Cell cycle checkpoint
3730562
213933506
2008-05-21T12:24:33Z
84.2.92.4
/* G2 Checkpoint */
'''Cell cycle checkpoints''' are control mechanisms that ensure the fidelity of cell division in [[eukaryotic]] [[cell (biology)|cells]]. These checkpoints verify whether the processes at each phase of the cell cycle have been accurately completed before progression into the next phase. Multiple checkpoints have been identified, but some of them are less well understood than others.
An important function of many checkpoints is to assess [[DNA damage]], which is detected by '''sensor''' mechanisms. When damage is found, the checkpoint uses a '''signal''' mechanism to either stall the cell cycle until repairs are made or, if repairs can't be made, to target the cell for destruction via apoptosis ('''effector''' mechanism). All the checkpoints that assess DNA damage appear to utilize the same '''sensor-signal-effector''' mechanism.
The cell cycle according to Temple and Raff, 1986, was meant to function as a clock, but if this was the case it would be expected that the stages of the cell cycle must function according to some sort of internal clock, that will determine how long a phase should take. Contradictorily, the cell cycle is now depicted like the falling dominoes, the preceding phase has got to "fall" before the next phase can take place. The cell cycle checkpoints are therefore made up of composites of protein kinases and adaptor proteins which all play salient roles in the maintenance of the integrity of the division.
Checkpoints are now accepted to exist at every single point in the cell cycle{{Fact|date=December 2007}}. The DNA damage checkpoint is always active. Nonetheless, most human cells for example are terminally differentiated and must exit the cell cycle. There is a phase late in [[G1 phase|G<sub>1</sub> phase]] called the restriction point (RP, or the restriction checkpoint); cells that should cease division exit the cell cycle and enter [[G0 phase|G<sub>0</sub>]]. The only cells that continually divide in the adult human include [[hematopoietic stem cell]]s and gut endothelial cells. The re-entrant into the cell cycle is therefore only possible by overcoming the RP. This is achieved by growth factor induced expression of cyclin D proteins. These then overcome the G<sub>0</sub> barrier and are able to enter the cell cycle.
The main checkpoints which control the cell division cycle in eukaryotes include:
== G1 (Restriction) Checkpoint ==
The first checkpoint is located at the end of the cell cycle's G<sub>1</sub> phase, just before entry into [[S phase]], making the key decision of whether the cell should divide, delay division, or enter a resting stage. Most cells stop at this stage and enter a resting state called G<sub>0</sub>. [[Liver]] cells, for instance, only enter mitosis around once or twice a year {{Fact|date=December 2007}}. The G<sub>1</sub> checkpoint is where eukaryotes typically arrest the cell cycle if environmental conditions make cell division impossible or if the cell passes into [[G0 phase|G<sub>0</sub>]] for an extended period. In animal cells, the G<sub>1</sub> phase checkpoint is called the [[restriction point]], and in [[yeast]] cells it is called the start point. The restriction point is mainly controlled by action of the CKI- p16. This protein inhibits the [[CDK4]]/6 and ensures that it can no longer interact with cyclin D1 to cause the cell cycle progression. In growth induced or oncogenic induced [[cyclin D]] expression, this checkpoint is overcome because the increased expression of cyclin D allows its interaction with CDK4/6 by competing for binding. Once active CDK4/6-CYCLIN D complexes form, they phosphorylate the tumour suppressor retinoblastoma (Rb), this relieves the inhibition of the transcription factor [[E2F]]. E2F is then able to cause expression of [[cyclin E]], which then interacts with [[Cyclin-dependent kinase 2|CDK2]] to allow for G1-S phase transition. That brings us to the end of the first checkpoint which allows the G0-G1-S-phase transition.
==G2 Checkpoint ==
The second checkpoint is located at the end of G<sub>2</sub> phase, triggering the start of the [[M phase]] ([[mitosis]]). In order for this checkpoint to be passed, the cell has to check a number of factors to ensure the cell is ready for mitosis. If this checkpoint is passed, the cell initiates the many [[molecule|molecular]] processes that signal the beginning of mitosis. The CDK's associated with this checkpoint are activated by phosphorylation of the CDK by the action of a "[[Maturation promoting factor]]" (or Mitosis Promoting Factor, MPF). The MPF activates the CDK in response to environmental conditions being right for the cell and allows the cell to begin DNA replication.
The molecular nature of this checkpoint involves an activating phosphatase, known as [[Cdc25]], which under favourable conditions removes the inhibitory phosphates present within the MPF complex. However, DNA is frequently damaged prior to mitosis, and to prevent transmission of this damage to daughter cells, the cell cycle is arrested via inactivation of the Cdc25 phosphatase (via phosphorylation with other protein kinases).
== Anaphase Checkpoint ==
The [[mitotic spindle]] checkpoint occurs at the point in [[metaphase]] where all the chromosomes have/should have aligned at the mitotic plate and be under bipolar tension. The tension created by this bipolar attachment is what is sensed and this is what initiates the anaphase entry. This sensing mechanism, allows the degradation of cyclin B which harbours a D-box(destruction box). Degradation of cyclin B, ensures it no longer inhibits the anaphase promoting complex, which in turn is now free to breakdown securin. The latter is a protein whose function it is to inhibit separase, the protein composite which is responsible for the separation of sister chromatids. Once this inhibitory protein is degraded via ubiquitination and subsequent proteolysis, separin then causes sister chromatid separation.<ref>{{cite book | last = Karp | first = Gerald | authorlink = Gerald Karp | title = Cell and Molecular Biology: Concepts and Experiments, 4th Edition | publisher = John Wiley and Sons | date= 2005 | location = Hoboken, NJ | pages = 598-599 | id = ISBN 0-471-16231-0}}</ref> After the cell has split into its two daughter cells, the cell enters G<sub>1</sub>.
==See also==
*[[Postreplication checkpoint]]
==References==
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[[Category:Cell cycle]]
[[de:Checkpoint (Molekularbiologie)]]
[[ko:세포 검문 지점]]