Wikipedia

Cell cycle

This article is about the eukaryotic cell cycle. For the prokaryotic cell cycle, seefission (biology).For the separation of chromosomes that occurs as part of the cell cycle, seemitosis.For the academic journal, seeCell Cycle(journal).
See also:Cell division

Thecell cycle,orcell-division cycle,is the sequential series of events that take place in acellthat causes it to divide into two daughter cells. These events include the growth of the cell, duplication of its DNA (DNA replication) and some of itsorganelles,and subsequently the partitioning of its cytoplasm, chromosomes and other components into two daughter cells in a process calledcell division.

Life cycle of the cell
Onion (Allium) cells in different phases of the cell cycle. Growth in an 'organism' is carefully controlled by regulating the cell cycle.
Cell cycle inDeinococcus radiodurans

Ineukaryotic cells(having acell nucleus) includinganimal,plant,fungal,andprotistcells, the cell cycle is divided into two main stages:interphase,and theM phasethat includes mitosis and cytokinesis.[1]During interphase, the cell grows, accumulating nutrients needed for mitosis, and replicates its DNA and some of its organelles. During the M phase, the replicatedchromosomes,organelles, and cytoplasm separate into two new daughter cells. To ensure the proper replication of cellular components and division, there are control mechanisms known ascell cycle checkpointsafter each of the key steps of the cycle that determine if the cell can progress to the next phase.

In cells without nuclei theprokaryotes,bacteriaandarchaea,thecell cycleis 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.[2]

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-celledfertilized egginto a mature organism, and is also the process by whichhair,skin,blood cells,and someinternal organsareregeneratedandhealed(with possible exception ofnerves;seenerve damage). After cell division, each of the daughter cells begin theinterphaseof 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

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The eukaryotic cell cycle consists of four distinct phases:G1phase,S phase(synthesis),G2phase(collectively known asinterphase) andM phase(mitosis and cytokinesis). M phase is itself composed of two tightly coupled processes: mitosis, in which the cell's nucleus divides, andcytokinesis,in which the cell'scytoplasmand 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 calledG0phaseor the resting phase.

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[3]
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. TheG1checkpointensures that everything is ready forDNAsynthesis.
Synthesis S DNA replication.
Gap 2 G2 Growth and preparation for mitosis. TheG2checkpointensures that everything is ready to enter the M (mitosis) phase and divide.
Cell division Mitosis M Cell division occurs. TheMetaphase Checkpointensures that the cell is ready to complete cell division.

G0phase (quiescence)

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Plant cell cycle
Animal cell cycle
Main article:G0 phase

G0is a resting phase where the cell has left the cycle and has stopped dividing. The cell cycle starts with this phase. Non-proliferative (non-dividing) cells in multicellulareukaryotesgenerally enter the quiescent G0state from G1and may remain quiescent for long periods of time, possibly indefinitely (as is often the case forneurons). This is very common for cells that are fullydifferentiated.Some cells enter the G0phase semi-permanently and are considered post-mitotic, e.g., some liver, kidney, and stomach cells. Many cells do not enter G0and continue to divide throughout an organism's life, e.g., epithelial cells.

The word "post-mitotic" is sometimes used to refer to bothquiescentandsenescentcells. 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 byapoptosis.

Interphase

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Main article: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.

G1phase (First growth phase or Post mitotic gap phase)

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Schematickaryogramof the human chromosomes, showing their usual state in the G0and G1phase of the cell cycle. At top center it also shows the chromosome 3 pair inmetaphase(annotated as "Meta." ), which takes place after having undergoneDNA synthesiswhich occurs in theS phase(annotated as S) of the cell cycle.
Further information:Karyotype
Main article:G1 phase

The first phase within interphase, from the end of the previous M phase until the beginning of DNA synthesis, is calledG1(G indicatinggap). 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 G1is highly variable, even among different cells of the same species.[4]In this phase, the cell increases its supply of proteins, increases the number of organelles (such as mitochondria, ribosomes), and grows in size. In G1phase, a cell has three options.

  • To continue cell cycle and enterS phase
  • Stop cell cycle and enterG0phase for undergoingdifferentiation.
  • Become arrested in G1phase hence it may enter G0phase or re-enter cell cycle.

