Cell Cycle- Definition, Phases, Regulation and Checkpoints

Cell Cycle Definition

The cell cycle is the sequence of events occurring in an ordered fashion which results in cell growth and cell division.

  • The cycle begins at the end of each nuclear division and ends with the beginning of the next.
  • A cell cycle acts as a unit of biological time that defines the life history of the cell. 
  • The cell cycle is a continuous process that includes all significant events of the cell, ranging from duplication of DNA and cell organelles to subsequent partitioning of the cytoplasm.
  • In addition, the process of cell growth where the cell absorbs nutrients and prepares for its cell division is also a part of the cell cycle.
  • The process of the cell cycle occurs in various phases, all of which are specialized for a particular stage of the cell.
  • The overall process and steps of the cell cycle might differ in eukaryotic and prokaryotic organisms as a result of the differences in their cell complexity.
  • Three main cycles are involved in the cell cycle; chromosome cycle, cytoplasmic cycle, and centrosome cycle.
    • The chromosome cycle involves DNA synthesis that alternates with mitosis. During this cycle, the double-helical DNA of the cell replicates to form two identical daughter DNA molecules. This is followed by mitosis to separate the cell into two daughter cells.
    • The cytoplasmic cycle involves cell growth that alternates with cytokinesis. During growth, the cell accumulates nutrients and growth factors and doubles the contents of the cytoplasm. Eventually, the cytoplasm divides via cytokinesis to equally divide the cytoplasmic contents into two cells.
    • The final cycle is the centrosome cycle where the centrosome is divided so that it can be inherited reliably and duplicated accordingly to form two poles of the mitotic spindle fibers.
  • The cell cycle is regulated by various stimulatory and inhibitory factors that decide whether the cell needs to divide or grow.
  • The cell cycle is divided into different phases (according to Howard and Pelc), each of which is defined by various processes.
Cell Cycle
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Phases of the Cell Cycle

1. Gap 0 Phase (G0)

  • Gap 0 phase or G0 phase of the cell cycle is a period of time where the cell is present in a quiescent stage or resting phase, as it neither divides nor grows.
  • The G0 phase can be considered either an extended G1 phase or a separate phase-out of the cell cycle.
  • Usually, cells enter the G0 phase when they reach maturity like in the case of muscle cells and nerve cells, but the cells continue to perform their function throughout their life.
  • In some cases, however, cells might enter the G0 phase from the checkpoint in the G1 phase due to the lack of growth factors or nutrients.
  • In the G0 phase, the cell cycle machinery of the cell is dismantled, and the cell continues to remain in the G0 phase until there is a reason for the cell to divide.
  • There are some cells like the parenchymal cells of the liver and kidneys that enter the G0 phase semi-permanently and can be induced to divide.
  • Even though the G0 phase is often associated as senescence, the G0 phase is a reversible stage where a cell can enter the cell cycle again to divide.
  • The cells in the G0 phase have different regulators that ensure the proper functioning of the cell.

2. Gap 1 Phase(G1)

  • The G1 phase of the cell cycle is a part of the interphase where the cell begins to prepare for cell division.
  • A cell enters the G1 phase after the M phase of the previous cycle, and thus, it is termed as the first gap phase of the first growth phase.
  • In this phase, no DNA synthesis takes place, but RNA synthesis occurs in order to produce proteins required for proper cell growth.
  • G1 phase is considered a time of resumption where the cell finally picks up normal cell metabolism that had slowed down during the M phase of the previous cycle.
  • The process and steps of the G1 phase are highly variable, even within the cells of the same species.
  • The most important event of the G1 phase, however, is the transcription of all three types of RNAs which then undergo translation to form proteins and enzymes necessary for other events in the cell cycle.
  • The duration of the G1 phase is also highly variable among cells. In some cells, it occupies about 50% of the total cell cycle time, whereas, in rapidly dividing cells, the phase is entirely omitted.
  • An important in the G1 phase is the G1/S checkpoint that determines if the cell is ready enough to proceed into the division phase. 
  • At this point, events like the detection of DNA damage and nutrient concentration are performed to ensure that the cell has enough machinery to undergo cell division.

