumulates nutrients needed for mitosis and also duplicates the DNA.  In the mitotic phase, the cell splits itself into two distinct cells known as 'daughter cells' and the final phase is cytokinesis, where the newly formed cells are completely divided. The cycle of cell division is a vital process by which a fertilized egg that is single-celled develops into a mature organism. It is by this process by which skin, hair, blood cells and some internal organs are renewed. 

Phases of Cell Cycle

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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.

Cell Cycle
 
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 G1. 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. 

Cell Cycle Control System

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Regulation of the cell cycle is a crucial process to the survival of the cell. Cell regulation includes the detection and repair of genetic damage and also prevention of uncontrolled cell division. The molecular events that control the cell cycle occurs in a sequential fashion and is impossible to reverse the cycle. 

Cell Cycle Control System

The major event in the cell cycle is the replication of DNA, which occurs in the S phase, and separation of the duplicated chromosomes and the constituents of the cell which occurs in the M phase. Regulation and the initiation and completion of S and M phases ensures the genetic information and other cellular components are duplicated and divided equally between the daughter cells with each cycle.  

Cell cycle checkpoints are the regulatory pathways that control the order and the timing of the transitions of the cell cycle. The checkpoints also ensures that critical events such as replication of DNA and segregation of chromosomes are completed before the cell progresses further through the cycle. The cell-cycle checkpoints respond to the cellular damage by slowing the cycle to provide time for repair and it also induces transcription of genes that facilitate the repair. The loss of the checkpoints results in instability of chromosomes and it can result in the transformation of normal cells into cancer cells.
 
Proteins like the cyclin dpendent kinase, kinases and cyclins control the switches for the cell cycle causing the cell to move from G1 to S or G2 to M.Regulatory molecules are of two classes cyclins and cyclin-dependent kinases. The genes encoding cyclins and CDKs are conserved among all eukaryotes. Proteins like the p27 and p53 prevent the cells from passing check points. They are also knwon as protein suppressors. P27 protein binds to cyclin and CDK blocking the entry into the S phase. P53 protein blocks the cell cycle at the M checkpoint if the DNA is damaged. P53 mutation is the most frequent mutation found in cancer cells. P53 fucntions by blocking the cycle giving the cell time to repair its DNA. If there is severe damage in the DNA the protein causes the cell to apoptosis.
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