The average human body is made up of an estimated 37.2 trillion cells (to wrap yourself around the idea, if they were seconds, it would equal to almost 12,000 centuries!), and over 200 different cell types.
Amongst this great variety of cells, some do not replicate in the whole span of a human life (such as egg cells), whilst others are constantly replicating to regenerate damaged tissue (such as red blood cells).
Some of the occasions where cells replicate include:
Embryonic development – before we are born, we develop from a single cell into a more developed organism through cell division and cell differentiation.
Post-embryonic development – after birth, the human body grows in size with the aid of new cells.
Maintaining tissue homeostasis – to maintain normal cell count after a wound is sustained or after cell death.
In disease – in some cases, cell replication can be harmful to the body. Uncontrolled cell proliferation (cancer) is an example of this.
What is the cell cycle?
Cell replication is controlled by the cell cycle, which is the ordered series of events that ensures duplication of the genome and division into two daughter cells. There are several tasks a cell needs to do to complete this cycle - it must firstly double in size, then copy its DNA, or genetic material, and lastly, split into two separate daughter cells. These processes are organised, predictable and constant.
In general, the cell cycle consists of four stages : the Gap 1 (G1) phase, S phase, Gap 2 (G2) phase, and mitotic (M) phase.
Cells that are not prepared for replication due to lack of nutrients or small size enter a phase named Gap 0 (G0) where they remain until they are suitable to enter the replication cycle.
Since the first three listed occur between two mitosis processes and are jointly named interphase, this means the cell cycle can be divided into two main sections: interphase and M phase (cell division).
There also are several checkpoints during the cell cycle that will be explained next. The different cell cycle phases and checkpoints are shown below in Figure 1.
Figure 1: An overview of the cell cycle. The cell cycle is divided into “interphase” and “M phase” (cell division). “Interphase” includes G1, S, and G2 phases where DNA is prepared for cell division whilst “M phase” includes the steps of mitosis: prophase, metaphase, anaphase, telophase, and cytokinesis. There are cell checkpoints between G1 and S phases, at the end of G2 and in between metaphase and anaphase. This figure was adapted from Online Biology Notes.
Interphase is the part of the cell cycle where cells prepare for replication. The cell grows in size and there are checkpoints included to make sure that replication is possible. It involves three phases:
G1 phase: This phase is when the cell grows in size. There is a checkpoint between the G1 and S phases, where the cell evaluates if it should move to synthesise DNA.
S phase: The chromosomes replicate to sister chromatids.
G2 phase: Cell growth, organelle replication, and preparation for mitosis occurs. At the end of G2, one more checkpoint is in place to ensure no mistakes have occurred and that it is safe for the cell to move on to replication.
The mitotic phase, on the other hand, is the section of the cell cycle where DNA is replicated, and the cell divides into daughter cells. It includes prophase, metaphase, anaphase, telophase, and cytokinesis.
Figure 2: An overview of all the phases of the cell cycle, and the details of each phase. It includes interphase (in green writing) with G1, S, and G2 phases as well as mitosis (in blue writing) with prophase, metaphase, anaphase, telophase and cytokinesis.
Before exploring each of these phases in more detail, let's introduce some basic concepts.
DNA packing
The DNA found inside a single cell, when stretched out, has a length of around 2 meters. In order for DNA to fit inside the nucleus of a cell, DNA must be organised and condensed into a smaller size. DNA is wrapped around histone proteins, forming 8-histone nucleosomes, allowing for organisation and chromatin condensation. Chromatin is defined as the uncondensed genetic material whilst chromosomes are the condensed genetic material. Sister chromatids are the two copies of a chromosome’s genetic material. They are identical to each other and are attached together at the centromere.
Figure 3: Representation of DNA inside the nucleus of a cell. The double helix is tightly packed around histone proteins, forming nucleosomes. These nucleosomes come together under uncompacted chromatin that later compresses into chromosomes. Chromosomes are in turn made of two sister chromatids.
Exploring the cell cycle and its checkpoints
Checkpoints are internal control mechanisms of the cell cycle that monitor the process of cell replication. They are “red lights” that trigger the next stage of the cell cycle or “red lights” that delay the next stage of the cell cycle. They depend on factors such as the external environment, cell size, access to nutrients, genetic mutations or cell health. A cell can only enter into cell division if they pass the designated checkpoints.
We will now explore the different phases of interphase and its checkpoints.
In order for DNA replication to take place, the cell must first make sure it is prepared for replication.
G1 phase and the G1/S checkpoint
During the first phase, G1, which occurs between the end of the cell division phase of mitosis and replication in S phase, the size of the cells gets larger, and their cellular content is duplicated. More specifically, the centrosomes are the ones being duplicated. You can think of the G1 as the training camp for the cells to be ready to be employed.
In order for the cell to move into the S phase, it must go past a checkpoint known as the G1/S restriction point. The cell can only go through this checkpoint if it has reached an appropriate size and enough energy resources. External factors such as mitogens (signals from outside a cell that trigger progression of the cell cycle) and growth factors play a key role in carrying the cell past this point.
