Cell cycle

The cell cycle, or cell-division cycle, is the sequential series of events that take place in a cell that causes it to divide into two daughter cells. These events include the growth of the cell, duplication of its DNA and some of its organelles, and subsequently the partitioning of its cytoplasm, chromosomes and other components into two daughter cells in a process called cell division.
In eukaryotic cells including animal, plant, fungal, and protist cells, the cell cycle is divided into two main stages: interphase, and the M phase that includes mitosis and cytokinesis. During interphase, the cell grows, accumulating nutrients needed for mitosis, and replicates its DNA and some of its organelles. During the M phase, the replicated chromosomes, organelles, and cytoplasm separate into two new daughter cells. To ensure the proper replication of cellular components and division, there are control mechanisms known as cell cycle checkpoints after each of the key steps of the cycle that determine if the cell can progress to the next phase.
In cells without nuclei the prokaryotes, bacteria and archaea, the cell cycle is divided into the B, C, and D periods. The B period extends from the end of cell division to the beginning of DNA replication. DNA replication occurs during the C period. The D period refers to the stage between the end of DNA replication and the splitting of the bacterial cell into two daughter cells.
In single-celled organisms, a single cell-division cycle is how the organism reproduces to ensure its survival. In multicellular organisms such as plants and animals, a series of cell-division cycles is how the organism develops from a single-celled fertilized egg into a mature organism, and is also the process by which hair, skin, blood cells, and some internal organs are regenerated and healed. After cell division, each of the daughter cells begin the interphase of a new cell cycle. Although the various stages of interphase are not usually morphologically distinguishable, each phase of the cell cycle has a distinct set of specialized biochemical processes that prepare the cell for initiation of the cell division.

Phases

The eukaryotic cell cycle consists of four distinct phases: G₁ phase, S phase, G₂ phase and M phase. M phase is itself composed of two tightly coupled processes: mitosis, in which the cell's nucleus divides, and cytokinesis, in which the cell's cytoplasm and cell membrane divides forming two daughter cells. Activation of each phase is dependent on the proper progression and completion of the previous one. Cells that have temporarily or reversibly stopped dividing are said to have entered a state of quiescence known as G₀ phase or resting phase.
Image:Cell Cycle 2-2.svg|thumb|300px|Schematic of the cell cycle. Outer ring: I = Interphase, M = Mitosis; inner ring: M = Mitosis, G₁ = Gap 1, G₂ = Gap 2, S = Synthesis; not in ring: G₀ = Gap 0/Resting

State	Phase	Abbreviation	Description
Resting	Gap 0	G₀	A phase where the cell has left the cycle and has stopped dividing.
Interphase	Gap 1	G₁	Cell growth. The G₁ checkpoint ensures that everything is ready for DNA synthesis.
Interphase	Synthesis	S	DNA replication.
Interphase	Gap 2	G₂	Growth and preparation for mitosis. The G₂ checkpoint ensures that everything is ready to enter the M phase and divide.
Cell division	Mitosis	M	Cell division occurs. The Metaphase Checkpoint ensures that the cell is ready to complete cell division.

G₀ phase (quiescence)

G₀ is a resting phase where the cell has left the cycle and has stopped dividing. The cell cycle starts with this phase. Non-proliferative cells in multicellular eukaryotes generally enter the quiescent G₀ state from G₁ and may remain quiescent for long periods of time, possibly indefinitely. This is very common for cells that are fully differentiated. Some cells enter the G₀ phase semi-permanently and are considered post-mitotic, e.g., some liver, kidney, and stomach cells. Many cells do not enter G₀ and continue to divide throughout an organism's life, e.g., epithelial cells.
The word "post-mitotic" is sometimes used to refer to both quiescent and senescent cells. Cellular senescence occurs in response to DNA damage and external stress and usually constitutes an arrest in G₁. Cellular senescence may make a cell's progeny nonviable; it is often a biochemical alternative to the self-destruction of such a damaged cell by apoptosis.

