Cell division or cell cycle is the cell’s growth and duplication process. One parent cell splits and engenders two identical daughter cells in this process. Those upon maturing create some more offspring in a cyclical manner. Cell division plays a vital role in bodily growth or healing internal or external wounds. All prokaryotes and some single-celled eukaryotes are known for asexual reproduction called binary fission, which is cell division itself. The cell cycle is divided between a preparatory and dividing phase, namely interphase and mitosis. 

The interphase has three stages- G1, S, and G2.

Phases of the cell cycle: The cell division is the passing of genetic materials from one cell to the next. The DNA is replicated in S phase of the interphase. The cell is divided into two in M phase or through mitosis. Besides the S phase (synthesis) and the M phase (mitosis), there are two gap phases: G1 and G2. Both the gap phases fall under the preparatory time and help produce materials for the cell’s sustenance and ability to reproduce.

• Interphase: This is the phase where plenty of protein is produced, DNA and centrosomes are replicated, and cells are nourished and equipped for mitosis. This is the phase where a cell grows and develops.
• Mitosis: In this phase, the chromosomes are separated to form two nuclei, keeping the total number of chromosomes intact. The identical genetic figuration of the mother and daughter cells is maintained. For this function, mitosis is also known as equational division. During this, the whole of cellular energy is focused on cell division. No further growth after the interphase takes place in the cell cycle. There are also five stages of mitosis known as prophase, prometaphase, metaphase, anaphase, and telophase.

First stage of interphase- G1 phase: The G1 phase takes the maximum time of the cell division compared to other phases. It continues for 30-40% of the cell cycle. During this, a cell is grown and passing the restriction point gets to the next phase of synthesis. For its crucial role in the cell’s nutrition, besides the gap 1 phase, we know it as the growth phase too. 

What happens in the G1 phase?

Proteins required to proliferate are synthesised in the first gap phase. Proteins like mRNA are produced in the G1 phase, which is essential to DNA replication.

For a better understanding, the G1 phase is classified into four stages- competence, entry, progression and assembly. They are termed as g1a, g1b, g1b and g1c, respectively. 

Competence is the ability to receive nutrients from outside of the cell membrane. The next step is the entry where the actual admittance takes place.

 After the necessary ingredients are allowed inside the cell membrane, they are absorbed and used for growth in the stage that is called progress. In the final stage, all elements are brought together and put in order to enter the next stage. This is called assembly. All these stages pass within three hours, and then the cell must face a restriction point that decides whether it will go along the cell cycle or be rested for an indefinite amount of time.

• Restriction point: This point is also known as the G1/S checkpoint. However, many argue that the term checkpoint does not capture the meaning entirely. From this point, a cell either moves forward in the cell cycle or continues to produce daughter cells or remains alive, only suspending their cell division. This state of living by cells is called a G0 phase. It is uncertain whether a cell in its lifetime would enter this phase. Moreover, equally unsettling is the question of whether they will come out. It measures if the cell size is big enough for division, whether it can receive growth signals, if it is rich in nutrition, and has any damaged DNA.
• G0 phase: G0 phase was discovered long ago, but there was debate stirring for long whether it was a part of the G1 phase or itself something else. Nowadays, new research has unearthed many interesting notions of this phase and its existence in our bodies. It is found that neuronal cells display this phase in an astonishingly large number. 

If a cell cannot overcome the restriction point, instead of entering the S phase, it enters the G0 phase, where the multiplication of cells is not performed. There are some cells that, after entering this stage, revert to G1 and pass the subsequent phases, while some remain in this position forever. External and internal factors determine whether a cell will enter this phase, nutrition deficit being one of them. 

• Quiescent phase: This is the G phase where the cells reside temporarily and reenter the cell division process via G1. 
• Senescent phase: The other variation of the G phase is where cells live for the rest of their time, functioning metabolically but stopping the replication altogether.

Conclusion:

The G1 phase thus discussed sets the ground for cell division. It is the first phase of the interphase that prepares the cell for division in mitosis, the next big phase. It is also responsible for choosing the fit cells to carry on the genetic materials.