In order for the restriction point checkpoint to be activated, a mitogen or a growth factor must interact with cell surface receptors. This interaction activates a signal transduction pathway. Let’s look at a common example of this:
A mitogen or a growth factor binds to receptors on the surface of a cell. This binding triggers a signal pathway that starts with the activation of GTPase Ras (regulatory proteins). GTPase Ras activates a MAP kinase cascade which in turn activates transcription regulatory proteins that enter the cell nucleus. This provokes an increase in immediate early gene expression, particularly of genes such as those coding for transcription regulation protein Myc. Myc promotes cell-cycle entry and stimulates the transcription of genes that accelerate cell growth. The MAP kinase cascade also activates delayed-response gene expression, such as those that activate the G1-Cdk, which in turn triggers the activation of the gene-regulating protein E2F. E2F triggers entrance into S-phase, overcoming the G1/S checkpoint and moving into the next stage of the cell cycle.
Figure 4: Representation of the overcoming of the G1/S checkpoint and entering into S phase.
If the cell is unable to overcome the G1/S checkpoint, it has two possible fates:
The cell moves into the G0 phase, a temporary phase where the cell is put into arrest to wait until more suitable conditions occur.
The cell undergoes apoptosis, which is the programmed death of a cell and is also known as “cellular suicide”. It differs from necrosis, which is where a cell dies due to irreparable damage. During apoptosis, the cell organelles are packaged into vesicles that are collected by immune cells for disposal. Apoptosis requires energy thus it is an active process, whereas necrosis doesn’t. Apoptosis is mediated by enzymes called caspases, which trigger cell death by cleaving specific proteins in the cytoplasm and nucleus.
CDKs also play a huge part. CDK, or cyclin-dependent kinase, which, in general, adds phosphate to proteins, cause the cell to move from one phase to another. This is due to the MPF, or maturation promotion factor, that utilises and includes CDK that inherently triggers the progress and movement to the next relative phase.
S phase
On passing the G1/S checkpoint, cells then move into the S-phase, or the synthesis phase, where the DNA-packed chromosomes can replicate. Just like during G1, cell growth continues in the S-phase as well, and there is “synthesis” of proteins and enzymes that are necessary and required for DNA synthesis. After the S-phase, the particular cell has double the number of chromosomes than it did originally. Once this is achieved, the cell can continue its life cycle to phase G2.
G2 phase and the G2/M checkpoint
During the G2 phase, the cell seriously prepares for mitosis by undergoing very rapid cell growth and protein synthesis. There is another checkpoint before moving onto mitosis to ensure that these processes are correctly done. It also needs to be stated that G2 is not a fully necessary part of the cell cycle as some cells go straight from DNA replication and begin mitosis. The G2 phase is also absent in meiosis as the first meiotic division reduces the ploidy, or the number of sets of chromosomes in the cell, by a factor of two.
Figure 5: The interphase stage of the cell cycle in detail, including the position of centrosomes and development in this phase.
Mitosis: the story of how a cell divides
Once checkpoints have ensured that it is safe for the cell to divide, the cell enters into mitosis.
Prophase and prometaphase
During the first phase of mitosis – prophase – the chromosomes become shorter and thicker due to chromatin condensation, resulting in visible sister chromatids. The centrosomes go to the poles as mitotic spindles, or microtubules, form. These mitotic spindle fibres interact with the sister chromatids. In each pair of these sister chromatids, there are 2 kinetochores (disc-like protein structures) attached to the microtubules on the opposite pole. The microtubule network is broken down by the release of tubulin dimers from the plus-end of the specific microtubule. These move to the cell poles from which mitotic spindles can grow from.
The mitotic spindle, sometimes referred to as the spindle apparatus, is a dynamic structure that moves sister chromatids apart during mitosis. They are also made up of microtubules. Centrosomes are also significant for mitotic spindles; since they determine the orientation of the spindles, they are technically responsible for creating the plane at which the cell divides.
Between prophase and metaphase, prometaphase occurs. This is when the nuclear envelope breaks down and the compacted chromosomes attach to the mitotic spindles. It is important to remember each of the chromosomes consist of two chromatids.
Figure 6: Representation of the development in prophase. Includes both the dissolving of the nuclear membrane and chromosome duplication.
Metaphase
During metaphase, pairs of sister chromatids align along the metaphase plate. Also, each pair of chromatids is attached to both poles by the kinetochore microtubules, as explained earlier. Kinetochores are large structures formed at the centromeric region of each eukaryotic chromosome. They capture the spindle microtubules during mitosis and serve to attach the chromosomes to the spindle poles. During metaphase, the structure of the chromosomes is slightly different. Chromatin is fully condensed; the chromosomes are compact, and each are fully visible as separate elements.
However, when the microtubules do not attach equally to each chromatid, they are referred to as unequal tensional chromosomes. The assistance of moving molecules along microtubules is achieved with kinesins (motor protein). Kinesin-5 pushes the two poles of the spindles apart by walking toward the plus-end of the antiparallel microtubules, lengthening the spindle. In direct opposition to this is the action of kinesin-14, which pulls the two poles toward each other by walking the minus-end of the interpolar microtubules, causing it to shorten.
