🧬 G₀ Phase – Quiescence in the Cell Cycle
In adult animals, not all cells keep dividing. Some cells, like neurons and heart muscle cells, stop dividing after development and enter a state called the G₀ phase, also known as the quiescent phase.
This phase is reached from the G1 phase — not directly from M phase. Once in G₀, cells remain metabolically active but stop dividing.
There are two categories of G₀ entry:
- Permanently in G₀ – e.g., neurons and heart muscle cells. They divide during development and permanently exit the cycle early in G1.
- Temporarily in G₀ (quiescent) – e.g., liver cells and lymphocytes. These cells can re-enter the cycle when needed, such as during tissue repair or immune response.
Important checkpoint: Cells exit into G₀ only if they do not cross the restriction point (R-point) in late G1. If they cross it, they are committed to completing the cycle and cannot go to G₀ from there.
🧠 Why do heart or neuron cells enter G₁ if they won’t divide?
This is a common question — and here’s the key reason:
- All post-mitotic daughter cells must enter G1 phase after mitosis. That’s how the cell cycle is structured in eukaryotes.
- From there, cells evaluate whether to continue to divide or exit into G₀.
- The G₀ pathway is a branch from G1, not from M or S phase.
- Heart cells and neurons, after their final division during development, enter early G1 — and then permanently exit into G₀ before the R-point.
Why not directly exit from M phase?
Because G₀ is molecularly defined as a branch off G1 — not from M. The decision-making molecules (e.g., Cyclin D, Rb-E2F pathway) are active only in G1.
🚉 Analogy – Cell Cycle Train Station
Imagine the cell cycle as a train journey. After mitosis (M phase), every new cell lands on the platform (G1). At G1, the cell chooses whether to:
- Board the next train (S phase, replication)
- Exit the station into a peaceful rest area (G₀)
But no one can exit the station directly from the train (M phase). You must step onto the platform (G1) first. This is exactly how cells behave.
Q1. Neurons enter G₀ phase from:
- A) End of M phase
- B) S phase
- C) Early G1 phase
- D) G2 phase
Neurons briefly enter G1 after final division, then permanently exit into G₀.
Q2. The restriction point in G1 is located at:
- A) Start of G1
- B) Middle of G1
- C) End of G1
- D) G1/S border
Restriction point lies in late G1. After crossing it, the cell is committed to divide.
Q3. Which of these cells can re-enter the cell cycle from G₀?
- A) Neurons
- B) Cardiomyocytes
- C) Liver cells
- D) RBCs
They are quiescent but can divide during regeneration or injury.
Q4. G₀ cells are:
- A) Dividing actively
- B) Metabolically inactive
- C) Dead cells
- D) Metabolically active but non-dividing
They are alive and functioning, but not dividing.
🔬 Summary
- 🌀 G₀ phase is a non-dividing but active state.
- 🧠 Permanent G₀: Neurons, heart muscle cells
- 🧪 Reversible G₀: Liver cells, lymphocytes
- ⛔ G₀ exit occurs before R-point in late G1
- ⚙️ Decision to divide or pause happens only in G1
G₀ is not an end — it’s a wise biological pause. And it starts only after stepping into G1.