During the G1 phase, the cell grows, builds materials it will need to copy its DNA, and decides whether to commit to another round of division.

What G1 Phase Is

  • G1 (Gap 1 or growth 1) is the first stage of interphase in the eukaryotic cell cycle, right after mitosis and before DNA synthesis (S phase).
  • Cells can spend a large fraction of their cycle time in G1, especially in tissues that divide slowly.

Key Things That Happen in G1

  • Cell growth: the cell increases in size and volume.
  • Biosynthesis “ramp‑up”:
    • Synthesis of mRNA needed for upcoming protein production.
* Synthesis of many proteins required for DNA replication and general metabolism.
  • Organelle production: cells make additional organelles and cellular components to support division (e.g., mitochondria, membrane components).
  • Energy buildup: the cell accumulates energy reserves (ATP, building blocks like nucleotides) so it can replicate all its DNA in S phase.
  • Preparation for DNA replication: enzymes and factors needed for DNA synthesis are produced and assembled.

Checkpoints and “Decision Point”

  • G1 contains a major control point (G1/S checkpoint or restriction point) where the cell “checks”:
* Cell size
* Nutrient availability
* Presence of growth factors
* DNA damage status
  • If conditions are good, G1/S cyclin–Cdk complexes activate and commit the cell irreversibly to enter S phase and duplicate its DNA.
  • If conditions are poor or signals are missing, the cell may pause in G1 or exit into G0 (a quiescent, non‑dividing state).

Why G1 Phase Matters

  • G1 determines whether a cell will divide again, stay quiescent, or differentiate.
  • Misregulation of G1 control (especially cyclins, Cdks, and checkpoints) is a major driver of uncontrolled proliferation and cancer.

In simple terms: G1 is the “grow, stock up, and decide” stage before a cell commits to copying its DNA and dividing.

TL;DR: In G1 phase, the cell grows, makes RNA, proteins, organelles, and energy reserves, checks size/nutrients/DNA, and then either commits to DNA replication in S phase or exits to G0.

Information gathered from public forums or data available on the internet and portrayed here.