Table of Contents
Introduction to Cell Cycle
Cell cycle or cell division is a sequence of events occurring in a eukaryotic cell leading to its division, DNA replication, and production of two daughter cells. Commonly called mitosis, this cell cycle differs from meiosis in which four cells with a haploid number are produced after a complete cycle.
The cell cycle is a vital phenomenon which leads to the formation of a fully developed organism from a single fertilized egg. Any future increase in a number of cells involves the cell cycle.
Stages of Cell Cycle
The cell cycle has two main phases which are further divided into distinct stages. The two main phases include:
- G1 phase
- S phase (Synthesis)
- G2 phase.
Mitotic (M) Phase
G0 is another phase where the cells have left the cell cycle temporarily and reversibly. The cells do not divide during this phase.
A summary of events taking place during these phases is tabulated below:
|G1 (Gap 1)||Cell prepares for DNA replication, and cell grows.|
|S (Synthesis)||DNA replication occurs|
|G2 (Gap 2)||Cell grows and prepares for final cell division|
|M (Mitosis)||Cell divides into two daughter cells|
Activation and progression of a phase are dependent on completion of the previous phase. For a successful completion of the cycle, the cell cycle is regulated and controlled by cell cycle checkpoints at different phases of a cycle.
Cell Cycle Checkpoints
The cell cycle is monitored and controlled by certain regulatory proteins called cell cycle checkpoints. These checkpoints are present at certain critical areas of the cycle to ensure that the previous phase has completed without any error or whether any repair is needed. Once all the necessary repairs of the previous phase have been completed, only then these checkpoints allow the cell to proceed to the next phase.
There are three checkpoints in each cell cycle:
- G1/S Checkpoint
- G2/M Checkpoint
- Metaphase Checkpoint
G1/S checkpoint, also called restriction point, is the first integral checkpoint of the cell cycle located between G1 and S phase. This checkpoint ensures that the cell has all the desired requirements to undergo DNA replication and hence decides where the cell delays G1 phase, enters G0 or proceeds to S phase.
This decision is based on both internal and external factors. A cell with any deficiency or defect that may hinder the DNA replication will not be allowed to go beyond this stage into S phase.
After the cell undergoes successful DNA replication in S phase and is growing in G2 phase, this checkpoint assesses:
- if the DNA replication was successful and there lies any need to repair defective replicated DNA or not.
- An adequate amount of cytoplasm and phospholipids are present for the cell to undergo complete division into two daughter cells.
- if it is the right time for the cell to proceed to mitosis phase for the final division.
The G2/M checkpoint is regulated by M-phase promoting factor (MPF) that consists of a group of enzymes and proteins called cyclin-CDK complex.
Metaphase Checkpoint (Spindle Checkpoint)
Once the cell is in metaphase, this checkpoint ensures:
- if the mitotic spindle has formed and
- if the chromosomes have aligned themselves at the mitotic plate and adequate tension exists in the spindles.
The presence of tension in the spindles ensures there is no inhibition of anaphase-promoting complex (APC/C) and thus the cell enters the next stage, anaphase.
Cell Cycle Checkpoints – A Cyclically Activated Protein Kinase System
The cell cycle checkpoints consist of a group of protein kinases called cyclin-dependent kinases (CDK). The activity of these CDKs is controlled by an array of enzymes and proteins collectively referred to as cyclins. This cyclin-CDK complex is the basis of the checkpoints that regulate the cell cycle.
The activity of CDKs oscillates throughout the cell cycle, thus, leading to cyclic phosphorylation of proteins that initiate, regulate, or inhibit the events of the cell cycle.
Out of four classes of cyclins that bind to CDKs, three of them that are required by eukaryotic cells include:
- G1/S cyclins (bind to CDKs at the end of G1 phase and let the cell decide for DNA replication)
- S-Cyclins (bind CDKs during S phase and initiate DNA replication)
- M-Cyclins (regulate and promote events of mitosis)
Cyclin-CDK complexes that are at play during different phases and checkpoints of the cell cycle are:
Cdk2, Cdk4, Cdk6
Checkpoints and their Cyclin-CDK Complexes
G1/S Checkpoint and Cyclin-CDKs
A rise in cyclin D cause it to bind to CDK4 and CDK6, and this complex causes phosphorylation of proteins. Meanwhile, accumulation of cyclin E cause it to form a complex with CDK2 and thus the cell enters an irreversible stage of DNA replication.
G2/M Checkpoint and cyclin-CDKs
At this checkpoint, cyclin B binds to CDK1 which in turn regulates the events and generates activation of subsequent proteins and the entry of the cell into the Mitotic phase.
During this stage DNA damage is also assessed and in case of defective or incomplete DNA replication, activation of CDK1 and CDK2 along with p53 occurs to halt the cell cycle and prevent it from entering mitosis.
Metaphase/Spindle Checkpoint and cyclin-CDKs
The tension built within the spindles activates anaphase-promoting complex (APC) which degrades cyclin B and in turn causes breakdown of proteins (securing, separase, cohesin) which hold the chromosomes together.
Role of p53 Gene
Damaged DNA during a cell cycle senses and causes activation of the tumor suppressor gene called p53. Commonly known as “the guardian of genome,” this gene ensures that no cell passes on any damaged DNA into the daughter cells.
During G1/S checkpoint if a damaged DNA is sensed, it halts the cycle by activating CDK inhibitor (CKI) that temporarily stops the cell cycle by binding to cyclin-CDK complex till the DNA is repaired. Then it activates production of DNA repairing enzymes. If the damage is unrepairable, the p53 gene allows the cell to undergo programmed cell death.
In case of defective or absence of p53 gene, damaged and mutated DNA is passed on to the cells and leads to the development of cancer. Mutated p53 gene causes the cell to lose control of the cell cycle.
These genes regulate cell growth, differentiation, and proliferation, and tend to inhibit apoptosis. Any mutation in the gene or overexpression will lead them to become oncogenes that lead to uncontrolled cell proliferation and in turn cancer.
Some proto-oncogenes include growth factors, RAS proteins, and Src kinase.
- Growth factors induce cell proliferation. Growth factor receptors are increased in cases of breast cancer.
- RAS protein is involved in signaling a pathway in cell proliferation. Mutation of this leads to myeloid leukemia, thyroid tumour, and adenocarcinoma of pancreas and colon. It is activated by mutation in 20 %—30 % of all cancers.
- Src kinase regulates cell proliferation, migration, differentiation, and apoptosis. Mutation leads to head and neck cancers, brain cancers, etc. It is activated by mutation in 2 %—5 % of all cancers.
These genes control cell division by slowing it down, managing DNA repair, and guiding the cell towards apoptosis. Mutation in this gene will lead to the development of cancer because damaged DNA will then accumulate and cells will escape apoptosis.
Tumor-suppressor genes are grouped into caretaker genes, gatekeeper genes, and landscaper genes. Common tumor-suppressor genes include Retinoblastoma protein (pRb) and p53 gene.
pRB is mutated in 40 % of all cancers, and p53 is mutated in 50 % of all cancers.
The Two-Hit Hypothesis
This hypothesis, which was first proposed by A.G. Knudson, states that both alleles that code for a protein must be affected for it to have its effect. If one allele is defective, the other one will still produce normal protein.
Thus this hypothesis indicates that mutated tumor-suppressor alleles are recessive while mutated oncogene alleles are dominant.