3. Synthesis Phase (S)

  • The S phase or synthesis phase of the cell cycle is a part of the interphase where important events like DNA replication and formation of histone proteins take place.
  • The processes of the S phase are tightly regulated as the synthesis of proteins and replication of DNA require utmost precision.
  • The production of histone proteins and other proteins are crucial in this phase as the newly replicated DNA molecules require histone proteins to form nucleosomes.
  • The entry into the S phase is regulated by the G1/S checkpoint that only allows cells with enough nutrients and healthy DNA to enter the next phase.
  • The phase is moderately long, occupying about 30% of the total cell cycle time.
  • During this phase, the content of DNA doubles in the cell, but the number of chromosomes remain the same as the division of chromosome doesn’t take place just yet.
  • The regulatory mechanism of the S phase also ensures that the process of DNA synthesis takes place before the M phase and with precision.
  • In order to preserve the epigenetic information, different regions of the DNA are replicated at different times.
  • Similarly, actively expressed genes tend to replicate during the first half of the S phase, whereas inactive genes and structural DNA tend to replicate during the latter half.
  • Therefore, at the end of the S phase, each chromosome of the cell has double the amount of DNA with a double set of genes.

4. Gap 2 Phase (G2)

  • The G2 phase or Gap Phase 2 or Growth Phase 2 is a phase of the cell cycle where the cell collects nutrients and releases proteins in order to prepare the cell for the M phase.
  • The G2 phase is also a part of the interphase when the cell is still in the resting phase while preparing for cell division.
  • The G2 phase is also important as it checks for DNA damage (during replication) to ensure that the cell is in proper condition to undergo division.
  • The phase might be skipped in some rapidly dividing cells that directly enter the mitotic phase after DNA replication.
  • It is, however, an essential phase that checks for mutations and DNA damage to prevent excessive cell proliferation.
  • Even though information on the regulation and working of the G2 phase has been studied, its role in cancer initiation and development is yet to be determined.
  • DNA repair is a crucial step in the G2 phase as it repairs breaks that might be present in the DNA strand after replication.
  • The entry of the cell from the G2 phase to the M phase is regulated by the G2 checkpoint, where different proteins and complexes are involved.
  • In the case of DNA damage or insufficient nutrients, the cell remains in the G2 phase and is not passed for cell division.

5. Mitosis Phase (M)

Mitosis Phase (M) of Cell Division Cycle

  • The M phase or Mitotic phase of the cell cycle is the most crucial and dramatic phase of the entire cycle where the cell divides to form identical daughter cells.
  • The most important event of this phase is the karyokinesis (nuclear division) where the chromosomes separate into form two distinct cells.
  • The process of mitosis might differ from one organism to another and even from one cell to another.
  • Mitosis begins with the condensation of chromosomes which then separate and move towards opposite poles.
  • A cell entering the M phase has a 4N concentration of genetic material and ends with two cells, each containing a 2N concentration of DNA.
  • Mitotic cell division occurs via four distinct steps; prophase, metaphase, anaphase, and telophase.
  • Prophase is the first stage of mitosis where the chromosome of the cell divides into two chromatids held together by a unique DNA region called the centromere. As the prophase progresses, the chromatids become shorter and thicker. Prophase also includes the division of centriole that move toward the two opposite ends of the cell.
  • Metaphase is the second and the longest stage of cell division where the chromatids are lined up on the metaphase plate. The chromatids are shorter and thicker and are still held together by a centromere. 
  • Anaphase is the next stage of mitosis involving the splitting of each chromosome into sister chromatids to form daughter chromosomes. After splitting, the chromatids are moved towards the pole due to the shortening of the microtubules.
  • Telophase is the final stage of mitosis which involves the reorganization of two nuclei and the entry of the cell into the next phase. During this phase, a nuclear envelope is formed around the chromosomes to form two distinct daughter nuclei.
  • Telophase indicates the end of the M phase, which initiates the division of cell organelles and separation of cytoplasm into two cells (cytokinesis).

