DNA is the primary structure replicated during interphase, but it’s not the only one. The centrosome (the cell’s microtubule-organizing center) also duplicates, organelles increase in number, and the cell roughly doubles its protein content. All of this happens across three distinct subphases, each with a specific job, that together account for about 95% of the total cell cycle.
DNA Replication in S Phase
DNA replication is the headline event of interphase, and it happens exclusively during the S phase (short for “synthesis”). Before S phase, a human somatic cell contains 46 chromosomes with a total DNA content described as 2c. By the end of S phase, every chromosome has been copied, doubling the DNA content to 4c. The chromosome count, however, stays at 46 because each replicated chromosome consists of two identical sister chromatids joined at the centromere, and a joined pair still counts as one chromosome. So after replication, a human cell has 46 chromosomes made up of 92 sister chromatids total.
The cell uses a built-in licensing system to make sure each stretch of DNA is copied exactly once. A group of proteins called MCM proteins bind to replication origins along the chromosomes during G1, acting as “licenses” that permit copying to begin. Once S phase starts and a section of DNA fires, that license is consumed and cannot be reissued until the cell completes division. This prevents dangerous double-copying of any gene.
Centrosome Duplication
The centrosome duplicates in step with DNA replication, beginning at the G1/S boundary. A newly divided cell inherits a single centrosome containing two centrioles arranged at right angles to each other. Early in G1, the centrioles disengage and drift apart while remaining loosely connected by protein fibers. Then, as the cell enters S phase, a new “procentriole” begins assembling perpendicular to each existing centriole. These daughter centrioles elongate throughout G2 until they reach roughly the same size as the originals.
This timing is not a coincidence. Centrosome duplication and DNA replication share a key molecular trigger: the enzyme complex Cdk2/cyclin E. Activating this enzyme essentially fires the starting gun for both processes at once, which keeps the two in sync. By the time the cell reaches mitosis, it has two complete centrosomes ready to anchor opposite ends of the mitotic spindle and pull sister chromatids apart.
What Happens in G1 and G2
The S phase is bookended by two gap phases, G1 and G2, that focus on growth and preparation. During G1, the cell increases in size, synthesizes proteins, and produces the molecular machinery it will need for DNA replication. This is also when the cell commits to dividing. If nutrient or growth signals are insufficient, the cell can exit into a resting state (G0) rather than proceeding.
G2 picks up after DNA replication is complete. The cell continues growing, replenishes energy stores, and synthesizes proteins involved in chromosome handling during mitosis. Some organelles, including mitochondria, are duplicated during this phase. The cytoskeleton also begins to be reorganized, freeing structural proteins that will later be used to build the mitotic spindle. By the end of G2, the cell has essentially doubled its contents so that each daughter cell will receive a full complement after division.
Protein Content Doubles Gradually
Unlike DNA, which is copied in a defined burst during S phase, protein content increases steadily throughout all of interphase. Studies tracking individual proteins in mammalian cells show that global synthesis rates remain relatively constant across G1, S, and G2, with no dramatic spikes in any single phase. What does change is the synthesis rate of specific proteins at specific times. Some proteins accumulate faster early in the cycle, others later, while degradation rates stay mostly flat. The net result is that by the end of G2, the cell has roughly twice the protein mass it started with in G1.
Ribosomal RNA production is a major part of this effort. The nucleolus, a dense structure inside the nucleus, is highly active throughout interphase, churning out the ribosomal components the cell needs to sustain elevated protein synthesis. Structural studies of nucleoli at different points in interphase show consistent changes in size and organization as the cell ramps up ribosome output to keep pace with growth demands.
How Long Interphase Takes
Interphase dominates the cell cycle in terms of time. In cultured human breast epithelial cells with an average cycle of about 21 hours, interphase lasts roughly 20 of those hours. G1 takes about 4 hours, S phase about 9 hours, and G2 about 6.5 hours. Mitosis itself, the dramatic division stage, occupies less than 1 hour, or about 5% of total cycle time. These numbers vary by cell type: rapidly dividing embryonic cells can have extremely short gap phases, while adult liver cells may spend years in G0 without entering S phase at all.
Checkpoints That Verify Replication
The cell doesn’t blindly move from one phase to the next. Molecular checkpoints at the G1/S boundary and the G2/M boundary inspect whether conditions are right before allowing progression. If DNA is damaged during S phase, a checkpoint kinase called Chk2 stabilizes the stalled replication machinery and pauses the process until repairs are made. At the G2/M boundary, the cell verifies that replication is fully complete before entering mitosis. This checkpoint works by keeping a key mitotic enzyme in its inactive, phosphorylated state. Only when replication is confirmed does the enzyme become active and trigger the onset of mitosis.
When these checkpoints fail, the consequences can be severe. In yeast experiments where checkpoint proteins were deleted, cells attempted to divide before finishing replication, a lethal scenario. In human cells, checkpoint failures contribute to the genomic instability seen in many cancers.