Cell cycle checkpoint
Encyclopedia
Cell cycle checkpoints are control mechanisms that ensure the fidelity of cell division in eukaryotic 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, though some of them are less understood than others.
The cell cycle according to Temple and Raff, 1986, was meant to function as a clock; but, if this were the case, it would be expected that the stages of the cell cycle must function according to some sort of internal clock, which would determine how long a phase should take. However, the cell cycle is now depicted like the falling dominoes: The preceding phase has 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 that all play salient roles in the maintenance of the integrity of the division.
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
called the restriction point (RP, or the restriction checkpoint); cells that should cease division exit the cell cycle and enter G0
. Cells that continually divide in the adult human include hematopoietic stem cells and gut epithelial cells. Therefore, the re-entrant into the cell cycle is possible only by overcoming the RP. This is achieved by growth factor-induced expression of cyclin D proteins. These then overcome the G0 barrier and are able to enter the cell cycle.
The main checkpoints that control the cell division cycle in eukaryotes include:
, making the key decision of whether the cell should divide, delay division, or enter a resting stage. Many cells stop at this stage and enter a resting state called G0. Liver
cells, for instance, enter mitosis only around once or twice a year. The G1 checkpoint is where eukaryotes typically arrest the cell cycle if environmental conditions make cell division impossible or if the cell passes into G0
for an extended period. In animal cells, the G1 phase checkpoint is called the restriction point
, and in yeast
cells it is called the Start point. The restriction point is controlled mainly by action of the CKI- p16 (CDK inhibitor 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), which relieves the inhibition of the transcription factor E2F
. E2F is then able to cause expression of cyclin E
, which then interacts with CDK2
to allow for G1-S phase transition. This brings the cell to the end of the first checkpoint, signaling the G0-G1-S-phase transition.
In simpler terms, the CDK inhibitor p16 inhibits another CDK from binding to its cyclin (D). When growth is induced, the expression of this cyclin is so high that they do bind. The new CDK/cyclin complex now phosphorylates retinoblastoma (a tumour suppressor). Un-phosphorylated retinoblastoma inhibits a transcription factor. This factor then brings about the G1-S phase transition.
). 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 molecular
processes that signal the beginning of mitosis. The CDKs associated with this checkpoint are activated by phosphorylation of the CDK by the action of a "Maturation promoting factor
" (Mitosis Promoting Factor, MPF).
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. This is done by the ATM kinase protein which phosphorylates Cdc25 which leads to its ubiquitinylation and destruction.
checkpoint occurs at the point in metaphase
where all the chromosomes have/should aligned at the mitotic plate and be under bipolar tension. The tension created by this bipolar attachment is what is sensed, which initiates the anaphase entry. To do this, the sensing mechanism ensures that the anaphase-promoting complex
(APC/C) is no longer inhibited, which is now free to degrade cyclin B
, which harbours a D-box (destruction box), and to break down securin
. The latter is a protein whose function is to inhibit separase
, which in turns cuts the cohesin
s, the protein composite responsible for cohesion of sister chromatids. Once this inhibitory protein is degraded via ubiquitination and subsequent proteolysis, separase then causes sister chromatid separation. After the cell has split into its two daughter cells, the cell enters G1.
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
. These checkpoints verify whether the processes at each phase of the cell cycle
Cell cycle
The cell cycle, or cell-division cycle, is the series of events that takes place in a cell leading to its division and duplication . In cells without a nucleus , the cell cycle occurs via a process termed binary fission...
have been accurately completed before progression into the next phase. Multiple checkpoints have been identified, though some of them are less understood than others.
Function
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 either to stall the cell cycle until repairs are made or, if repairs cannot 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 were the case, it would be expected that the stages of the cell cycle must function according to some sort of internal clock, which would determine how long a phase should take. However, the cell cycle is now depicted like the falling dominoes: The preceding phase has 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 that all play salient roles in the maintenance of the integrity of the division.
