S phase
/ ESS fayz /
Before a cell can split into two, it faces a non-negotiable rule: each daughter must walk away with a complete, correct copy of the genome. There is no second chance to fetch missing pages later. So the cell sets aside a dedicated stretch of time to photocopy its entire DNA library, letter by letter. That stretch is S phase, the part of interphase devoted to copying the cell's DNA.
S phase ('S' for synthesis) sits in the middle of interphase, between G1 and G2. During it the cell carries out DNA replication: the double helix is unzipped and each old strand serves as a template to build a new partner strand, so one chromosome becomes two identical copies joined together. These joined copies are called sister chromatids, held together by a protein glue called cohesin. By the end of S phase the cell has doubled its DNA but still counts as one cell with the normal chromosome number — it just has two copies of each chromosome temporarily stuck together.
S phase matters because the accuracy of this single copying step determines whether the next generation of cells inherits a faithful genome or a corrupted one. DNA polymerase proofreads as it goes, but errors and damage still slip through, which is why the cell guards S phase tightly and refuses to start it twice in one cycle. A common misconception is that S phase 'creates new chromosomes' — it does not change the chromosome number, it just duplicates the DNA so it can later be split evenly between two cells.
A human cell copies all 3.2 billion base-pairs of its DNA during S phase in roughly 8 hours — by firing thousands of replication start-sites at once, rather than copying the whole genome end-to-end from a single point.
Thousands of start-sites copy the genome in parallel during S phase.
The cell has a strict licensing system to ensure each stretch of DNA is copied exactly once per cycle — re-replicating DNA in the same cycle would over-duplicate genes and is a recipe for genomic chaos.