The deciding point is calledcheck point(Restriction point). Thischeck pointis called the restriction point or START and is regulated by G1/S cyclins, which cause transition from G1to S phase. Passage through the G1check point commits the cell to division.

S phase (DNA replication)

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Main article:S phase

The ensuingS phasestarts whenDNA synthesiscommences; when it is complete, all of thechromosomeshave been replicated, i.e., each chromosome consists of two sisterchromatids.Thus, during this phase, the amount of DNA in the cell has doubled, though theploidyand number of chromosomes are unchanged. Rates of RNAtranscriptionandprotein synthesisare very low during this phase. An exception to this ishistoneproduction, most of which occurs during the S phase.[5][6][7]

G2phase (growth)

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Main article:G2 phase

G2phase 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 (preprophase). Before proceeding tomitotic phase,cells must be checked at the G2checkpoint for any DNA damage within the chromosomes. The G2checkpoint is mainly regulated by the tumor proteinp53.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)

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Main article:Mitosis

The relatively briefM phaseconsists of nuclear division (karyokinesis) and division of cytoplasm (cytokinesis). It is a relatively short period of the cell cycle. 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:

A diagram of the mitotic phases

Mitosis is the process by which aeukaryoticcell separates thechromosomesin itscell nucleusinto two identical sets in two nuclei.[8]During the process of mitosis the pairs ofchromosomescondense and attach tomicrotubulesthat pull thesister chromatidsto opposite sides of the cell.[9]

Mitosis occurs exclusively ineukaryoticcells, but occurs in different ways in different species. For example, animal cells undergo an "open" mitosis, where thenuclear envelopebreaks down before the chromosomes separate, whilefungisuch asAspergillus nidulansandSaccharomyces cerevisiae(yeast) undergo a "closed" mitosis, where chromosomes divide within an intactcell nucleus.[10]

Cytokinesis phase (separation of all cell components)

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Main article:Cytokinesis

Mitosis is immediately followed bycytokinesis,which divides the nuclei,cytoplasm,organellesandcell membraneinto 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, acell plateis formed to separate it in plant cells. The position of the cell plate is determined by the position of a preprophase band of microtubules andactinfilaments. Mitosis and cytokinesis together define thedivisionof 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 calledendoreplication.This occurs most notably among thefungiandslime molds,but is found in various groups. Even in animals, cytokinesis and mitosis may occur independently, for instance during certain stages offruit flyembryonic development.[11]Errors in mitosis can result in cell death throughapoptosisor causemutationsthat may lead tocancer.

Regulation of eukaryotic cell cycle

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Levels of the three major cyclin types oscillate during the cell cycle (top), providing the basis for oscillations in the cyclin–Cdk complexes that drive cell-cycle events (bottom). In general, Cdk levels are constant and in large excess over cyclin levels; thus, cyclin–Cdk complexes form in parallel with cyclin levels. The enzymatic activities of cyclin–Cdk complexes also tend to rise and fall in parallel with cyclin levels, although in some cases Cdk inhibitor proteins or phosphorylation introduce a delay between the formation and activation of cyclin–Cdk complexes. Formation of active G1/S–Cdk complexes commits the cell to a new division cycle at the Start checkpoint in late G1. G1/S–Cdks then activate the S–Cdk complexes that initiate DNA replication at the beginning of S phase. M–Cdk activation occurs after the completion of S phase, resulting in progression through the G2/M checkpoint and assembly of the mitotic spindle. APC activation then triggers sister-chromatid separation at the metaphase-to-anaphase transition. APC activity also causes the destruction of S and M cyclins and thus the inactivation of Cdks, which promotes the completion of mitosis and cytokinesis. APC activity is maintained in G1 until G1/S–Cdk activity rises again and commits the cell to the next cycle. This scheme serves only as a general guide and does not apply to all cell types.

Regulation of the cell cycle involves processes crucial to the survival of a cell, including the detection and repair of genetic damage as well as the prevention of uncontrolled cell division. 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.