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
G1 phase
The G1 phase is a period in the cell cycle during interphase, before the S phase. For many cells, this phase is the major period of cell growth during its lifespan. During this stage new organelles are being synthesized, so the cell requires both structural proteins and enzymes, resulting in great...
called the restriction point (RP, or the restriction checkpoint); cells that should cease division exit the cell cycle and enter G0
G0 phase
The G0 phase is a period in the cell cycle in which cells exist in a quiescent state. G0 phase is viewed as either an extended G1 phase, where the cell is neither dividing nor preparing to divide, or a distinct quiescent stage that occurs outside of the cell cycle...
. Cells that continually divide in the adult human include hematopoietic stem cells and gut epithelial cells. Therefore, the re-entrant into the cell cycle is possible only by overcoming the RP. This is achieved by growth factor-induced expression of cyclin D proteins. These then overcome the G0 barrier and are able to enter the cell cycle.
The main checkpoints that control the cell division cycle in eukaryotes include:
G1 (Restriction) Checkpoint
The first checkpoint is located at the end of the cell cycle's G1 phase, just before entry into S phaseS phase
S-phase is the part of the cell cycle in which DNA is replicated, occurring between G1 phase and G2 phase. Precise and accurate DNA replication is necessary to prevent genetic abnormalities which often lead to cell death or disease. Due to the importance, the regulatory pathways that govern this...
, making the key decision of whether the cell should divide, delay division, or enter a resting stage. Many cells stop at this stage and enter a resting state called G0. Liver
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
cells, for instance, enter mitosis only around once or twice a year. The G1 checkpoint is where eukaryotes typically arrest the cell cycle if environmental conditions make cell division impossible or if the cell passes into G0
G0 phase
The G0 phase is a period in the cell cycle in which cells exist in a quiescent state. G0 phase is viewed as either an extended G1 phase, where the cell is neither dividing nor preparing to divide, or a distinct quiescent stage that occurs outside of the cell cycle...
for an extended period. In animal cells, the G1 phase checkpoint is called the restriction point
Restriction point
The restriction point is a G1 phase checkpoint in the cell cycle of animal cells. Prior to the restriction point, a cell exits the cell cycle if specific mitogenic and growth signals are absent. Cells that progress past the restriction point are committed to enter S phase, where DNA synthesis and...
, and in yeast
Yeast
Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with 1,500 species currently described estimated to be only 1% of all fungal species. Most reproduce asexually by mitosis, and many do so by an asymmetric division process called budding...
cells it is called the Start point. The restriction point is controlled mainly by action of the CKI- p16 (CDK inhibitor 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
Cyclin D
Cyclin D is a member of the cyclin protein family that is involved in regulating cell cycle progression. The synthesis of cyclin D is initiated during G1 and drives the G1/S phase transition...
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), which relieves the inhibition of the transcription factor E2F
E2F
E2F is a group of genes that codifies a family of transcription factors in higher eukaryotes. Three of them are activators: E2F1, 2 and E2F3a. Six others act as suppressors: E2F3b, E2F4-8. All of them are involved in the cell cycle regulation and synthesis of DNA in mammalian cells...
. E2F is then able to cause expression of cyclin E
Cyclin E
Cyclin E is a member of the cyclin family.Cyclin E binds to G1 phase Cdk2, which is required for the transition from G1 to S phase. The Cyclin E/CDK2 complex phosphorylates p27Kip1 , tagging it for degradation, thus promoting expression of Cyclin A, allowing progression to S phase....
, which then interacts with CDK2
Cyclin-dependent kinase 2
Cyclin-dependent kinase 2 also known as cell division protein kinase 2 is an enzyme that in humans is encoded by the CDK2 gene.-Function:...
to allow for G1-S phase transition. This brings the cell to the end of the first checkpoint, signaling the G0-G1-S-phase transition.
In simpler terms, the CDK inhibitor p16 inhibits another CDK from binding to its cyclin (D). When growth is induced, the expression of this cyclin is so high that they do bind. The new CDK/cyclin complex now phosphorylates retinoblastoma (a tumour suppressor). Un-phosphorylated retinoblastoma inhibits a transcription factor. This factor then brings about the G1-S phase transition.
G2 Checkpoint
The second checkpoint is located at the end of G2 phase, triggering the start of the M phase (mitosisMitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
). 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 molecular
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
processes that signal the beginning of mitosis. The CDKs associated with this checkpoint are activated by phosphorylation of the CDK by the action of a "Maturation promoting factor
Maturation promoting factor
Maturation-promoting factor is a heterodimeric protein composed of cyclin B and cyclin-dependent kinase that stimulates the mitotic and meiotic cell cycles...