Figure 7: Representation of metaphase. Includes the alignment of chromosomes in the metaphase plate.
Anaphase
The next phase is anaphase, in which sister chromatids are pulled apart to create two identical single-chromatid chromosomes. The sister chromatids are pulled apart to opposite poles of the cell by the mitotic spindle, resulting in microtubule shrinkage and kinesin motors. As in prophase, the centrosome positioning determines the plane of the division of the cell. It is important to mention that during anaphase, the poles move further away from one another.
Figure 8: Representation of anaphase, showing the pull of chromosomes to opposite poles by microtubules.
Telophase
The structure of chromosomes during the next phase, telophase, includes having two separate bundles of single-chromatid chromosomes. Chromatin also begins to de-condense once it reaches the poles of the cell. During telophase, the nuclear membrane reforms to form two separate nuclei.
Figure 9: Representation of telophase, including the redevelopment of the nuclear envelope surrounding the chromosomes of the two future daughter cells. Also shows the change in the shape of the cell ahead of cytokinesis.
Cytokinesis: breaking apart from the mother cell
Once the nuclear membrane has reformed and chromosomes decondense into chromatin, cytokinesis takes place. In this final stage, the mother cell physically divides into two daughter cells. How this division takes place depends on the cell type.
In animal cells, a cleavage furrow forms on the original metaphase plate, separating the nuclei of both daughter cells. The cleavage furrow forms in the original metaphase plate and “pinches” off to separate the daughter cells. This is possible thanks to the contractile ring, a ring of protein filaments that shrinks the center of the mother cell, displacing the plasma membrane until the cells separate.
In plant cells, a different process takes place, due to the rigidness of the cell wall so a septum grows on the metaphase plate to form the two new cells. A cell plate forms from Golgi apparatus produced vesicles that gather in the equator. These vesicles diffuse together to form a cell plate, which later evolves into the cell wall of the daughter cells.
Figure 10: Representation of cytokinesis, including the difference between animal and plant cells. In animal cells, a cleavage furrow pinches the daughter cells off whilst in plant cells, a cell plate forms to separate daughter cells.
After cytokinesis, the cell cycle starts again at the G1 phase, where it remains until a need for replication appears or apoptosis (programmed cell death) takes place.
The cell cycle and its role in cancer treatment
Cancer cells are abnormal and do not respond to common signals needed for normal cell cycles. These cells also grow and divide, clearly showing that with time, the cells get further and further away from the standard. This uncontrolled replication accelerates and can cause tumors to form in different areas of the body.
As explained by Nature, “When a mutation gives a cancer cell a growth advantage, it can make more copies of itself than a normal cell can — and its offspring can outperform their noncancerous counterparts in the competition for resources.” This is the first process of the multi-phase process of the formation of cancerous cells. This process may be followed by a few other similar ones, giving the cell even more of an advantage and gain toward what it is trying to achieve. This is especially harmful and accelerated if the genes responsible for cell repair such as BRCA1, BRCA2 and PALB2 are damaged.
Within the normal cell cycle, there are many different genes that control and manage the entire cell division process. Cancer cells become dormant due to mutations forming in various genes during the growth process. One of the most commonly mutated cells in the process of becoming cancer is the protein family Ras. All of the Ras protein family members are also a part of a protein class named small GTPase. These proteins control cellular signal transduction, or transmission of signals, in the cells. They are responsible for ‘turning on’ other genes during the necessary growth and survival processes.
Proteases have the ability to degrade extracellular matrices. This accentuates the effects of invasion and metastasis. Metastases develop when the dormant cancer cell leaves the main cancerous tumor and even goes to the bloodstream of the patient. In some cases, the metastasis may break away from the main tumor and enter the lymphatic system as well. This is particularly perilous as these are both systems that carry moving fluids around the body and cancer has easier access to different parts of the body. This is due to proteases. Different forms of cancer have different effects on the proteolytic activity and a smaller activity of “opposing endogenous inhibitors,” as stated by Mayo.
With regard to treatments, targeted therapies and immunotherapies have presented hopeful endeavours towards treatment and curing of cancer. Numerous challenges have been presented with these new endeavours including drug resistance in patients. Cancer cells may be more dangerous as some patients experience cancer relapse, where the cancer cells ‘hide’ out in other organs before reactivating to cause harm to the patient.
The takeaway message
The cell cycle is a vital process of any living organism, able to regenerate damaged tissue, growth and reproduction. Understanding the cell cycle is the basis of the study of biological systems. The cell cycle is a complex and regulated process where mutations are usually avoided. Nonetheless, when mutations take place, cells can become cancerous and reproduce at very fast rates. The study of the cell cycle is a fundamental pillar to understanding cancer and many other diseases.
Author
Covadonga Piquero Lanciego
BSc Biological and Biomedical Sciences
University of Dundee