How Does Interphase Prepare a Cell to Divide

The life cycle of the cell is known as the cell cycle. It consists of a series of events occurred between the cell’s birth and division into new daughter cells. In order to divide, a cell should complete several tasks. The most important two targets are DNA replication and the protein synthesis. These two targets are completed through a series of sequential events found in the cell cycle. The eukaryotic cell cycle is composed of three sequential periods called interphase, mitotic phase, and the cytokinesis.

This article explains, 

1. What is Interphase
2. How Does Interphase Prepare a Cell to Divide
      – G₁ phase
      – S phase
      – G₂ phase
      – G₀ phase

What is Interphase

Interphase is the first phase of cell cycle, where the cell prepares for the upcoming nuclear division. It consists of three phases, which are called G₁ phase, S phase and G₂ phase. G₀ phase is another special phase where the cell rests before entering the cell cycle is found. During G₁ phase, the cell synthesizes more ribosomes and proteins in order to grow to its proper size. During S phase, DNA is replicated and the proteins that package DNA are synthesized along with more cell membrane material. During G₂ phase, organelles divide. The cell can also enter G₀ phase while it is in its G₁ phase. Generally, a cell which enters G₀ would be either matured into a special function or no longer re-enter the cell cycle. A cell in its interphase is shown in figure 1.

Figure 1: An Interphase Cell

How Does Interphase Prepare a Cell to Divide

In the following section, we’ll examine how interphase prepares a cell to divide by analyzing the different phases of interphase.

G₁ phase

G₁ phase is the first gap phase of the interphase. During the G₁ phase, the cell synthesizes proteins in order to increase the size of the cell. The concentration of proteins in a cell at G₁ phase is estimated around100 mg/mL. Ribosomes are considered as the molecular machines, which synthesize proteins in the cell. The number of ribosomes in the cell is also increased during the G₁ phase. A cell only enters its S phase when it is composed of enough ribosomes in order to synthesize DNA packaging proteins required during the S phase. During the late G₁ phase, mitochondria are fused together, forming a mitochondrial network in order to produce energy for the cell efficiently. Mechanism of protein synthesis is shown in figure 2.

Figure 2: Protein Synthesis

A G₁ phase cell is prepared by the G₁ cyclin-CDK complex to enter into the S phase by promoting the expression of transcription factors which promotes the S phase cyclins. G₁ cyclin-CDK complex also degrades the S phase inhibitors. The timing of the G₁ phase is regulated by cyclin D-CDK4/6, which is activated by G₁ cyclin-CDK complex. The cyclin E-CDK2 complex pushes the cell from G₁ to S phase (G₁/S transition). Cyclin A-CDK2 inhibits the DNA replication of the S phase by disassembling the replication complex when the cell is at G₁ phase. On the other hand, by the G₁/S checkpoint, the presence of the enough row materials along with the ribosomes for the DNA replication at S phase is checked. The transition of G₁/S is the rate-limiting step of the cell cycle which is known as the restriction point.

S phase

The synthesis phase during which the DNA replication of the cell takes place is called the S phase. Since DNA is packaged in the nucleus by proteins, these packaging proteins are also synthesized during the S phase in a linked manner. The packaging proteins are histones. During the S phase, the cell produces a large number of phospholipids. Phospholipids are involved in the synthesis of the cell membrane as well as the membrane of organelles. The amount of phospholipid is doubled during S phase in order to achieve two daughter cells, which are enclosed by membranes. Mechanism of DNA replication is shown in figure 3.

Figure 3: DNA Replication

A large pool of cyclin A-CDK2 activates the occurrence of G₂ phase by terminating the S phase by regulating the timing of S phase.

G₂ phase

The second gap phase of the interphase is G₂ phase, where the replication of organelles occurs in the cell. Cell allows further synthesis of proteins during the G₂ phase. A cell at the G₂ phase consists of twice the amount of DNA than in G₁ phase. G₂ phase ensures that DNA is intact without any breaks or nicks. Cyclin B-CDK2 pushes G₂ phase to the M phase (G₂/M transition). The G₂/M transition is the final checkpoint before the cell’s entering to mitosis. The simultaneous replication of DNA in a growing embryo is checked by G₂/M checkpoint, obtaining a symmetrical cell distribution in the embryo.

G₀ phase

G₀ phase may occur either just after mitosis or just before G₁ phase. A G₁ phase cell may also enter G₀ phase. The entry into G₀ phase is considered as leaving the cell cycle. That means, G₀ phase is the resting phase, and the cell leaves the cell cycle and stops its division. Some of the cells, which enter the G₀ phase are differentiated into highly specialized cells. Terminally differentiated cells never enter the cell cycle again. Some cells like neurons remain dormant permanently. However, some cells may leave G₀ phase and re-enter G₁ phase, allowing the cell division. Cells like kidney, liver and stomach cells remain semi-permanently at the G₀ phase. Some cells like epithelial cells never enter the G₀ phase. An overview of phases in the eukaryotic cell cycle is shown in figure 4.

Figure 4: Cell Cycle Phase in Eukaryotes

After the successful completion of interphase, a cell will enter into its mitotic division phase, in order to undergo the nuclear division. Nuclear division is followed by cytokinesis, which is the cytoplasmic division, resulting in two daughter cells genetically and functionally identical to their parent cell.

Conclusion

The interphase is the period of the cell cycle which prepares the cell to divide by providing the space for the nucleus and organelles. Space is provided by enlarging the cell.  Hence, the cell is capable of functioning and dividing later by its own. Three phases can be identified in the interphase: G₁ phase, S phase, and G₂ phase. During G₁ phase, the cell uptakes necessary nutrients into the cell and increases the number of ribosomes inside the cell. Hence, the protein synthesis is induced during the G₁ phase. The cell replicates its genetic material in order to maintain a uniform ploidy throughout its progeny. The number of ribosomes is also increased in order to synthesize histones which are required for the packaging of newly replicating DNA. During G₂ phase, the cell increases the number of organelles or simply doubles the number of organelles, which is required for its division into two new cells. The sequential nature of the each phase and final outcome of the interphase is regulated by cyclin-CDks and checkpoints at each phase.

The metabolic rate of the cell is also high throughout the interphase. After the completion of interphase in a successful manner, the cell enters to its mitotic phase where the nuclear division of the cell takes place. Nuclear division is followed by cytokinesis. After the completion of cell division, the ultimate result is the two daughter cells which are genetically and metabolically identical to the parent cell.

 Reference:
1. Nguyen D. H., Leaf Group. “What Happens in the Interphase of the Cell Cycle?”

Image Courtesy:
1. “Schinterphase” By Ymai assumed (based on copyright claims) – Own work assumed (based on copyright claims)., (CC BY-SA 2.5) via Commons Wikimedia
2. “Proteinsynthesis”By Mayera at the English language Wikipedia (CC BY-SA 3.0) via Commons Wikimedia
3. “0323 DNA Replication” By OpenStax – (CC BY 4.0) via Commons Wikimedia
4. “Eukaryotic replication cycle” By Boumphreyfr – Own work (CC BY-SA 3.0) via Commons Wikimedia