Role of cyclins and CDKs

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Nobel Laureate
Paul Nurse
Nobel Laureate
Tim Hunt

Two key classes of regulatory molecules,cyclinsandcyclin-dependent kinases(CDKs), determine a cell's progress through the cell cycle.[12]Leland H. Hartwell,R. Timothy Hunt,andPaul M. Nursewon the 2001Nobel Prize in Physiology or Medicinefor their discovery of these central molecules.[13]Many of the genes encoding cyclins and CDKs areconservedamong 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 by studying yeast, especiallySaccharomyces cerevisiae;[14]genetic nomenclature in yeast dubs many of these genescdc(for "cell division cycle" ) followed by an identifying number, e.g.cdc25orcdc20.

Cyclins form the regulatory subunits and CDKs the catalytic subunits of an activatedheterodimer;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 calledphosphorylationthat 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. CDKs are constitutively expressed in cells whereas cyclins are synthesised at specific stages of the cell cycle, in response to various molecular signals.[15]

General mechanism of cyclin-CDK interaction

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Upon receiving a pro-mitotic extracellular signal, G1cyclin-CDKcomplexes become active to prepare the cell for S phase, promoting the expression oftranscription factorsthat in turn promote the expression of S cyclins and of enzymes required forDNA replication.The G1cyclin-CDK complexes also promote the degradation of molecules that function as S phase inhibitors by targeting them forubiquitination.Once a protein has been ubiquitinated, it is targeted for proteolytic degradation by theproteasome.However, results from a recent study of E2F transcriptional dynamics at the single-cell level argue that the role of G1 cyclin-CDK activities, in particular cyclin D-CDK4/6, is to tune the timing rather than the commitment of cell cycle entry.[16]

Active S cyclin-CDK complexes phosphorylate proteins that make up thepre-replication complexesassembled during G1phase on DNAreplication 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'sgenomewill 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 togene copy numbereffects, possession of extra copies of certain genes is also deleterious to the daughter cells.

Mitotic cyclin-CDK complexes, which are synthesized but inactivated during S and G2phases, promote the initiation ofmitosisby stimulating downstream proteins involved in chromosome condensation andmitotic spindleassembly. A critical complex activated during this process is aubiquitin ligaseknown as theanaphase-promoting complex(APC), which promotes degradation of structural proteins associated with the chromosomalkinetochore.APC also targets the mitotic cyclins for degradation, ensuring that telophase and cytokinesis can proceed.[17]

Specific action of cyclin-CDK complexes

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Cyclin Dis the first cyclin produced in the cells that enter the cell cycle, in response to extracellular signals (e.g.growth factors). Cyclin D levels stay low in resting cells that are not proliferating. Additionally,CDK4/6andCDK2are also inactive because CDK4/6 are bound byINK4family members (e.g., p16), limiting kinase activity. Meanwhile, CDK2 complexes are inhibited by the CIP/KIP proteins such as p21 and p27,[18]When it is time for a cell to enter the cell cycle, which is triggered by a mitogenic stimuli, levels of cyclin D increase. In response to this trigger, cyclin D binds to existingCDK4/6, forming the active cyclin D-CDK4/6 complex. Cyclin D-CDK4/6 complexes in turn mono-phosphorylates theretinoblastomasusceptibility protein (Rb) to pRb. The un-phosphorylated Rb tumour suppressor functions in inducing cell cycle exit and maintaining G0 arrest (senescence).[19]

In the last few decades, a model has been widely accepted whereby pRB proteins are inactivated by cyclin D-Cdk4/6-mediated phosphorylation. Rb has 14+ potential phosphorylation sites. Cyclin D-Cdk 4/6 progressively phosphorylates Rb to hyperphosphorylated state, which triggers dissociation of pRB–E2Fcomplexes, thereby inducing G1/S cell cycle gene expression and progression into S phase.[20]

