The stages, in order

  1. G₁ phase (first gap). Little changes under the microscope, but the cell is biochemically busy, accumulating chromosomal DNA building blocks, associated proteins, and the energy reserves needed to replicate every chromosome.
  2. S phase (synthesis). DNA replication produces identical pairs of sister chromatids attached at the centromere. The centrosome is duplicated here too, giving rise later to the mitotic spindle.
  3. G₂ phase (second gap). The cell replenishes energy stores, synthesizes the proteins needed to move chromosomes, duplicates some organelles, and dismantles the cytoskeleton to free resources for the mitotic phase.
  4. M phase, karyokinesis. Mitosis divides the nucleus through prophase, prometaphase, metaphase, anaphase and telophase, aligning the duplicated chromosomes and segregating them into two identical daughter nuclei.
  5. M phase, cytokinesis. The cytoplasm is physically divided between the two daughter cells, by a contractile actin ring in animal cells and by a cell plate growing outward in plant cells.
  6. G₀ phase. A quiescent state outside the cycle. Cells in G₀ are not actively preparing to divide, and some enter it temporarily in response to conditions such as nutrient availability.

Division is the short part

It is easy to come away from a diagram thinking the cell cycle is mostly mitosis, because mitosis is what the diagram draws in detail. The proportions are the other way around. In human cells in culture, which divide roughly every 24 hours, mitosis and cytokinesis last only about an hour, so approximately 95 percent of the cycle is spent in interphase. During interphase the chromosomes are decondensed and the nucleus looks uniform, but this is when the cell grows at a steady rate and, in S phase only, replicates its DNA. The timing of DNA synthesis is precisely what divides the cycle into its four discrete phases.

Where students go wrong

The first error is calling interphase a resting phase. Nothing rests: G₁ accumulates the raw materials and energy for replication, S copies every chromosome, and G₂ builds the proteins that will move them. The genuinely quiescent state is G₀, which sits outside the cycle rather than inside it, and cells enter it when they are not actively preparing to divide. The second error is putting DNA replication in mitosis. Chromosomes arrive at M phase already duplicated as sister chromatids, which is why mitosis can separate them without copying anything.

Where this leads next

The stages of mitosis page takes M phase apart one stage at a time, and it is the natural follow on once the overall shape of the cycle is clear. In the other direction, progression between these phases is governed by a conserved regulatory apparatus that also links the cycle to external signals controlling cell proliferation, which is where checkpoints, cyclins and the cancer connection begin. Meiosis is also easier from here, because it too begins with one round of DNA replication in S phase before its two divisions.

Common mistakes

  • Describing interphase as a resting phase. The genuinely quiescent state is G₀, which lies outside the cycle.
  • Placing DNA replication in mitosis. It happens in S phase, before M phase begins.
  • Assuming mitosis takes up most of the cycle. In human cells in culture, mitosis and cytokinesis last about an hour out of roughly 24.
  • Forgetting that the centrosome is duplicated in S phase alongside the DNA, which is what supplies the two poles of the spindle.

Related concepts

  • Mitosis vs meiosis: Mitosis is a single nuclear division that produces two nuclei genetically identical to the original and at the same ploidy level.
  • Haploid vs diploid: A haploid cell contains a single set of chromosomes.
  • Stages of mitosis: OpenStax divides mitosis, or karyokinesis, into five stages: prophase, prometaphase, metaphase, anaphase and telophase.

Sources