Interphase: Definition, G1, S and G2 Phases Explained
By Dr. Zubair Khalid, DVM, MS, PhD ·
Interphase is the longest portion of the cell cycle, during which a cell grows, carries out its normal functions, and prepares for division. It is not a quiet time. The cell is actively building proteins, copying its DNA, and doubling some organelles. Interphase is divided into three stages: G1, S, and G2. After interphase, the cell enters the mitotic phase, where it divides into two daughter cells [1]. This preparation period is essential for ensuring that each new cell receives a complete set of genetic material.
Key Takeaways
- Interphase is the preparation stage of the cell cycle, consisting of G1, S, and G2 phases.
- During the S phase, the cell replicates its DNA, producing sister chromatids.
- Checkpoints at the end of G1 and G2 verify that conditions are favorable for division.
- G0 is a resting state where cells pause their cycle, sometimes permanently.
- Under a microscope, an interphase cell shows an intact nucleus and uncondensed chromatin.
What Is Interphase in the Cell Cycle?
The cell cycle is the ordered series of events that leads to cell growth and division [1]. It has two major parts: interphase and the mitotic phase. Interphase is the stage where the cell grows and replicates its DNA. The mitotic phase is when the replicated DNA and cytoplasm are separated into two new cells [1]. Interphase takes up most of the cell cycle, because the cell needs time to prepare. Without proper preparation, the cell would not be able to divide correctly. This is why interphase is so heavily regulated by internal checkpoints.
The Three Stages of Interphase: G1, S, and G2
Interphase is not a single uniform step. It is broken into three distinct stages, each with a specific job. The stages are G1, S, and G2. The "G" stands for gap, and the "S" stands for synthesis. The gaps are not empty pauses. They are periods of intense biochemical activity where the cell grows and checks its readiness [1].
| Stage | Main Activity | Key Events |
|---|---|---|
| G1 (First Gap) | Cell growth and preparation | Accumulates building blocks for DNA, produces proteins, builds energy reserves [1] |
| S (Synthesis) | DNA replication | Copies each chromosome, produces sister chromatids, duplicates the centrosome [1] |
| G2 (Second Gap) | Final preparation | Replenishes energy stores, synthesizes proteins for chromosome movement, duplicates some organelles [1] |
G1 Phase: The First Gap
During G1, the cell is biochemically very active, even though little change is visible under a microscope [1]. The cell is accumulating the building blocks of DNA and associated proteins. It is also building up energy reserves to complete the task of replicating each chromosome [1]. This phase can last a long time, depending on the cell type and external conditions. If conditions are not favorable, the cell may exit the cycle and enter G0.
S Phase: Synthesis of DNA
The S phase is the stage where DNA replication happens. Throughout interphase, nuclear DNA remains in a semi-condensed chromatin configuration [1]. In the S phase, the cell copies each chromosome, producing identical pairs of DNA molecules called sister chromatids [1]. These sister chromatids are firmly attached at a region called the centromere. The centrosome is also duplicated during this phase [1]. The two centrosomes will later form the mitotic spindle, the apparatus that moves chromosomes during mitosis. For more on how DNA is copied, see our guide on DNA Replication Occurs in S Phase.
G2 Phase: The Second Gap
In the G2 phase, the cell replenishes its energy stores and synthesizes proteins needed for chromosome manipulation and movement [1]. Some organelles are duplicated, and the cytoskeleton is dismantled to provide resources for the mitotic phase [1]. There may be additional cell growth during G2 [1]. The cell must complete all final preparations before it can enter mitosis. This is the last chance to fix any problems before the cell commits to dividing.
G0 Phase: The Quiescent State
Not all cells are always dividing. Some cells exit the cell cycle and enter a quiescent stage called G0 [1]. In G0, the cell is not actively preparing for division. It is performing its normal functions in the body. Some cells stay in G0 for a very long time, such as mature neurons or cardiac muscle cells [2]. Other cells can be signaled to leave G0 and re-enter the cycle if conditions change. This is a reversible state for many cells. The ability to enter G0 is a protective mechanism that prevents unnecessary division.
Checkpoints at G1 and G2
The cell cycle is controlled by internal checkpoints that ensure conditions are favorable before the cell moves forward [2]. A checkpoint is a point in the cycle where progression can be halted until conditions are met [2]. There are three main checkpoints, but the two that occur during interphase are the G1 checkpoint and the G2 checkpoint.
- The G1 checkpoint occurs near the end of G1. It checks for cell size, adequate reserves, and DNA damage [2]. If the DNA is damaged, the cell cycle is halted. The cell can try to repair the damage or enter G0 [2]. This checkpoint is also called the restriction point, because passing it commits the cell to division [2].