Read More: Mitosis- definition, purpose, stages, applications with diagram

6. Cytokinesis

  • Cytokinesis is the division of cytoplasm into two halves, indicating the end of cell division.
  • Cytokinesis occurs immediately after the M phase to separate the nucleus, cell membrane and the rest of the cytoplasm into two halves to form two distinct and complete cells.
  • The phase begins with the constriction of the cell membrane, which ultimately leads to cleavage and division.
  • The constriction is first observed during anaphase, which continues to grow deeper to finally cause cleavage.
  • The process and mechanism of cytokinesis might be different in different cells. 
  • In some cases, cytokinesis is often considered to be a part of the M phase, but in the case of animal cells, cytokinesis and mitosis might occur independently.
  • The contraction of the cell membrane during cytokinesis is brought about by the contraction of actin fibres that form a bundle, called a contractile ring.
  • In the case of a plant cell, however, a distinct cell plate is formed at the middle of the dividing cell which separates the cytoplasm and cell organelles into equal halves.
  • Cytokinesis, like the rest of the cell cycle, is also regulated by several factors that are responsible for the initiation of division as well as the termination.

Read More: Cytokinesis- Definition and Process (in animal and plant cells)

Cell Cycle Regulation

1. Cyclins

  • Cyclins are a group of proteins that together work to regulate different phases of the cell cycle as core regulators.
  • These proteins regulate the various phases of the cell cycle by either activating the cyclin-dependent kinases or by activating some other enzymes or complexes.
  • Cyclins are specific to different phases as work to regulate different phases of the cycle.
  • In humans, four different cyclins are known, G1 cyclins, G1/S cyclins, S cyclins, and M cyclins. These cyclins, as the name suggests, regulate different phases.
  • The term ‘cyclin’ was given to this class of proteins because of the varying concentration of these proteins in the cell during the cell cycle.

Cyclins- Cell Cycle Regulators

Mechanism 

  • The concentration of these cyclins usually remains low for the most part but peaks dramatically if they are needed during the cycle.
  • The activation of the cyclin proteins is stimulated by the binding of the growth factors to the receptors on the cell, which activate the transcription of the cyclin genes.
  • Most of the cyclin proteins act by binding themselves to the cyclin-dependent kinases, which form a complex. The complex is then responsible for the regulation of the cell cycle.
  • Some cyclin proteins like the cyclin D of the G1 phase (or G1 cyclin) act as rate-limiting proteins for cell cycle progression. G1 cyclins accelerate G1 transition by the overexpression of the cyclin genes. 
  • Even though cyclins do not have any enzymatic activity on their own, they induce different processes in the cell cycle by providing binding sites for other enzymes.

2. Cyclin-dependent kinases (CDKs)

  • Cyclin-dependent kinases (CDKs) are a group of enzymes that work to regulate different processes in the cell cycle after activation by the binding of a cyclin molecule.
  • CDKs are a part of the CMGC group of enzymes consisting of serine or threonine units that are characterized by their dependency on protein subunits.
  • The activity of these enzymes is only observed after the binding of a cyclin molecule followed by the phosphorylation of the threonine unit.

Mechanism

  • The cyclin molecules that bind to these kinases provide additional sequences to the enzymes that are required for their enzymatic activity.
  • The CDKs usually have specificity towards different cyclin molecules, and the binding of cyclin to the CDK molecule determines the specificity of the enzyme towards its substrate.
  • The mechanism of action of these enzymes might differ among different kinases regulation different phases of the cell cycle.
  • The activated CDKs in the interphase undergo phosphorylation and cause inactivation of the retinoblastoma protein (Rb).
  • The inactivation of Rb causes depression of multiple genes encoding proteins that are necessary for DNA synthesis.
  • The regulation of the cell cycle is also brought by the inhibition of the CDKs in which case, CDK inhibitors are involved.
  • CDK regulating the cell cycle is negatively regulated by the binding of other smaller proteins of the Cip/Kip families of inhibitors.
  • These are also specific to the enzymes and act by distorting the cyclin interface and the ATP-binding pocket of the enzyme.
  • These prevent the activation of CDKs, which causes a negative regulation of the cell cycle.

