During the S phase of interphase, a cell copies its entire DNA so that both future daughter cells will receive a complete set of genetic instructions. S stands for “synthesis,” and this phase is devoted almost entirely to DNA replication. In a typical rapidly dividing human cell with a 24-hour cycle, S phase lasts about 8 hours, making it the second longest phase after G1.
But DNA copying is not the only thing happening. The cell also doubles its histone proteins, begins duplicating its centrosomes, and runs an active surveillance system to catch errors before they become permanent. Here’s how it all works.
How DNA Replication Begins
Your DNA doesn’t start copying from one end of a chromosome to the other like reading a book. Instead, replication launches from hundreds or thousands of specific sites along each chromosome called origins of replication. These origins don’t all fire at once. Early in S phase, origins in loosely packed, gene-rich regions activate first because the DNA there is more accessible. Tightly packed regions replicate later because the machinery has a harder time reaching those origins. This staggered timing follows a predictable schedule that was largely set up during the preceding G1 phase.
The cell also keeps a reserve of “dormant” origins that were licensed in G1 but aren’t normally needed. If the replication machinery runs into trouble at any point during S phase, these backup origins can fire to make sure the job gets finished.
Firing an origin requires two key enzymes working together. A kinase paired with cyclin E handles the early origins, while the same kinase paired with cyclin A takes over in mid-to-late S phase to activate the remaining origins. A second kinase called CDC7 also contributes. Together, these enzymes act like ignition switches, triggering replication at each origin in sequence.
The Machinery That Copies DNA
Once an origin fires, a team of proteins assembles at that spot to form a structure called a replication fork. The process starts with an enzyme called helicase, which grabs onto the double helix and pries the two strands apart at speeds up to 1,000 base pairs per second. As the helix unwinds, single-strand binding proteins coat the exposed DNA to keep it from snapping back together or getting damaged.
With the strands separated, the actual copying can begin. DNA polymerase is the enzyme that reads each exposed strand and builds a new complementary strand, one nucleotide at a time. Two polymerase molecules work simultaneously at each fork, one on each strand. But there’s a catch: DNA polymerase can only build in one direction (5′ to 3′). One strand, called the leading strand, runs in the right direction and gets copied in a smooth, continuous stretch. The other strand, the lagging strand, runs the opposite way. To handle this, the cell copies it in short segments, stitching them together afterward.
The lagging strand also needs a helper enzyme called primase, which lays down tiny RNA primers to give polymerase a starting point for each new segment. One polymerase lays the initial primer and a short stretch of DNA, then hands the job off to a second polymerase that extends the segment. These short pieces are eventually joined into one continuous strand.
By the end of S phase, every chromosome has been faithfully duplicated. The two copies, called sister chromatids, stay physically connected until the cell is ready to divide.
Histone Production Keeps Pace With DNA
Raw DNA doesn’t just float loose in the nucleus. It wraps around clusters of histone proteins to form a tightly organized structure called chromatin. When the cell doubles its DNA, it also needs to double its supply of histones so the new DNA can be properly packaged right away.
Histone production and DNA synthesis are tightly linked during S phase. In mammalian cells, the two processes are mutually dependent: if DNA replication stalls, histone production shuts down almost immediately. The cell destabilizes histone messenger RNA within minutes of a replication block, preventing a dangerous buildup of unpackaged histones. The reverse is also true: sustained DNA synthesis depends on a steady supply of new histones.
This coordination matters because unpackaged DNA is vulnerable. Prompt chromatin assembly protects the new genetic material and preserves the patterns of gene activation and silencing that the cell needs to maintain its identity.
Centrosome Duplication
While DNA replication is the headline event, the cell is also quietly duplicating its centrosome, the structure that will later organize the spindle fibers needed to pull chromosomes apart during division. Centrosome duplication shares some of the same regulatory signals as DNA replication, particularly the cyclin E-CDK2 complex that drives entry into S phase. This shared control helps ensure that both processes happen roughly in sync: one round of DNA copying, one round of centrosome duplication.
If this coordination breaks down and centrosomes duplicate more than once, the cell can end up with extra copies. That leads to disorganized spindle formation during division, which increases the risk of chromosomes being distributed unevenly between daughter cells.
How the Cell Catches Errors
Copying 6 billion base pairs of DNA in 8 hours is an enormous task, and things can go wrong. The replication fork might stall when it hits damaged DNA. Ultraviolet light, chemical exposures, or even normal metabolic byproducts can create lesions that block the polymerase. When that happens, the cell activates a surveillance system called the intra-S phase checkpoint.
The trigger works like this: when a polymerase stalls at a damaged site, the helicase keeps unwinding DNA ahead of it. This creates a growing stretch of exposed single-stranded DNA, which gets coated by a protective protein called RPA. That RPA-coated DNA acts as an alarm signal, recruiting a sensor kinase called ATR to the site. ATR then activates a relay kinase called Chk1, which carries the checkpoint signal throughout the cell.
The checkpoint response does two important things. First, it suppresses the firing of late replication origins that haven’t activated yet, slowing down the overall pace of replication so the cell can deal with the problem. Early origins that have already fired continue working, since those are the forks that actually detect the damage in the first place. Second, the checkpoint stabilizes stalled replication forks to prevent them from collapsing. A collapsed fork can break the chromosome, so keeping stalled forks intact is critical for protecting the genome.
The replication fork itself is remarkably sensitive to damage. Studies have shown that extremely low concentrations of DNA-damaging agents are enough to trigger the checkpoint during S phase, suggesting that the fork acts as a highly efficient damage sensor.
What Happens When S Phase Ends
By the time S phase is complete, the cell has gone from having 46 chromosomes (in humans) to having 46 chromosomes each made of two identical sister chromatids, effectively doubling its DNA content. It has also produced enough histones to package all of that new DNA and duplicated its centrosome in preparation for division. The cell then moves into G2, a shorter phase lasting about 4 hours, where it checks for any remaining replication errors and prepares the final molecular machinery needed to enter mitosis.