PHASE G1 (CELL CYCLE): DESCRIPTION AND IMPORTANCE - BIOLOGY - 2024

The G1 phase is one of the stages in which the interface of the life cycle of a cell is divided. Many authors refer to this as the "growth phase", since during it the most significant growth of a cell occurs.

During the G1 phase, therefore, various intracellular metabolic changes occur that prepare the cell for division. At a certain point in this phase, known in some texts as the "restriction point", the cell engages in division and continues to the S phase of synthesis.

Cell cycle phases (Source: Richard Wheeler (Zephyris). Spanish labels by Alejandro Porto. / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0) via Wikimedia Commons)

The cell cycle

The cell cycle consists of the ordered sequence of events that occur in a cell in preparation for its division. It is generally defined as a process divided into 4 stages by which cells:

- increase in size (phase G1)

- copy their DNA and synthesize other important molecules (synthesis phase or S phase)

- prepare for division (G2 phase) and

- divide (M phase or mitosis)

In accordance with the above, the cell cycle can be divided into two great "moments": the interface and mitosis. The interface consists of the G1, S and G2 phases, which comprise all the processes between one mitotic division and another, which is why it is said that a cell spends most of its life in interface.

Regulation

According to the "stimulatory" or "inhibitory" messages that a cell receives during the interface, it can "decide" whether or not to enter the cell cycle and divide.

These "messages" are carried by some specialized proteins, including growth factors, receptors for these growth factors, signal transducers, and nuclear regulatory proteins.

In addition, cells also have checkpoints or restriction points in the different phases, which allow them to ensure that the cell cycle progresses correctly.

Many of the "non-reproductive" cells are constantly dividing, which is why they are said to be always in an active cell cycle.

Cells that do not divide or that are quiescent cells enter from the G1 phase to a phase called G0, during which they can remain viable for many months and even years (many of the cells of the human body are in this phase).

Terminally differentiated cells cannot leave the G0 phase and enter the cell cycle, as is the case with some neuronal cells, for example.

Description of phase G1

As mentioned, the G1 phase of the cell cycle can be considered a growth phase, since after a cell divides, its daughter cells enter this phase and begin to synthesize the enzymes and nutrients necessary for the subsequent replication of DNA and cellular division.

During this phase, a large quantity of proteins and messenger RNA is also produced, and their duration is highly variable, depending, generally, on the amount of nutrients available to the cell.

Subphases of G1

The G1 phase can be described as consisting of four “sub-phases”: competition (g1a), entry or entry (g1b), progression (g1c) and assembly (g1d).

Competition refers to the process by which a cell entering G1 absorbs nutrients and extracellular elements through its plasma membrane. The entry or entry consists of the entry of these "materials", which contribute to the growth of the cell.

This growth occurs during the sub-phase of progression, which ends when these materials assemble to form other cellular structures and complete the cell's progress into the G1 phase and toward the checkpoint.

Control or "restriction" points

All cells have regulators that allow them to monitor their growth. At the end of the G1 phase there is a checkpoint that ensures that protein synthesis has occurred properly and that all cellular DNA is "intact" and "ready" for subsequent phases.

The specialized "safeguards" found at this checkpoint are proteins known as cyclin-dependent kinases or CDKs, from Cyclin-Dependent Kinases, proteins that also participate in the beginning of DNA division during S phase.

Cyclin-dependent kinases are protein kinases that are characterized by requiring a separate subunit (a cyclin) that provides the essential domains for enzyme activity.

Cell cycle checkpoints (Source: WassermanLab / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0), via Wikimedia Commons and modified by Raquel Parada)

They are responsible for the addition of phosphate groups in serine and threonine residues located in specific domains of their target proteins, altering their activity.

They have very important functions both in the control of cell division and in the modulation of gene transcription in response to different extra- and intracellular signals. Thanks to these proteins, not only the G1 phase, but also the S phase and the G2 phase function as a “clock” of the cell cycle.

The G1 / S checkpoint

The checkpoint in the G1 phase is one of the most important and it is there where the cell “decides” if it has grown enough and if the nutritional conditions around it and inside it are adequate to start the genomic replication process.

The cyclin-dependent protein kinases of subfamily 2 (Cdk2), which depend on cyclin E., participate at this phase transition point.

Once the cell "passes" this checkpoint and enters the next phase, the activity of Cdk1 is "turned off" again by destroying its cyclin portion, which is why it has been shown that these proteins are inactive until that there are cyclins available in the cytosol.

Importance

The G1 phase is not only essential for cell growth and for the preparation of subcellular structures for division, but its control point is critical from the point of view of the regulation of cell proliferation.

The "deregulation" of proliferation control is one of the main drivers of tumor development in different types of tissues, since many of the checkpoints of the cell cycle are "bypassed" during tumorigenesis.

References

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