DNA replication is the process that must occur before both mitosis and meiosis can take place. Every cell preparing to divide first needs to make a complete copy of its entire genome so that each daughter cell receives a full set of genetic instructions. This copying happens during a specific window called S phase (short for synthesis phase), which is part of the larger preparatory period known as interphase.
What Happens During S Phase
Before a cell can split its chromosomes into two groups, it needs two identical copies of each chromosome to work with. During S phase, the cell unwinds its double-stranded DNA and builds a new complementary strand along each original strand. The result: every single chromosome is duplicated, producing pairs of identical copies called sister chromatids that stay physically connected to each other.
The process starts when enzymes called helicases pry apart the two strands of the DNA double helix, creating a Y-shaped structure known as a replication fork. Once the strands are separated, DNA polymerases move along each exposed strand and assemble a matching strand nucleotide by nucleotide. Because polymerases can’t start from scratch, a short RNA primer is laid down first to give them a starting point. From there, the polymerases work their way along billions of base pairs until the entire genome has been copied.
In a typical human cell with a 24-hour cycle, S phase lasts roughly 8 hours. That’s a significant chunk of time, but the cell isn’t just passively waiting. It’s running thousands of replication forks simultaneously across all 46 chromosomes to get the job done within that window.
How the Cell Keeps Copies Accurate
Copying 6 billion base pairs of DNA is an enormous task, and errors would be dangerous for daughter cells. Replicative DNA polymerases have a built-in proofreading ability: they can detect when a wrong nucleotide has been inserted, reverse direction, remove the mistake, and then continue forward with the correct base. This proofreading alone dramatically reduces the error rate.
On top of that, a second layer of quality control called mismatch repair scans newly synthesized DNA for errors that proofreading missed. Together, these systems bring the final error rate down to roughly one mistake per billion nucleotides copied, which is remarkably precise for a process happening at high speed across the entire genome.
Holding Sister Chromatids Together
Once a chromosome is replicated, the two identical copies need to stay physically linked until the cell is ready to pull them apart during division. A ring-shaped protein complex called cohesin wraps around both sister chromatids, essentially lassoing them together. This connection is critical: without it, the cell would have no reliable way to ensure each daughter cell gets exactly one copy of every chromosome. Cohesin stays in place through the rest of interphase and into the early stages of division, only releasing at the precise moment the chromatids need to separate.
What Else the Cell Prepares
DNA replication is the headline event, but the cell handles several other tasks during interphase to get ready for division. During G1 phase, which comes before S phase, the cell grows in size and produces the proteins and organelles it will need. It also stockpiles the raw materials for DNA synthesis. Building new nucleotides (the individual units of DNA) is energy-intensive and draws on multiple metabolic pathways, so the cell ramps up production ahead of and during S phase.
After S phase wraps up, the cell enters G2 phase, a shorter period of about 4 hours in a typical human cell. During G2, the cell continues to grow and begins duplicating its centrosomes, the structures that will anchor the spindle fibers responsible for pulling chromosomes apart. By the time the cell actually enters mitosis or meiosis, it has two centrosomes positioned on opposite sides, ready to organize the spindle. The cell also runs internal checkpoints during G2 to verify that DNA replication finished completely and that no significant damage remains unrepaired.
Why the Cell Waits for a Green Light
The transition from G1 into S phase is one of the most tightly controlled moments in the entire cell cycle. The cell doesn’t begin replicating DNA until specific signaling proteins confirm that conditions are right. Growth-dependent enzymes called cyclin-dependent kinases (CDKs), paired with proteins called cyclins, act as the switches. In mammalian cells, cyclin D paired with CDK4 or CDK6 initiates the cascade during G1, and cyclin E paired with CDK2 pushes the cell across the threshold into S phase.
These molecular switches exist because starting DNA replication at the wrong time, or without adequate resources, could produce incomplete or damaged copies. The gap phases on either side of S phase give the cell time to monitor both its internal state and external signals like nutrient availability and growth factors. If something is wrong, the cell can pause or even exit the cycle entirely rather than attempt a division that could go badly.
S Phase Before Meiosis Takes Longer
Both mitosis and meiosis require DNA replication beforehand, and both use the same core molecular machinery to get it done. But the pre-meiotic S phase is notably slower. In rats, for example, the mitotic S phase lasts around 7 to 8 hours, while the pre-meiotic S phase in sperm-producing cells stretches to about 84 hours. In humans, pre-meiotic S phase runs roughly 62 hours.
The reason for the extended timeline isn’t fully pinned down, but it likely relates to the additional preparation meiotic cells need. During meiosis, homologous chromosomes must find each other and pair up, and the cell begins assembling specialized protein structures along the chromosomes even as replication is finishing. These extra steps may slow the process or require more careful coordination than a standard mitotic replication cycle.
Despite the difference in timing, the core requirement is identical. No cell, whether it’s headed for mitosis or meiosis, can proceed to divide its chromosomes without first making a faithful copy of every strand of DNA it carries.