Interphase

Interphase represents the phase between two successive M phases. Interphase is a series of changes that takes place in a newly formed cell and its nucleus before it becomes capable of division again. It is also called preparatory phase or intermitosis. Typically interphase lasts for at least 91% of the total time required for the cell cycle.
Interphase proceeds in three stages, G₁, S, and G₂, followed by the cycle of mitosis and cytokinesis. The cell's nuclear DNA contents are duplicated during S phase.

G₁ phase (First growth phase or Post mitotic gap phase)

The first phase within interphase, from the end of the previous M phase until the beginning of DNA synthesis, is called G₁. It is also called the growth phase. During this phase, the biosynthetic activities of the cell, which are considerably slowed down during M phase, resume at a high rate. The duration of G₁ is highly variable, even among different cells of the same species. In this phase, the cell increases its supply of proteins, increases the number of organelles, and grows in size. In G₁ phase, a cell has three options.

To continue cell cycle and enter S phase
Stop cell cycle and enter G₀ phase for undergoing differentiation.
Become arrested in G₁ phase hence it may enter G₀ phase or re-enter cell cycle.

The deciding point is called check point. This check point is called the restriction point or START and is regulated by G₁/S cyclins, which cause transition from G₁ to S phase. Passage through the G₁ check point commits the cell to division.

S phase (DNA replication)

The ensuing S phase starts when DNA synthesis commences; when it is complete, all of the chromosomes have been replicated, i.e., each chromosome consists of two sister chromatids. Thus, during this phase, the amount of DNA in the cell has doubled, though the ploidy and number of chromosomes are unchanged. Rates of RNA transcription and protein synthesis are very low during this phase. An exception to this is histone production, most of which occurs during the S phase.

G₂ phase (growth)

G₂ phase occurs after DNA replication and is a period of protein synthesis and rapid cell growth to prepare the cell for mitosis. During this phase microtubules begin to reorganize to form a spindle. Before proceeding to mitotic phase, cells must be checked at the G₂ checkpoint for any DNA damage within the chromosomes. The G₂ checkpoint is mainly regulated by the tumor protein p53. If the DNA is damaged, p53 will either repair the DNA or trigger the apoptosis of the cell. If p53 is dysfunctional or mutated, cells with damaged DNA may continue through the cell cycle, leading to the development of cancer.

Mitotic phase (chromosome separation)

The relatively brief M phase consists of nuclear division and division of cytoplasm. M phase is complex and highly regulated. The sequence of events is divided into phases, corresponding to the completion of one set of activities and the start of the next. These phases are sequentially known as:

prophase
prometaphase
metaphase
anaphase
telophase

Image:Mitosis Stages.svg|center|900px|A diagram of the mitotic phases
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two nuclei. During the process of mitosis the pairs of chromosomes condense and attach to microtubules that pull the sister chromatids to opposite sides of the cell.
Mitosis occurs exclusively in eukaryotic cells, but occurs in different ways in different species. For example, animal cells undergo an "open" mitosis, where the nuclear envelope breaks down before the chromosomes separate, while fungi such as Aspergillus nidulans and Saccharomyces cerevisiae undergo a "closed" mitosis, where chromosomes divide within an intact cell nucleus.

Cytokinesis phase (separation of all cell components)

Mitosis is immediately followed by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly equal shares of these cellular components. Cytokinesis occurs differently in plant and animal cells. While the cell membrane forms a groove that gradually deepens to separate the cytoplasm in animal cells, a cell plate is formed to separate it in plant cells. The position of the cell plate is determined by the position of a preprophase band of microtubules and actin filaments. Mitosis and cytokinesis together define the division of the parent cell into two daughter cells, genetically identical to each other and to their parent cell. This accounts for approximately 10% of the cell cycle.
Because cytokinesis usually occurs in conjunction with mitosis, "mitosis" is often used interchangeably with "M phase". However, there are many cells where mitosis and cytokinesis occur separately, forming single cells with multiple nuclei in a process called endoreplication. This occurs most notably among the fungi and slime molds, but is found in various groups. Even in animals, cytokinesis and mitosis may occur independently, for instance during certain stages of fruit fly embryonic development. Errors in mitosis can result in cell death through apoptosis or cause mutations that may lead to cancer.