- The G2 checkpoint occurs at the G2/M transition. It checks that all chromosomes have been replicated and that the replicated DNA is not damaged [2]. If problems are found, the cell cycle halts and the cell attempts to complete replication or repair the DNA [2].
These checkpoints are regulated by proteins such as cyclin-dependent kinases and cyclins [3]. When these regulators fail, the cell may divide with damaged DNA, which can lead to cancer [3]. For a deeper look, read about Cell Cycle Checkpoints: A Guide to G1, G2, and M Phase Regulation.
Why Interphase Is Not a Resting Phase
Many textbooks used to call interphase a resting phase. This is misleading. The cell is not resting at all. It is highly active, growing, and preparing for division [1]. The term "resting" came from early microscopists who could not see much activity happening. They only saw a quiet nucleus and uncondensed chromatin. But biochemically, the cell is working hard. It is building proteins, replicating DNA, and duplicating organelles. Calling it resting gives the wrong impression. A better description is that interphase is the preparation phase of the cell cycle.
Recognizing an Interphase Cell Under a Microscope
When you look at a cell under a light microscope, you can often tell if it is in interphase. The key features are:
- An intact nucleus with a visible nuclear envelope.
- A visible nucleolus, which is the site of ribosome production.
- Chromatin that is uncondensed, appearing as a diffuse, grainy mass rather than distinct chromosomes.
During mitosis, the nuclear envelope breaks down and the chromosomes condense into visible structures. In interphase, neither of those things has happened yet. The cell looks "quiet" under the microscope, even though it is biochemically busy. This is why early scientists mistakenly called it a resting phase. To see what comes next, review our Phases of Mitosis: Stages in Order, With Diagrams guide.
Frequently Asked Questions
What happens in an interphase?
During interphase, the cell grows, carries out normal metabolic functions, and prepares for division. In G1, the cell accumulates building blocks and energy. In S phase, it replicates its DNA and duplicates the centrosome. In G2, it synthesizes proteins needed for mitosis and replenishes energy stores [1]. The cell also passes through checkpoints that verify its readiness. If all conditions are met, the cell proceeds to the mitotic phase.
What is the G1, G2, and S phase?
G1, S, and G2 are the three stages of interphase. G1 is the first gap, where the cell grows and prepares for DNA replication. S is the synthesis phase, where DNA is replicated. G2 is the second gap, where the cell makes final preparations for mitosis [1]. Each stage has specific tasks and checkpoints. Together, they ensure the cell is ready to divide correctly.
What does the term interphase mean?
The term interphase means "between phases." It refers to the period between one mitotic division and the next. The prefix "inter" means between, and "phase" refers to a stage. Interphase is the part of the cell cycle where the cell is not actively dividing, but is preparing for the next division [1]. It includes G1, S, and G2 phases.
Why is interphase called resting phase?
Interphase is called the resting phase because of early microscope observations. Under a light microscope, the nucleus appears quiet and the chromosomes are not visible as distinct structures. Early scientists did not see obvious activity, so they assumed the cell was resting. This name has persisted in some textbooks, even though it is inaccurate. The cell is actually very active during interphase.
Why is it inappropriate to call interphase a resting stage?
It is inappropriate because the cell is performing intense biochemical work during interphase. It is accumulating building blocks, replicating DNA, synthesizing proteins, and duplicating organelles [1]. The cell is also passing through checkpoints that regulate its progression [2]. None of this is resting. The term "resting" only describes what is visible under a basic microscope, not what is actually happening inside the cell.
How long does interphase typically last?
The length of interphase varies widely depending on the cell type and organism. In rapidly dividing cells, the cell cycle can be completed in a short time, while other cells may spend a long time in interphase or enter G0 permanently [2]. Some cells, such as neurons, can remain in G0 for an entire lifetime [2]. There is no single answer, because the duration depends on the specific conditions and the cell's role in the body.
For a broader overview of the cell cycle and division, see our Cell Biology 101 guide. You can also compare interphase with division in our Mitosis vs Meiosis: Differences, Stages and Comparison Chart article.
References
- Clark MA, Douglas M, Choi J (2018). 10.2 The Cell Cycle. Biology 2e, OpenStax.
- Clark MA, Douglas M, Choi J (2018). 10.3 Control of the Cell Cycle. Biology 2e, OpenStax.
- Schafer KA (1998). The Cell Cycle: A Review. Veterinary Pathology 35:461-478.