" (Mitosis Promoting Factor, MPF).
The molecular nature of this checkpoint involves an activating phosphatase, known as Cdc25
Cdc25
Cdc25 is a dual-specificity phosphatase first isolated from the yeast Schizosaccharomyces pombe as a cell cycle defective mutant. As with other cell cycle proteins such as Cdc2 and Cdc4, the "cdc" in its name refers to "cell division cycle".Dual-specificity phosphatases are considered a sub-class...
, 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. This is done by the ATM kinase protein which phosphorylates Cdc25 which leads to its ubiquitinylation and destruction.
Metaphase Checkpoint
The mitotic spindleMitotic spindle
In cell biology, the spindle fibers are the structure that separates the chromosomes into the daughter cells during cell division. It is part of the cytoskeleton in eukaryotic cells...
checkpoint occurs at the point in metaphase
Metaphase
Metaphase, from the ancient Greek μετά and φάσις , is a stage of mitosis in the eukaryotic cell cycle in which condensed & highly coiled chromosomes, carrying genetic information, align in the middle of the cell before being separated into each of the two daughter cells...
where all the chromosomes have/should aligned at the mitotic plate and be under bipolar tension. The tension created by this bipolar attachment is what is sensed, which initiates the anaphase entry. To do this, the sensing mechanism ensures that the anaphase-promoting complex
Anaphase-promoting complex
Anaphase-Promoting Complex, also called cyclosome , is an E3 ubiquitin ligase that marks target cell cycle proteins for degradation by the 26S proteasome. The APC/C is a large complex of 11–13 subunit proteins, including a cullin and RING subunit much like SCF...
(APC/C) is no longer inhibited, which is now free to degrade cyclin B
Cyclin B
Cyclin B is a member of the cyclin family.Cyclin B is a mitotic cyclin. The amount of cyclin B and the activity of the cyclin B-Cdk complex rise through the cell cycle until mitosis, where they fall abruptly due to degradation of cyclin B...
, which harbours a D-box (destruction box), and to break down securin
Securin
Securin is a protein involved in control of the metaphase-anaphase transition and anaphase onset. Following bi-orientation of chromosome pairs and inactivation of the spindle checkpoint system, the underlying regulatory system, which includes securin, produces an abrupt stimulus that induces highly...
. The latter is a protein whose function is to inhibit separase
Separase
Separase is a cysteine protease responsible for triggering anaphase by hydrolysing cohesin which is the protein responsible for binding sister chromatids during metaphase. In humans, separase is encoded by the ESPL1 gene.- Discovery :...
, which in turns cuts the cohesin
Cohesin
Cohesin is a protein complex that regulates the separation of sister chromatids during cell division, either mitosis or meiosis.- Structure :...
s, the protein composite responsible for cohesion of sister chromatids. Once this inhibitory protein is degraded via ubiquitination and subsequent proteolysis, separase then causes sister chromatid separation. After the cell has split into its two daughter cells, the cell enters G1.
See also
- Postreplication checkpointPostreplication checkpoint-Postreplication checkpoint:When the genomic DNA of eukaryotic cells becomes damaged by spontaneous processes, chemical mutagens, or sunlight exposure, the replication of damaged DNA triggers a cellular response called a postreplication checkpoint...
- Biochemical switches in the cell cycleBiochemical switches in the cell cycleA series of biochemical switches control transitions between and within the various phases of the cell cycle. The cell cycle is a series of complex, ordered, sequential events that control how a single cell divides into two cells, and involves several different phases...
- Cell cycle analysisCell cycle analysisCell cycle analysis is a method in cell biology that employs flow cytometry to distinguish cells in different phases of the cell cycle. Before analysis, the cells are permeabilised and treated with a fluorescent dye that stains DNA quantitatively, usually propidium iodide...
- G2-M DNA damage checkpointG2-M DNA damage checkpointThe G2-M DNA damage checkpoint is an important cell cycle checkpoint in eukaryotic organisms ranging from yeast to mammals. This checkpoint insures that cells don't initiate mitosis before they have a chance to repair damaged DNA after replication...