Cell cycle

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The cell cycle, or cell-division cycle (CDC), is the series of events in a eukaryotic cell between one cell division and the next. Thus, it is the process by which a single-cell fertilized egg develops into a mature organism and the process by which hair, skin, blood cells, and some internal organs are renewed. A specialized form of cell division is responsible for cellular differentiation during embryogenesis and morphogenesis, as well as for the maintenance of stem cells during adult life.

The cell cycle consists of four distinct phases: G₁ phase, S phase, G₂ phase (collectively known as interphase) and M phase. M phase is itself composed of two tightly coupled processes: mitosis, in which the cell's chromosomes are divided between the two daughter cells, and cytokinesis, in which the cell's cytoplasm physically divides. Cells that have temporarily or reversibly stopped dividing are said to have entered a state of quiescence called G₀ phase, while cells that have permanently stopped dividing due to age or accumulated DNA damage are said to be senescent. Some cell types in mature organisms, such as parenchymal cells of the liver and kidney, enter the G₀ phase semi-permanently and can only be induced to begin dividing again under very specific circumstances; other types, such as epithelial cells, continue to divide throughout an organism's life.

The molecular events that control the cell cycle are ordered and directional; that is, each process occurs in a sequential fashion and it is impossible to "reverse" the cycle. There are two key classes of regulatory molecules that determine a cell's progress through the cell cycle: cyclins and cyclin-dependent kinases. Leland H. Hartwell, R. Timothy Hunt, and Paul M. Nurse won the 2001 Nobel Prize in Physiology or Medicine for their discovery of these central molecules in the regulation of the cell cycle.

1 Phases of the cell cycle
2 Cyclins and cyclin-dependent kinases
3 Checkpoints
4 Synchronisation of cell cultures
5 Observation
6 References
7 External links

[edit] Phases of the cell cycle

Schematic of the cell cycle. outer ring: I=Interphase, M=Metaphase; inner ring: M=Mitosis. The duration of mitosis in relation to the other phases has been exaggerated in this diagram

The cell cycle of a typical eukaryotic cell has four phases. The relatively brief M phase consists of nuclear division (mitosis) and cytoplasmic division (cytokinesis). After M phase, the daughter cells each begin interphase of a new 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 cell division. The first phase within interphase is called G₁ (G indicating gap); during this phase the biosynthetic activities of the cell, which had been considerably slowed down during M phase, resume at a high rate. The ensuing S phase starts when DNA synthesis commences; when it is complete, all of the chromosomes have been replicated. The cell then enters the G₂ phase, which lasts until the cell enters the next round of mitosis. Metabolic activity, cell growth, and cell differentiation all occur during interphase.

The term "post-mitotic" is sometimes used to refer to both quiescent and senescent cells. Nonproliferative cells in multicellular eukaryotes generally enter the quiescent G₀ state from G₁ and may remain quiescent for long periods of time, possibly indefinitely (as is often the case for neurons). This is very common for cells that are fully differentiated. Cellular senescence is a state that occurs in response to DNA damage or degradation that would make a cell's progeny nonviable; it is often a biochemical alternative to the self-destruction of such a damaged cell by apoptosis.

[edit] Cyclins and cyclin-dependent kinases

Cyclins and cyclin-dependent kinases (CDKs) are the two critical classes of molecules in regulation of cell cycle progression. Cyclins form the regulatory subunits and CDKs the catalytic subunits of an activated heterodimer; cyclins have no catalytic activity and CDKs are inactive in the absence of a partner cyclin. When activated by a bound cyclin, CDKs perform a common biochemical reaction called phosphorylation that activates or inactivates target proteins to orchestrate coordinated entry into the next phase of the cell cycle. Different cyclin-CDK combinations determine the downstream proteins targeted.

Many of the genes encoding cyclins and CDKs are conserved among all eukaryotes, but in general more complex organisms have more elaborate cell cycle control systems that incorporate more individual components. Many of the relevant genes were first identified studying yeast, especially Saccharomyces cerevisiae; genetic nomenclature in yeast dubs many of these genes cdc (for "cell division cycle") followed by an identifying number, e.g., cdc25. In the following discussion generic names such as "S cyclin" will be used to maintain generality, with the understanding that this may refer to one or to several homologous molecules in any given organism, and that some organisms may combine multiple functions in one molecule.

Upon receiving a pro-mitotic extracellular signal, G₁ cyclin-CDK complexes become active to prepare the cell for S phase, promoting the expression of transcription factors that in turn promote the expression of S cyclins and of enzymes required for DNA replication. The G₁ cyclin-CDK complexes also promote the degradation of molecules that function as S phase inhibitors by targeting them for ubiquitination. Once a protein has been ubiquitinated, it is targeted for proteolytic degradation by the proteasome. Active S cyclin-CDK complexes phosphorylate proteins that make up the pre-replication complexes assembled during G₁ phase on DNA replication origins. The phosphorylation serves two purposes: to activate each already-assembled pre-replication complex, and to prevent new complexes from forming. This ensures that every portion of the cell's genome will be replicated once and only once. The reason for prevention of gaps in replication is fairly clear, because daughter cells that are missing all or part of crucial genes will die. However, for reasons related to gene copy number effects, possession of extra copies of certain genes would also prove deleterious to the daughter cells.

Mitotic cyclin-CDK complexes, which are synthesized but inactivated during S and G₂ phases, promote the initiation of mitosis by stimulating downstream proteins involved in chromosome condensation and mitotic spindle assembly. A critical complex activated during this process is a ubiquitin ligase known as the anaphase-promoting complex (APC), which promotes degradation of structural proteins associated with the chromosomal kinetochore. APC also targets the mitotic cyclins for degradation, ensuring that telophase and cytokinesis can proceed.

[edit] Checkpoints

Checkpoints are used by the cell to monitor and regulate the progress of the cell cycle. If a cell fails to meet the requirements of a phase it will not be allowed to proceed to the next phase until the requirements have been met. Several checkpoints are designed to ensure that damaged or incomplete DNA is not passed on to daughter cells. At the end of the G1 phase, G2 phase and after DNA has been replicated in the S phase it is checked for damages. At the end of the M phase a checkpoint is present to stop cytokinesis in case the chromosomes are not properly aligned on the mitotic spindle.

[edit] Synchronisation of cell cultures

Several methods can be used to synchronise cell cultures

Serum starvation - G1 phase
Mitotic shake-off - M phase
Treatment with colchicine - M phase
Treatment with 5-fluorodeoxyuridine - S phase
FACS

[edit] Observation

There are numerous ways to observe the cell cycle occurring. Normally onion bulbs or garlic root tips are used.

A sample of root tip is fixed in a mixture of 99% of 70% aqueous industrial methylated spirit and 1% glacial ethanoic acid for two hours. Treat the root tips in 1M hydrochrolic acid at 60C for 6 -7 minutes. Rinse thoroughly with water. Add Schiff's reagent and leave for one hour. Rinse again in distilled water. Observe under a microscope.

[edit] References

Morgan DO. (2007) The Cell Cycle: Principles of Control. New Science Press: London.
Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P. (2003). Molecular Biology of the Cell. Ch 17. Garland Science: New York. 4th ed.
Lodish H, Berk A, Matsudaira P, Kaiser CA, Krieger M, Scott MP, Zipursky SL, Darnell J. (2004). Molecular Cell Biology. WH Freeman: New York, NY. 5th ed.
Watson JD, Baker TA, Bell SP, Gann A, Levine M, Losick R. (2004). Molecular Biology of the Gene, Ch. 7. Peason Benjamin Cummings; CSHL Press. 5th ed.