However, scientific observations from a recent study show that Rb is present in three types of isoforms: (1) un-phosphorylated Rb in G0 state; (2) mono-phosphorylated Rb, also referred to as "hypo-phosphorylated' or 'partially' phosphorylated Rb in early G1 state; and (3) inactive hyper-phosphorylated Rb in late G1 state.[21][22][23]In early G1 cells, mono-phosphorylated Rb exists as 14 different isoforms, one of each has distinctE2Fbinding affinity.[23]Rb has been found to associate with hundreds of different proteins[24]and the idea that different mono-phosphorylated Rb isoforms have different protein partners was very appealing.[25]A recent report confirmed that mono-phosphorylation controls Rb's association with other proteins and generates functional distinct forms of Rb.[26]All different mono-phosphorylated Rb isoforms inhibit E2F transcriptional program and are able to arrest cells in G1-phase. Importantly, different mono-phosphorylated forms of Rb have distinct transcriptional outputs that are extended beyond E2F regulation.[26]

In general, the binding of pRb to E2F inhibits the E2F target gene expression of certain G1/S and S transition genes includingE-type cyclins.The partial phosphorylation of Rb de-represses the Rb-mediated suppression of E2F target gene expression, begins the expression of cyclin E. The molecular mechanism that causes the cell switched to cyclin E activation is currently not known, but as cyclin E levels rise, the active cyclin E-CDK2 complex is formed, bringing Rb to be inactivated by hyper-phosphorylation.[23]Hyperphosphorylated Rb is completely dissociated from E2F, enabling further expression of a wide range of E2F target genes are required for driving cells to proceed into S phase [1]. Recently, it has been identified that cyclin D-Cdk4/6 binds to a C-terminal Alpha -helix region of Rb that is only distinguishable to cyclin D rather than other cyclins,cyclin E,AandB.[27]This observation based on the structural analysis of Rb phosphorylation supports that Rb is phosphorylated in a different level through multiple Cyclin-Cdk complexes. This also makes feasible the current model of a simultaneous switch-like inactivation of all mono-phosphorylated Rb isoforms through one type of Rb hyper-phosphorylation mechanism. In addition, mutational analysis of the cyclin D- Cdk 4/6 specific Rb C-terminal helix shows that disruptions of cyclin D-Cdk 4/6 binding to Rb prevents Rb phosphorylation, arrests cells in G1, and bolsters Rb's functions in tumor suppressor.[27]This cyclin-Cdk driven cell cycle transitional mechanism governs a cell committed to the cell cycle that allows cell proliferation. A cancerous cell growth often accompanies with deregulation of Cyclin D-Cdk 4/6 activity.

The hyperphosphorylated Rb dissociates from the E2F/DP1/Rb complex (which was bound to theE2Fresponsive genes, effectively "blocking" them from transcription), activating E2F. Activation of E2F results in transcription of various genes likecyclin E,cyclin A,DNA polymerase,thymidine kinase,etc. Cyclin E thus produced binds toCDK2,forming the cyclin E-CDK2 complex, which pushes the cell from G1to S phase (G1/S, which initiates the G2/M transition).[28]Cyclin B-cdk1 complex activation causes breakdown ofnuclear envelopeand initiation ofprophase,and subsequently, its deactivation causes the cell to exit mitosis.[15]A quantitative study of E2F transcriptional dynamics at the single-cell level by using engineered fluorescent reporter cells provided a quantitative framework for understanding the control logic of cell cycle entry, challenging the canonical textbook model. Genes that regulate the amplitude of E2F accumulation, such as Myc, determine the commitment in cell cycle and S phase entry. G1 cyclin-CDK activities are not the driver of cell cycle entry. Instead, they primarily tune the timing of E2F increase, thereby modulating the pace of cell cycle progression.[16]

Inhibitors

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Endogenous

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Overview of signal transduction pathways involved inapoptosis,also known as "programmed cell death"

Two families of genes, thecip/kip(CDK interacting protein/Kinase inhibitory protein) family and the INK4a/ARF (Inhibitor ofKinase 4/AlternativeReadingFrame) family, prevent the progression of the cell cycle. Because these genes are instrumental in prevention oftumorformation, they are known astumor suppressors.

Thecip/kipfamilyincludes the genesp21,p27andp57.They halt the cell cycle in G1phase by binding to and inactivating cyclin-CDK complexes. p21 is activated byp53(which, in turn, is triggered by DNA damage e.g. due to radiation). p27 is activated by Transforming Growth Factor β (TGF β), a growth inhibitor.