3. Maturation-promoting factor (MPF)

  • Maturation-promoting factor or M-phase promoting factor (MPF) is a large-sized diffusible protein that regulates the M-phase of a cell cycle.
  • The protein consists of two subunits; an inert subunit and a kinase subunit. The kinase subunit is capable of activating the inert subunit as well as other molecules.
  •  MPF is the regulator of the G2/M transition where it activates activities like nuclear envelope breakdown and chromosome condensation.

Mechanism

  • During the interphase, the inert subunit of MPF is inactive due to the presence of an enzyme, Wee1.  
  • The activation of the MPF unit is brought about by CDC25, which results in the binding of the cyclin molecule to the kinase subunit.
  • After the binding of cyclin to cyclin-dependent kinase, and the activation of CDK, transition into the M phase begins.
  • The MPF molecules then act by adding phosphate molecules to the nuclear envelope, which causes the breakdown of the membrane.
  • Besides, it also triggers the formation of spindle fibers as a result of microtubule instability.
  • The MPF kinase also phosphorylases several substances like histone H1, which then promotes chromosome condensation.
  • The activity of MPF is further regulated by other components like p34. The phosphorylation of p34 regulates the activity of MPF. 

4. Anaphase-promoting complex/cyclosome (APC/C)

  • Anaphase-promoting complex (APC) is a protein that regulates the M phase of the cell cycle by inhibiting the action of MPF and causes the destruction of cyclin molecules.
  • This molecule is important during the transition of a cell from metaphase to anaphase of the M phase.
  • The APC is an enzyme that functions in the cell cycle by a different mechanism than CDKs.

Mechanism

  • Instead of activation by phosphorylation and addition of phosphate group to the targets, APC adds ubiquitin on the target molecules. The target molecules are either S and M cyclins or securing.
  • In the case of cyclins, the binding of ubiquitin on the surface causes the movement of the cell to the proteasome. In the proteasome, the cyclins are degraded, which allows the newly formed daughter cell to enter the G1 phase.
  • Besides, it also triggers the separation of sister chromatids during the metaphase. It binds the ubiquitin tag to a protein, called securing.
  • The binding of the tag causes the destruction of securin, which then releases the separase enzyme.
  • The separase enzyme acts on the cohesion protein present at the site of connection between two sister chromatids. The separation of sister chromatids indicates anaphase.

5. p53

  • p53, also called TP53 or tumor protein, is a gene that encodes for the protein that regulates cell proliferation and also acts as a tumor suppressor.
  • The p53 gene is often termed the ‘guardian of the genome’ as it helps in conserving stability of the genome by preventing genome mutation.
  • In eukaryotic organisms, it is important as it suppresses cancer.
  • It also stimulates apoptosis if DNA damage is detected that is irreparable.

P53 Regulation and Signalling

Mechanism

  • The presence of p53 ensures proper cell cycle as it prevents the division of cells with damaged DNA.
  • The concentration of p53 in a normal cell is quite low; however, it increases due to DNA damage or stress signals.
  • The p53 gene can perform one of three functions, cell cycle arrest, DNA repair, and apoptosis.
  • The cell cycle arrest by p53 is mediated by the activation of p21/WAF1. The p21 binds to the G1 cyclin which arrests the cell in the G1 phase as the cyclin can no longer bind to its CDK.
  • The p21 also interacts with proliferating cell nuclear antigen that inhibits DNA replication, causing cell-cycle arrest.
  • Further, it also regulates the G2/M transition as p21 inhibits cyclin B, which is responsible for the activation of CDK in the G2/M checkpoint.
  • In the case of DNA damage, the cell cycle arrest by p53 activates the transcription of proteins involved in DNA repair.

6. Retinoblastoma protein (Rb)

  • Retinoblastoma protein is a nuclear phosphoprotein that helps in cell cycle regulation while also acting as a tumor suppression protein.
  • The primary function of Rb is to prevent excessive cell growth during the cell cycle progression.
  • It acts as a negative regulator of the cell cycle as inhibiting the process.
  • The protein is expressed in both cycling and resting cells which functions by inhibiting a variety of nuclear proteins involved in the cell cycle.
  • It regulates the transition of a cell from the G1 phase to the S phase by inhibiting DNA replication.

