AP® Biology · Unit 4 · Topic 4.5
Interphase: G1, S, and G2
Interphase is the long, busy stretch of the cell cycle where a cell grows, copies its DNA, and checks its own work before attempting to divide. It splits into three parts: G1 (growth), S (DNA synthesis), and G2 (final checks).
A cell spends roughly 90% of its life in interphase because division is risky and expensive. Splitting into two cells only works if there's enough cytoplasm, organelles, and energy to make two functional daughter cells, and only if the DNA is copied perfectly. Rushing that would produce broken or starved cells. So the cell takes its time: G1 for growth, S for copying, G2 for proofing the result.
In G1, the cell carries out its normal job and builds mass: more ribosomes, more mitochondria, more proteins, enough material to eventually support two cells instead of one. In S phase, every chromosome is replicated. Each chromosome, a single long DNA molecule, is copied into two identical DNA molecules called sister chromatids, which stay physically attached to each other at a region called the centromere. They aren't separate chromosomes yet; they're one chromosome with double the DNA, held together until division pulls them apart. In G2, the cell grows more, makes the proteins needed for mitosis like tubulin for spindle fibers, and checkpoints scan the new DNA for copying errors or damage before allowing entry into mitosis.
This sets up everything the rest of the unit depends on: mitosis only makes sense once you know S phase already doubled the DNA, and the G1, G2, and M checkpoints covered later are the quality control built into exactly these three phases. Some cells exit the cycle entirely into G0, a non-dividing state, rather than ever reaching another S phase.
Key terms
- G1 phase
- The first gap phase, when the cell grows and carries out normal metabolic activity before copying DNA.
- S phase
- The synthesis phase, when the entire genome is replicated, turning each chromosome into two sister chromatids.
- G2 phase
- The second gap phase, when the cell grows further and checks replicated DNA for errors before mitosis.
- Sister chromatids
- Two identical copies of a replicated chromosome, joined together at the centromere.
- G0
- A non-dividing state some cells enter, either temporarily or permanently, instead of continuing through the cell cycle.
Watch it explained
- How it worksWalk me through what a cell builds during G1, copies during S, and checks during G2.
- Common mistakeWhy isn't interphase a resting phase where nothing much is going on?
- How it worksHow does one chromosome become two sister chromatids joined at a centromere?
- Going furtherWhat happens to a neuron that enters G0 and never comes back out?
Check yourself
End of topic.