TheINK4a/ARF familyincludesp16INK4a,which binds to CDK4 and arrests the cell cycle in G1phase, andp14ARFwhich prevents p53 degradation.

Synthetic

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Synthetic inhibitors ofCdc25could also be useful for the arrest of cell cycle and therefore be useful as antineoplastic and anticancer agents.[29]

Many human cancers possess the hyper-activated Cdk 4/6 activities.[30]Given the observations of cyclin D-Cdk 4/6 functions, inhibition of Cdk 4/6 should result in preventing a malignant tumor from proliferating. Consequently, scientists have tried to invent the synthetic Cdk4/6 inhibitor as Cdk4/6 has been characterized to be a therapeutic target for anti-tumor effectiveness. Three Cdk4/6 inhibitors –palbociclib,ribociclib,andabemaciclib– currently received FDA approval for clinical use to treat advanced-stage ormetastatic,hormone-receptor-positive(HR-positive, HR+),HER2-negative(HER2-) breast cancer.[31][32]For example, palbociclib is an orally active CDK4/6 inhibitor which has demonstrated improved outcomes for ER-positive/HER2-negative advanced breast cancer. The main side effect isneutropeniawhich can be managed by dose reduction.[33]

Cdk4/6 targeted therapy will only treat cancer types where Rb is expressed. Cancer cells with loss of Rb have primary resistance to Cdk4/6 inhibitors.

Transcriptional regulatory network

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Current evidence suggests that a semi-autonomous transcriptional network acts in concert with the CDK-cyclin machinery to regulate the cell cycle. Several gene expression studies inSaccharomyces cerevisiaehave identified 800–1200 genes that change expression over the course of the cell cycle.[14][34][35]They are transcribed at high levels at specific points in the cell cycle, and remain at lower levels throughout the rest of the cycle. While the set of identified genes differs between studies due to the computational methods and criteria used to identify them, each study indicates that a large portion of yeast genes are temporally regulated.[36]

Many periodically expressed genes are driven bytranscription factorsthat are also periodically expressed. One screen of single-gene knockouts identified 48 transcription factors (about 20% of all non-essential transcription factors) that show cell cycle progression defects.[37]Genome-wide studies using high throughput technologies have identified the transcription factors that bind to the promoters of yeast genes, and correlating these findings with temporal expression patterns have allowed the identification of transcription factors that drive phase-specific gene expression.[34][38]The expression profiles of these transcription factors are driven by the transcription factors that peak in the prior phase, and computational models have shown that a CDK-autonomous network of these transcription factors is sufficient to produce steady-state oscillations in gene expression).[35][39]

Experimental evidence also suggests that gene expression can oscillate with the period seen in dividing wild-type cells independently of the CDK machinery. Orlandoet al.usedmicroarraysto measure the expression of a set of 1,271 genes that they identified as periodic in both wild type cells and cells lacking all S-phase and mitotic cyclins (clb1,2,3,4,5,6). Of the 1,271 genes assayed, 882 continued to be expressed in the cyclin-deficient cells at the same time as in the wild type cells, despite the fact that the cyclin-deficient cells arrest at the border betweenG1andS phase.However, 833 of the genes assayed changed behavior between the wild type and mutant cells, indicating that these genes are likely directly or indirectly regulated by the CDK-cyclin machinery. Some genes that continued to be expressed on time in the mutant cells were also expressed at different levels in the mutant and wild type cells. These findings suggest that while the transcriptional network may oscillate independently of the CDK-cyclin oscillator, they are coupled in a manner that requires both to ensure the proper timing of cell cycle events.[35]Other work indicates thatphosphorylation,a post-translational modification, of cell cycle transcription factors byCdk1may alter the localization or activity of the transcription factors in order to tightly control timing of target genes.[37][40][41]

While oscillatory transcription plays a key role in the progression of the yeast cell cycle, the CDK-cyclin machinery operates independently in the early embryonic cell cycle. Before themidblastula transition,zygotictranscription does not occur and all needed proteins, such as the B-type cyclins, are translated from maternally loadedmRNA.[42]

DNA replication and DNA replication origin activity

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Analyses of synchronized cultures ofSaccharomyces cerevisiaeunder conditions that prevent DNA replication initiation without delaying cell cycle progression showed that origin licensing decreases the expression of genes with origins near their 3' ends, revealing that downstream origins can regulate the expression of upstream genes.[43]This confirms previous predictions from mathematical modeling of a global causal coordination between DNA replication origin activity and mRNA expression,[44][45][46]and shows that mathematical modeling of DNA microarray data can be used to correctly predict previously unknown biological modes of regulation.

