Cell Cycle

We explain what the cell cycle is, its phases, control points and regulation. Also, its implication in the development of cancer.

What is the cell cycle?

The cell cycle is the ordered and sequential set of events that take place within all cells in general. They involve their growth and eventual reproduction in two “daughter” cells. This process is fundamental for the existence of multicellular beings.

It begins with the appearance of a young cell and ends with its maturation and cell division that is, the creation of two new cells. It is carried out according to a set of stimuli and biochemical responses interpreted by the cell nucleus, which guarantee the orderly reproduction of the body's tissues.

Therefore, cells normally begin their cell cycle when environmental conditions are conducive to doing so. However, the cycle does not always occur in the same way, with important variations existing in animal and plant cells or prokaryotes and eukaryotes. However, occurs in all living things with similar purposes and similar stages.

Cell cycle phases

The stages of the cell cycle are described according to the formula:

• g_1. From English Gap 1 or Interval 1
• S. Synthesis or Synthesis
• g_2. Gap 2 or Interval 2
• M. M-phase or Phase M, whose name is due to the fact that it includes mitosis or meiosis, before the cytoplasmic division or cytokinesis.

Cells, before starting the cell cycle, are called “quiescent” (meaning that they choose to be still), and once they have started the cell cycle, they are called “proliferating” (meaning that they multiply rapidly).

The cell cycle is not linear, but circular, since young cells can choose to repeat the process, thus creating two new ones each, as needs dictate. And broadly speaking, the different stages that comprise it are organized based on two separate phases, which are:

• The interface This first phase includes the G1-S-G2 stages, and during them it grows to its appropriate level to begin the duplication of its genetic material, copying it completely according to its DNA.
  • Gap Stage 1 The cell physically grows, duplicating its organelles and the proteins necessary for the following stages.
  • Stage S A complete copy of the cell's DNA is synthesized, as well as a duplicate of the centrosome, which will help separate the DNA at later stages.
  • Gap Stage 2 The cell grows even larger, generates new proteins and organelles, and prepares for mitosis, cell division.
• The M phase The mitotic phase begins when the cell has already duplicated its genetic material and organelles, ready to divide into two identical individuals. The beginning of mitosis begins with the separation of DNA into two double strands, and the two new cell nuclei move away from each other, towards opposite poles.

The M phase is divided into four different phases: prophase, metaphase, anaphase, telophase.

Thus, when cytokinesis begins, which is the preparation for the definitive separation of the two new cells, each nucleus remains separate. A barrier begins to be generated between both cells, which will later be part of the plasma membrane itself, and finally the physical separation occurs.

Cell cycle regulation

The cell cycle must be given under very specific conditions which merit very specific control and regulation instances. So without precise instructions, not only does the entire cycle not begin, but the transition from one stage to the next will not occur.

In the first instance, control is exercised by genes in the cell's own genetic code. There are the instructions to make or modify proteins to trigger each stage of the cycle. The set of enzymes that activate, facilitate or complete each phase are cyclins and kinases cyclin dependent.

Cell cycle checkpoints

There are, especially during mitosis, a series of cell cycle checkpoints, in which the process is monitored and ensures that no errors have been made. These are transitory existence verification routes that is, once their function has been fulfilled and it has been verified that the process continues without errors, they disappear.

Furthermore, if the problem has not been satisfactorily resolved after a period of time, these checkpoints prepare the cell to undertake self-destruction or apoptosis.

The checkpoints during mitosis are:

• At the end of the G1 stage and before the S This is the checkpoint for unreplicated DNA, which inhibits the Cdc25 gene, which in turn activates Cyclin A/B Cdk1. Thus, it prevents the cycle from continuing.
• Before anaphase in mitosis It is a checkpoint that guarantees the separation of chromosomes, and operates by activating the Mad2 protein that prevents the degradation of segurin, until conditions are appropriate.
• DNA damage checkpoints in G1, S or G2 In the event that cellular damage occurs, specifically to the genetic material, the p53 protein will be activated, which allows DNA repair. If this fails, apoptosis processes are immediately activated.

Importance of the cell cycle

The cell cycle is the fundamental cycle of cell reproduction, which allows the growth of multicellular organisms and tissue repair. In addition, it causes the proliferation necessary to, for example, generate the critical cell mass for form embryos of future new individuals of the species.

It is a process that is carried out constantly. It is encoded in our DNA itself, so it is one of the fundamental and original cycles of eukaryotic cellular life.

Cancer and the cell cycle

As is known, Cancer is a disease in which certain cells in certain tissues begin abnormal reproduction unstoppable, of dysfunctional cells. This process, which may well cause death if not stopped in time, is not interrupted by the natural process of apoptosis cellular, which requires medical intervention.

Many specialists suggest that the beginning of carcinogenic process is in certain cell cycle regulatory genes that do not work well or were damaged, subjecting the process to a lack of control that in turn generates other failures and culminates in the formation of a tumor. These genes are known as oncogenes, and their precursors are known as protoncogenes.

References

• “Cell cycle” in Wikipedia.
• “The cell cycle – Biology” (video) in Educatina.
• “Phases of the cell cycle” in Khan Academy.
• “Cell cycle” in Hypertexts of the Biology Area (UNNE).
• “The Cell Cycle” at the University of Leicester.
• “Cell Cycle” in The Encyclopaedia Britannica.