Cell cycle: an overview

Cell cycle is the name by which two new cells are replicated and produced. The cell cycle contains several phases known as G1, S, G2, and M. G1 is the stage the cell is about to split. It next progresses to the S phase, which duplicates the whole DNA in the cell. Thus, S is the synthesis of DNA. After the DNA is copied and the whole of genetic material is additional, the cell enters the G2 phase, arranges and condenses the genetic material or begins to condense and split the genetic material. M is the next phase. M is synonymous with mitosis. A cell cycle is a set of occurrences in a cell that grows and splits. A cell spends most of its time in what is called the interphase and it grows, replicates and prepares for cell division. It also grows at this period. The cell subsequently passes out from interphase to mitosis and finishes its division. The resulting cells, called daughter cells, enter their own interphases and start a new cycle round.

Cancer is characterized by unregulated and rapid proliferation of cells resulting from aberrant activity of various cell cycle proteins which makes cell cycle regulators attractive targets in cancer therapy.

Eukaryotic cell cycle:

The division of most eukaryotic cells consists of four coordinated processes: cell growth, DNA replication, distribution of the duplicated chromosomes to daughter cells and finally cell division. The cell cycle in eukaryotes is more complex as compared to prokaryotes, although the cell growth is usually a continuous process, DNA is only synthesized during one phase of the cell cycle and then replicated chromosomes are distributed to daughter nuclei via complex series of events before the actual cell division takes place. Progression between stages of the cell cycle is controlled by a regulatory apparatus, which not only coordinates the different events of the cell cycle but also links the cell cycle with extracellular signals that control cell proliferation.

Phases of the cell cycle: The eukaryotic cell cycle is divided into two basic parts that are mitosis and interphase. Mitosis corresponds to the separation of daughter chromosomes and usually ends with cell division i.e. cytokinesis. A typical eukaryotic cell cycle takes approximately 24 hours. But mitosis and cytokinesis last only for about an hour, which means that, approximately 95% of the cell cycle is spent in interphase. During interphase, chromosomes are distributed throughout the nucleus in order to make the nucleus appear morphologically uniform. However, at a molecular level, interphase is the time during which both DNA replication and cell growth occur in an orderly manner in preparation for cell division. As mentioned earlier, DNA is only synthesized during only a portion of interphase. Hence, the timing of DNA synthesis divides the eukaryotic cell cycle into four discrete phases. The M phase of the cell cycle corresponds to mitosis which is usually followed by cytokinesis. The M phase is followed by the G1 phase which corresponds to the interval or gap between mitosis and initiation of DNA replication. During this phase, the cell is metabolically active and grows continuously but does not replicate its DNA. G1 phase is followed by S phase during which DNA replication takes place. This phase is followed by the G2 phase during which cell growth continues and proteins are synthesized in preparation for mitosis. The duration for which cell cycle phases varies significantly in different types of cells. For a human cell, the G1 phase might last about 11 hours, S phase about 8 hours, G2 phase about 4 hours and M phase about 1 hour, which adds up to 24 hours

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