Checkpoints

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Main article:Cell cycle checkpoint

Cell cycle checkpointsare used by the cell to monitor and regulate the progress of the cell cycle.[47]Checkpoints prevent cell cycle progression at specific points, allowing verification of necessary phase processes and repair ofDNA damage.The cell cannot proceed to the next phase until checkpoint requirements have been met. Checkpoints typically consist of a network of regulatory proteins that monitor and dictate the progression of the cell through the different stages of the cell cycle.

It is estimated that in normal human cells about 1% ofsingle-strand DNA damagesare converted to about 50 endogenous DNA double-strand breaks per cell per cell cycle.[48]Although such double-strand breaks are usuallyrepairedwith high fidelity, errors in their repair are considered to contribute significantly to the rate of cancer in humans.[48]

There are several checkpoints to ensure that damaged or incomplete DNA is not passed on to daughter cells. Three main checkpoints exist: the G1/S checkpoint, the G2/M checkpoint and the metaphase (mitotic) checkpoint. Another checkpoint is the Go checkpoint, in which the cells are checked for maturity. If the cells fail to pass this checkpoint by not being ready yet, they will be discarded from dividing.

G1/S transition is a rate-limiting step in the cell cycle and is also known asrestriction point.[15]This is where the cell checks whether it has enough raw materials to fully replicate its DNA (nucleotide bases, DNA synthase, chromatin, etc.). An unhealthy or malnourished cell will get stuck at this checkpoint.

The G2/M checkpoint is where the cell ensures that it has enough cytoplasm and phospholipids for two daughter cells. But sometimes more importantly, it checks to see if it is the right time to replicate. There are some situations where many cells need to all replicate simultaneously (for example, a growing embryo should have a symmetric cell distribution until it reaches the mid-blastula transition). This is done by controlling the G2/M checkpoint.

The metaphase checkpoint is a fairly minor checkpoint, in that once a cell is in metaphase, it has committed to undergoing mitosis. However that's not to say it isn't important. In this checkpoint, the cell checks to ensure that the spindle has formed and that all of the chromosomes are aligned at the spindle equator before anaphase begins.[49]

While these are the three "main" checkpoints, not all cells have to pass through each of these checkpoints in this order to replicate. Many types of cancer are caused by mutations that allow the cells to speed through the various checkpoints or even skip them altogether. Going from S to M to S phase almost consecutively. Because these cells have lost their checkpoints, any DNA mutations that may have occurred are disregarded and passed on to the daughter cells. This is one reason why cancer cells have a tendency to exponentially acquire mutations. Aside from cancer cells, many fully differentiated cell types no longer replicate so they leave the cell cycle and stay in G0until their death. Thus removing the need for cellular checkpoints. An alternative model of the cell cycle response to DNA damage has also been proposed, known as thepostreplication checkpoint.

Checkpoint regulation plays an important role in an organism's development. In sexual reproduction, when egg fertilization occurs, when the sperm binds to the egg, it releases signalling factors that notify the egg that it has been fertilized. Among other things, this induces the now fertilized oocyte to return from its previously dormant, G0,state back into the cell cycle and on to mitotic replication and division.

p53plays an important role in triggering the control mechanisms at both G1/S and G2/M checkpoints. In addition to p53, checkpoint regulators are being heavily researched for their roles in cancer growth and proliferation.

Fluorescence imaging of the cell cycle

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Fluorescent proteins visualize the cell cycle progression. IFP2.0-hGem(1/110) fluorescence is shown in green and highlights the S/G2/M phases.smURFP-hCdtI(30/120) fluorescence is shown in red and highlights the G0/G1phases.

