AP Syllabus focus:
‘Cells may enter G0, leaving the cycle or being held at a particular stage, where they remain metabolically active but do not divide unless signals trigger reentry.’
Cells do not always move continuously through the cell cycle.
This labeled cell-cycle schematic shows the main proliferative phases (G1, S, G2, and M) and the branch into G0 (quiescence). It visually reinforces that G0 is not part of DNA replication or mitosis, but an alternative state that cells can enter from the cycle. Source
Many cells pause division to conserve resources, maintain specialised functions, or await environmental cues. The G0 phase explains how nondividing cells persist and when they can rejoin proliferation.
What the G0 phase is
Core idea
Cells may exit the active cell cycle (G1, S, G2, M) into a nondividing state. This can be a temporary “pause” or a long-term, sometimes permanent, withdrawal from division.
G0 phase: A nondividing state in which a cell has left the cell cycle (often from G1) and remains metabolically active, but does not replicate DNA or undergo mitosis unless appropriate signals cause reentry.
A key AP Biology emphasis is that G0 cells are not “inactive”; they maintain homeostasis and often perform essential tissue-specific roles while not dividing.
Entering G0: leaving the cycle or being held
Two common routes into nondivision
Leaving the cycle: A cell exits the cycle into G0 when division is not needed or conditions are unfavorable.
Being held at a stage: Regulatory signals can keep a cell from passing the point of commitment to division, effectively keeping it out of the replicative sequence.
Typical triggers for entering G0
Absence of external signals required for proliferation (e.g., missing growth factors)
Nutrient/energy limitation, where biosynthesis and DNA replication would be costly
Contact inhibition and space constraints in dense tissues
Differentiation cues that prioritise specialised function over division
Developmental timing, when proliferation must stop in specific cell populations
What G0 cells do while not dividing
Metabolic activity without cell division
The syllabus specifies that cells in G0 remain metabolically active. Practically, this means they:
Continue ATP production and basic metabolism
Maintain membrane transport, ion gradients, and pH balance
Synthesize proteins needed for cell maintenance and specialised tasks
Perform tissue-specific functions (for example, secretion, contraction, or signal processing) depending on cell type
G0 is therefore best understood as a shift in priorities: from duplication and division to maintenance and function.
Temporary vs long-term (nondividing) states
Reversible quiescence (can reenter)
Some G0 cells are quiescent: they can resume the cell cycle when conditions change.
This OpenStax figure depicts G0 as an alternate phase outside the main cycle, with arrows indicating both exit from G1 and potential reentry back into G1. The layout emphasizes that quiescence is compatible with ongoing cell function, while progression through S and M occurs only after reentry into the active cycle. Source
Common features include:
Retained capacity to respond to mitogenic signals (pro-division cues)
Intact cellular machinery for DNA replication, activated only upon reentry
Strong dependence on external signals to restart division
Long-term or permanent nondivision
Other cells remain in G0 for extremely long periods and may never reenter division, especially when:
Highly specialised structure/function makes division impractical
Tissue integrity depends on stable, nonproliferating cells
For AP Biology, the critical distinction is whether signals can trigger reentry; some cell types are poised to rejoin the cycle, while others are effectively locked into a nondividing identity.
Reentry into the cell cycle: signals matter
“Unless signals trigger reentry”
A defining property in the syllabus statement is conditional reentry: G0 cells do not divide unless they receive appropriate cues. In general terms, reentry involves:
Detection of external signalling molecules (e.g., growth-promoting cues in the local environment)
Activation of internal regulators that restore progression toward DNA replication
Commitment to division only when conditions support successful completion of replication and mitosis
Why reentry control is biologically important
Supports tissue maintenance by allowing division only when needed
Prevents unnecessary proliferation that could disrupt tissue architecture
Conserves energy by avoiding DNA replication in poor conditions
Why organisms use G0 strategically
Advantages of pausing division
Resource efficiency: avoids costly replication when growth is not required
Functional specialisation: supports cells that must prioritise stable function
Flexibility: allows some cells to “wait” and rapidly respond to appropriate cues
Protection: reduces exposure to replication-associated errors when division is not necessary
FAQ
They often combine DNA-content assays with markers of proliferation. For example, low DNA content alone can’t distinguish G0 from G1, but absence of proliferation markers (e.g., Ki-67) supports G0.
Yes. Many reduce expression of DNA replication and mitosis genes, while maintaining or increasing transcripts needed for maintenance, stress resistance, and specialised cell functions.
No. Senescence is a largely irreversible growth arrest with distinctive secretory and stress-response changes. G0 is often reversible and is defined by the capacity to re-enter upon signalling.
Nutrient and energy-sensing pathways can bias cells toward quiescence, including signalling that downshifts biosynthesis and promotes maintenance programmes when resources are limited.
Quiescence helps preserve stem cell function by limiting replication-associated damage. Niche-derived signals can maintain G0 and later stimulate activation for regeneration when needed.
Practice Questions
State two characteristics of a cell in G0 phase. (2 marks)
Any two of:
Metabolically active (1)
Not undergoing cell division/mitosis (1)
Not replicating DNA (1)
Has left the cell cycle or is held outside progression (1)
Can re-enter the cell cycle if it receives the correct signals (1)
Explain how G0 helps coordinate cell numbers in a tissue, referring to (i) why cells enter G0, (ii) what they do while in G0, and (iii) how they resume division. (5 marks)
Cells enter G0 when division is not needed or conditions/signals are unfavourable (1)
Entry can involve leaving the cycle or being held at a stage before commitment to division (1)
Cells in G0 remain metabolically active and maintain normal cellular functions (1)
Cells do not replicate DNA or undergo mitosis while in G0 (1)
Re-entry requires signals (e.g., growth-promoting cues) that restart progression through the cell cycle (1)
