By John McDaniel | Updated Aug 30, 2022
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Scientists describe the phases of a cell’s growth and development as the cell cycle. All non‑reproductive cells continuously progress through this cycle, which comprises four stages: G1, S, G2, and M. Except for M, the other stages belong to interphase—the period during which cells grow, accumulate nutrients, and prepare for division.
The G1 phase, often called the “growth phase,” is when a cell expands in size and synthesizes the proteins and enzymes required for subsequent DNA replication and cell division. Its duration is variable and largely depends on nutrient availability. During G1, protein production peaks, setting the stage for the S phase.
Cells possess regulatory checkpoints that monitor growth and genome integrity. At the end of G1, the “restriction point” ensures that protein synthesis is complete and the genome is intact before progression to S phase. Cyclin‑dependent kinases (CDKs) and their cyclin partners orchestrate this transition and trigger the initiation of DNA synthesis.
Although G1 is a single phase, it can be subdivided into four sub‑phases that describe specific cellular activities: G1a (Competence), G1b (Entry), G1c (Progression), and G1d (Assembly). In the competence sub‑phase, the cell uptakes nutrients and external molecules. The entry sub‑phase incorporates these materials into the cytoplasm. Progression involves growth and biosynthesis, while assembly consolidates the components and passes the cell through the restriction point.
The nomenclature of the cell‑cycle stages reflects their functions: “G” for “gap,” “S” for “synthesis,” and “M” for “mitosis.” G1 and G2 denote the first and second gap periods, respectively. The sub‑phases within G1 are denoted as G1a, G1b, G1c, and G1d.