Pioneering work by Atsushi Miyawaki and coworkers developed the fluorescent ubiquitination-based cell cycle indicator (FUCCI), which enablesfluorescenceimaging of the cell cycle. Originally, agreen fluorescent protein,mAG, was fused to hGem(1/110) and an orangefluorescent protein(mKO2) was fused to hCdt1(30/120). Note, these fusions are fragments that contain anuclear localization signalandubiquitinationsites fordegradation,but are not functional proteins. Thegreen fluorescent proteinis made during the S, G2,or M phase and degraded during the G0or G1phase, while the orangefluorescent proteinis made during the G0or G1phase and destroyed during the S, G2,or M phase.[50]A far-red and near-infrared FUCCI was developed using acyanobacteria-derivedfluorescent protein(smURFP) and abacteriophytochrome-derivedfluorescent protein(movie found at this link).[51] Several modifications have been made to the original FUCCI system to improve its usability in several in vitro systems and model organisms. These advancements have increased the sensitivity and accuracy of cell cycle phase detection, enabling more precise assessments of cellular proliferation[52]

Role in tumor formation

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A disregulation of the cell cycle components may lead totumorformation.[53]As mentioned above, when some genes like the cell cycle inhibitors,RB,p53etc. mutate, they may cause the cell to multiply uncontrollably, forming a tumor. Although the duration of cell cycle in tumor cells is equal to or longer than that of normal cell cycle, the proportion of cells that are in active cell division (versus quiescent cells in G0phase) in tumors is much higher than that in normal tissue.[54]Thus there is a net increase in cell number as the number of cells that die by apoptosis or senescence remains the same.

The cells which are actively undergoing cell cycle are targeted in cancer therapy as the DNA is relatively exposed during cell division and hence susceptible to damage bydrugsorradiation.This fact is made use of in cancer treatment; by a process known asdebulking,a significant mass of the tumor is removed which pushes a significant number of the remaining tumor cells from G0to G1phase (due to increased availability of nutrients, oxygen, growth factors etc.). Radiation or chemotherapy following the debulking procedure kills these cells which have newly entered the cell cycle.[15]

The fastest cycling mammalian cells in culture, crypt cells in the intestinal epithelium, have a cycle time as short as 9 to 10 hours. Stem cells in resting mouse skin may have a cycle time of more than 200 hours. Most of this difference is due to the varying length of G1,the most variable phase of the cycle. M and S do not vary much.

In general, cells are most radiosensitive in late M and G2phases and most resistant in late S phase. For cells with a longer cell cycle time and a significantly long G1phase, there is a second peak of resistance late in G1.The pattern of resistance and sensitivity correlates with the level of sulfhydryl compounds in the cell. Sulfhydryls are natural substances that protect cells from radiation damage and tend to be at their highest levels in S and at their lowest near mitosis.

Homologous recombination(HR) is an accurate process forrepairing DNAdouble-strand breaks. HR is nearly absent inG1 phase,is most active inS phase,and declines in G2/M.[55]Non-homologous end joining,a less accurate and more mutagenic process for repairing double strand breaks, is active throughout the cell cycle.

Cell cycle evolution

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Evolution of the genome

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The cell cycle must duplicate all cellular constituents and equally partition them into two daughter cells. Many constituents, such as proteins andribosomes,are produced continuously throughout the cell cycle (except duringM-phase). However, the chromosomes and other associated elements likeMTOCs,are duplicated just once during the cell cycle. A central component of the cell cycle is its ability to coordinate the continuous and periodic duplications of different cellular elements, which evolved with the formation of the genome.

The pre-cellular environment contained functional and self-replicatingRNAs.[56]All RNA concentrations depended on the concentrations of other RNAs that might be helping or hindering the gathering of resources. In this environment, growth was simply the continuous production of RNAs. These pre-cellular structures would have had to contend with parasitic RNAs, issues of inheritance, and copy-number control of specific RNAs.[56][57]

