DNA replication in meiosis occurs during the S phase of interphase, before meiosis I begins. This is the only time the cell copies its DNA during the entire meiotic process. There is no second round of replication between meiosis I and meiosis II, which is exactly how a cell that starts with two sets of chromosomes ends up producing four cells with just one set each.
The Pre-Meiotic S Phase
Before a cell enters meiosis, it passes through interphase, the same preparatory period that precedes ordinary cell division. Interphase has three stages: G1 (growth), S (synthesis, where DNA is copied), and G2 (final preparation). During the S phase, every chromosome is duplicated, producing two identical sister chromatids joined together at a structure called the centromere. By the time the cell reaches prophase I, the first stage of meiosis I, each chromosome already exists as a pair of connected copies.
One detail that makes meiotic replication distinctive is what gets loaded onto the chromosomes while the DNA is being copied. Ring-shaped protein complexes called cohesins are placed onto the DNA during replication, and they act like molecular clips holding the two sister chromatids together. These cohesins must be present while the DNA is still being copied, because they can only be “activated” to grip both sister chromatids during the replication process itself. In meiosis, the cell uses a special version of cohesin that plays a critical role later, when homologous chromosomes pair up and exchange segments during crossing over.
Why No Replication Happens Before Meiosis II
The brief period between meiosis I and meiosis II is called interkinesis. It looks superficially like an interphase, but the cell skips DNA replication entirely. This is the key structural difference between meiosis and two back-to-back rounds of ordinary cell division. If the cell did replicate its DNA again, the chromosome number wouldn’t be halved, and the whole point of meiosis (producing cells with half the genetic material) would be lost.
The cell prevents this extra replication by keeping the activity of certain cell-cycle enzymes high between the two divisions. These enzymes normally drop to low levels after a division, which is what signals a cell to re-enter the growth and replication cycle. By staying elevated, they effectively tell the cell to proceed straight into meiosis II. Experiments have shown that artificially shutting off these enzymes after meiosis I causes the cell to replicate its DNA again, confirming that active suppression is what keeps replication from happening.
A Built-In Safety Check
The cell doesn’t just replicate its DNA and move on. A checkpoint mechanism called the meiotic replication checkpoint ensures that copying is fully complete before the next major event: the deliberate breaking and rejoining of DNA strands that drives genetic recombination. This checkpoint works by blocking the production or activation of the molecular machinery responsible for creating those DNA breaks. The enforced timing matters because crossovers, the exchanges of genetic material between paired chromosomes, should only happen between fully replicated chromosomes. If breaks occurred on half-copied DNA, the result could be irreparable damage.
What Happens When the Process Goes Wrong
Errors during or after meiotic replication can lead to aneuploidy, a condition in which a resulting egg or sperm ends up with the wrong number of chromosomes. In humans, aneuploidy in eggs is the leading genetic cause of miscarriage and developmental conditions like Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), and Patau syndrome (trisomy 13). Most autosomal monosomies and many trisomies are incompatible with life, which is why aneuploidy accounts for a large share of early pregnancy losses.
The cohesin proteins loaded during the pre-meiotic S phase are central to this problem. In human eggs, meiosis can be paused for decades. A woman’s oocytes begin meiosis during fetal development but don’t complete it until ovulation, sometimes 40 or more years later. Over that time, the cohesin complexes holding sister chromatids together gradually deteriorate. This loss of “grip” is strongly linked to the well-documented increase in chromosomal errors with maternal age: the rate of trisomy in recognized pregnancies sits around 2% to 3% for women in their twenties but climbs to roughly 35% for women in their forties. Most of these errors trace back to problems in meiosis I, where weakened cohesion allows chromosomes to separate incorrectly.
Putting It All Together
The timeline is straightforward. A cell destined for meiosis copies all of its DNA once, during the S phase of interphase, before meiosis I. It then divides twice in succession, with no replication in between. The first division separates homologous chromosome pairs; the second separates sister chromatids. The result is four cells, each with half the original DNA content. Every step after replication, from crossing over in prophase I to the final separation in meiosis II, depends on that single round of DNA copying and the molecular structures established during it.