Mechanism

  • The family of transcription factors, E2F is the primary target of Rb. These factors regulate the timing and levels of expression of different genes involved in the cell-cycle process.
  • E2F factors target the proteins involved in replication like DNA polymerase and thymidine kinase.
  • In the G0/G1 phase hypophosphorylated Rb binds to E2F which inactivates and prevents cell-cycle progression,
  • Similarly, in the S phase, the chronic activation of Rb leads to downregulation of the necessary DNA replication factors.

Cell cycle checkpoints

1. G1 Checkpoint

  • The G1 checkpoint is the first checkpoint in the cell cycle of a mammalian cell and the start point in the yeast cell that determines whether the cell enters the cell cycle or not.
  • The checkpoint is present between the G1 phase and S phase and is responsible for the entry of the cell in the division phase.
  • Depending on the external and internal factors and stimuli, the decision of whether the cell enters the cell cycle or undergoes the G0 phase is determined.
  • The checkpoints are essential in the cell cycle as they limit the chances of genomic instability arising due to DNA damage during the cycle.
  • The G1 checkpoint is regulated by p53 which aids in the downregulation of tumors and cell lines.
  • In order to cause G1 checkpoint arrest, the p53 regulates the transcription of CDK inhibitor p21.
  • The arrest is stimulated by factors like a break in the DNA double-strand, which prevents the proliferation of irreparably damaged cells.
  • The G1 checkpoint arrest is a positive feedback mechanism where the presence of breaks in the DNA strand enhances the expression of the p53 gene.
  • Because of the proteins involved in the checkpoint, the G1 checkpoint is an important checkpoint during tumor suppression and prevention of excessive cell proliferation.
  • Cells with reparable DNA damage are held at the checkpoint to provide time for repair while others are either signaled for apoptosis or moved to the G0 phase.

Cell cycle checkpoints

2. G2 Checkpoint

  • The G2 checkpoint is the second checkpoint in the cell cycle where is present at the transition between G2 and S phase.
  • The checkpoint prevents the entry of cells into the S phase of the cycle by preventing the activation of regulators like cyclins and CDKs.
  • This checkpoint, like the G1 checkpoint, looks for DNA damage and breaks to prevent the proliferation of mutated or damaged cells.
  • As the checkpoint helps maintain genomic stability, studies on the checkpoint help to understand the molecular mechanism of cancer.
  • The target of the G2 checkpoint arrest is the CDK2 that usually drives the transition from G2 to the S phase.
  • In the checkpoint, DNA damage triggers the activation of the ATM pathway, which causes phosphorylation of ATM and inactivation of checkpoint kinases.
  • The checkpoint also involves the p53 genes which inactivate enzymes by the expression of p21 proteins.
  • Additional pathways in the G2 checkpoint ensure the stability of the arrest by the expression of proteins like Rb and downregulation of several genes that code for proteins required for the S phase. 

3. Metaphase Checkpoint (Spindle checkpoint)

  • The metaphase checkpoint or M phase checkpoint or Spindle checkpoint is the checkpoint during mitosis which checks if all the sister chromatids are correctly attached to the spindle fibers.
  • The checkpoint ensures that all the chromosomes of cells entering the anaphase are firmly attached to at least two spindle fibers from opposite poles of the cell.
  • The separation of chromosomes in anaphase is an irreversible process, which is why this checkpoint is crucial in mitosis.
  • The proteins in the checkpoint look for straggler chromosomes that can be detected in the cytoplasm.
  • The checkpoint acts by negative regulation of CDC20 which prevents the activation of ubiquitin tag by the anaphase-promoting complex.
  • There are different mechanisms to deactivate the checkpoint once all chromosomes are correctly attached.
  • One of the important mechanisms is by transporting the motor complex proteins away from the kinetochores. The proteins are then redistributed to the spindle poles.

References

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