Partitioning "genomic" RNA from "functional" RNA helped solve these problems.[58]The fusion of multiple RNAs into a genome gave a template from which functional RNAs were cleaved. Now, parasitic RNAs would have to incorporate themselves into the genome, a much greater barrier, in order to survive. Controlling the copy number of genomic RNA also allowed RNA concentration to be determined through synthesis rates and RNA half-lives, instead of competition.[56]Separating the duplication of genomic RNAs from the generation of functional RNAs allowed for much greater duplication fidelity of genomic RNAs without compromising the production of functional RNAs. Finally, the replacement of genomic RNA withDNA,which is a more stable molecule, allowed for larger genomes. The transition from self-catalysis enzyme synthesis to genome-directed enzyme synthesis was a critical step in cell evolution, and had lasting implications on the cell cycle, which must regulate functional synthesis and genomic duplication in very different ways.[56]

Cyclin-dependent kinase and cyclin evolution

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Cell-cycle progression is controlled by the oscillating concentrations of differentcyclinsand the resulting molecular interactions from the variouscyclin-dependent kinases(CDKs). In yeast, just one CDK (Cdc28 inS. cerevisiaeand Cdc2 inS. pombe) controls the cell cycle.[59]However, in animals, whole families of CDKs have evolved.[60][61]Cdk1 controls entry to mitosis and Cdk2, Cdk4, and Cdk6 regulate entry into S phase. Despite the evolution of the CDK family in animals, these proteins have related or redundant functions.[62][63][64]For example,cdk2 cdk4 cdk6triple knockout mice cells can still progress through the basic cell cycle.[65]cdk1knockouts are lethal, which suggests an ancestral CDK1-type kinase ultimately controlling the cell cycle.[65]

Arabidopsis thalianahas a Cdk1 homolog called CDKA;1, howevercdka;1A. thalianamutants are still viable,[66]running counter to theopisthokontpattern of CDK1-type kinases as essential regulators controlling the cell cycle.[67]Plants also have a unique group of B-type CDKs, whose functions may range from development-specific functions to major players in mitotic regulation.[68][69]

G1/S checkpoint evolution

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Overviews of the G1/S transition control networks in plants, animals, and yeast. All three show striking network topology similarities, even though individual proteins in the network have very little sequence similarity.[67]

TheG1/S checkpointis the point at which the cell commits to division through the cell cycle. Complex regulatory networks lead to the G1/S transition decision. Across opisthokonts, there are both highly diverged protein sequences as well as strikingly similar network topologies.[67][70]

Entry into S-phase in both yeast and animals is controlled by the levels of two opposing regulators.[67]The networks regulating thesetranscription factorsare double-negative feedback loops and positive feedback loops in both yeast and animals.[67][70][71]Additional regulation of the regulatory network for the G1/S checkpoint in yeast and animals includes thephosphorylation/de-phosphorylation of CDK-cyclin complexes. The sum of these regulatory networks creates ahystereticand bistable scheme, despite the specific proteins being highly diverged.[72][73]For yeast,Whi5must be suppressed by Cln3 phosphorylation for SBF to be expressed,[74]while in animalsRbmust be suppressed by the Cdk4/6-cyclin D complex forE2Fto be expressed.[75]Both Rb and Whi5 inhibit transcript through the recruitment of histone deacetylase proteins to promoters.[76][77]Both proteins additionally have multiple CDK phosphorylation sites through which they are inhibited.[78][75]However, these proteins share no sequence similarity.

Studies inA. thalianaextend our knowledge of the G1/S transition acrosseukaryotesas a whole. Plants also share a number of conserved network features with opisthokonts, and many plant regulators have direct animal homologs.[79]For example, plants also need to suppress Rb for E2F translation in the network.[80]These conserved elements of the plant and animal cell cycles may be ancestral in eukaryotes. While yeast share a conserved network topology with plants and animals, the highly diverged nature of yeast regulators suggests possible rapid evolution along the yeast lineage.[67]

See also

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References

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Further reading

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  • Morgan DO (2007).The Cell Cycle: Principles of Control.London: Published by New Science Press in association with Oxford University Press.ISBN978-0-87893-508-6.
  • Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P (2008). "Chapter 17".Molecular Biology of the Cell(5th ed.). New York: Garland Science.ISBN978-0-8153-4111-6.
  • Krieger M, Scott MP, Matsudaira PT, Lodish HF, Darnell JE, Zipursky L, Kaiser C, Berk A (2004).Molecular cell biology.New York: W.H. Freeman and CO.ISBN978-0-7167-4366-8.
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