When Does DNA Replication Occur During Meiosis? S Phase Guide
DNA replication during meiosis happens once, before the process begins, during the S phase of the cell cycle. This single round of replication is followed by two consecutive divisions—meiosis I and meiosis II—which together produce four haploid cells. The timing matters because it explains how chromosomes get halved while genetic diversity is generated.
The Short Answer: Replication Precedes Both Divisions
Meiosis starts with a diploid cell carrying two copies of each chromosome, one from each parent. Before any division takes place, the cell copies all of its DNA during the S phase, sometimes called the premeiotic or meiotic S phase. After this replication, each chromosome consists of two identical sister chromatids held together by cohesion proteins. No further DNA synthesis occurs at any later point in meiosis, so a single replication event fuels two rounds of division. The Wikipedia overview of meiosis describes this as replication followed by meiosis I and meiosis II.
Why One Round Is Enough
Think of it as a preparation-then-split model. The S phase doubles the DNA so the cell has enough material to divide twice. In meiosis I, homologous chromosomes (the maternal and paternal versions of each pair) separate. In meiosis II, the sister chromatids finally split apart—mirroring what happens in mitosis. Because replication already created two chromatids per chromosome, the cell can apportion them across two divisions without needing to copy DNA again.
A quick worked example helps. A human cell about to undergo meiosis starts with 46 chromosomes. After S phase, it still has 46 chromosomes, but now each is composed of two sister chromatids—so 92 chromatids total. Meiosis I separates homologous pairs, leaving two cells with 23 chromosomes each. Meiosis II separates the sister chromatids, yielding four cells, each with 23 single-chromatid chromosomes.
What Happens After Replication Ends
Once DNA replication is complete, the cell does not launch straight into division. Instead, it enters a prolonged G2-like stage known as meiotic prophase. During this stage, homologous chromosomes pair up and genetic recombination takes place. Enzymes cut and repair DNA to exchange segments between maternal and paternal chromosomes, generating new combinations of genetic material. A subset of these recombination events produces crossovers—physical links called chiasmata—that help hold homologs together and later guide their proper separation.
This recombination is unique to meiosis and is precisely why copies made during S phase matter so much: the cell must have two chromatids per chromosome for crossing over and segregation to work correctly. Recombination is explained in more detail by Nature Scitable’s coverage of recombination.
Replication Versus Recombination
It’s easy to confuse these two events, so it helps to keep them straight:
- Replication (S phase): Copies each chromosome into two identical sister chromatids. Happens once, before meiosis I.
- Recombination (prophase I): Exchanges segments between homologous chromosomes. Happens after replication, during meiotic prophase. It creates variety, not extra DNA.
Replication makes identical copies; recombination reshuffles them. Together they set the stage for the two divisions.
How the Two Divisions Use the Replicated DNA
After replication and recombination, meiosis proceeds through two distinct phases.
Meiosis I is called the reductional division because it separates homologous chromosomes. The two homologs within a pair move to opposite poles, so each resulting cell receives one chromosome from each original pair—but that chromosome still has both sister chromatids.
Meiosis II is the equational division. Here, cohesion between sister chromatids is released and the chromatids segregate, much like in mitosis. This produces four haploid cells, each containing a single set of chromosomes.
In some organisms, all four products become gametes such as sperm or spores. In female animals, three of the four are typically discarded as polar bodies, leaving one cell to mature into an ovum.
Timing in Different Organisms and Sexes
The core rule—replicate once, divide twice—holds across nearly all sexually reproducing organisms, including animals, plants, and fungi. What varies is the timing around it. In many organisms, meiosis pauses at points and resumes later; in mammals, some meiotic steps in females can pause for years and only complete after fertilization. The molecular sequence, however, stays the same: S phase first, then the two divisions.
Because meiosis is fundamental to reproduction, questions about its timing are common in biology coursework and clinical genetics alike. If you have specific concerns about reproductive health or genetic conditions, consult a qualified medical professional.
Key Takeaways
- DNA replication in meiosis occurs exactly once, during the S phase of the cell cycle, before meiosis I begins.
- After replication, each chromosome has two identical sister chromatids held together by cohesion.
- No further DNA synthesis happens between meiosis I and meiosis II—one replication event supports both divisions.
- Meiotic prophase follows replication and is when homologous chromosomes recombine, creating genetic diversity.
- Meiosis I separates homologous chromosomes; meiosis II separates sister chromatids, yielding four haploid cells.
- Timing details vary across organisms and between sexes, but the “replicate once, divide twice” sequence is universal.
