When Is DNA Replicated in Meiosis? The S Phase Explained
DNA is replicated during the S phase of interphase, before meiosis begins. That single round of replication happens in the parental cell while it is still diploid, and it is finished well before the first meiotic division starts. In other words, the answer to when is DNA replicated in meiosis is: not during meiosis at all, but just before it, in the S phase of the cell cycle. Meiosis then carries out two consecutive nuclear divisions using the DNA copies that were already made, ultimately producing four haploid daughter cells.
To understand why the timing works this way, it helps to separate replication from division and see where each one fits in the cell cycle.
The Short Answer
Replication occurs during the S phase of interphase, the stage that precedes meiosis. The “S” stands for synthesis, and it refers specifically to the synthesis of new DNA. During this phase, each chromosome is duplicated so that it consists of two sister chromatids joined at a centromere. The cell is still diploid at this point — it has not yet divided — and it has simply doubled its DNA content in preparation for division.
That means replication is not a step within meiosis. Instead, it is a preliminary step in interphase, and meiosis proper begins afterward. The distinction matters because it explains how a single duplication event can support two rounds of division, which is the central trick behind meiosis.
What “S phase” actually means
The S phase is one part of interphase, the period of growth and preparation before a cell divides. Interphase is traditionally described in three stages: G1 (cell growth), S (DNA synthesis), and G2 (further growth and preparation for division). Replication is confined to S — it is not spread across interphase, and it does not continue into meiosis. For a general reference on the phases of the cell cycle, Nature Scitable lays out the sequence clearly.
S Phase vs. Meiosis
Interphase and meiosis are sequential, not overlapping. Interphase comes first and includes the S phase, where the genome is duplicated. Meiosis comes after, and it consists of two divisions: meiosis I and meiosis II. By the time meiosis I begins, replication is complete.
The order looks like this:
- Interphase (G1, S, G2): The cell grows, replicates its DNA in S, and prepares for division.
- Meiosis I: The homologous chromosomes separate.
- Meiosis II: The sister chromatids separate.
Because replication is finished before meiosis I, the two meiotic divisions operate on DNA that is already duplicated. No new replication occurs between meiosis I and meiosis II. This is a key difference from mitosis, where a single division is preceded by a single S phase.
Where the two meiotic divisions fit
Meiosis I is often called the reductional division because it reduces the chromosome number from diploid (2n) to haploid (n) by separating homologous chromosomes. Meiosis II is the equational division because it separates sister chromatids, similar to mitosis. The National Center for Biotechnology Information provides detailed overviews of these divisions. Both divisions rely on the DNA copies produced earlier, during S phase — there is no second round of replication to provide fresh copies.
Why Replication Precedes Meiosis
Replication must happen first because each of the four final cells needs a complete set of genetic instructions. If the DNA were not duplicated beforehand, the divisions would leave daughter cells with incomplete genomes.
A diploid cell entering meiosis has two copies of each chromosome — one from each parent — for a total of 2n chromosomes. In the S phase, each of those chromosomes is copied, so the cell temporarily holds 4n DNA content, arranged as pairs of sister chromatids. Meiosis I then separates the homologous pairs, and meiosis II separates the sister chromatids. The result is four haploid cells, each with n chromosomes.
The arithmetic only works because duplication happened first. Without it, the two divisions would halve the DNA twice, producing cells with too little genetic material.
A short worked example
Imagine a hypothetical organism with a diploid number of 4 chromosomes (2n = 4). Before meiosis, during S phase, each of the 4 chromosomes is replicated, giving 8 chromatids paired as 4 sets of sister chromatids.
- After meiosis I: 2 cells, each with 2 chromosomes (still as sister chromatids).
- After meiosis II: 4 cells, each with 2 chromosomes (now single chromatids).
Each of the four cells ends up haploid, with one complete set of chromosomes — made possible entirely by the replication that occurred before meiosis started.
Replication vs. Meiotic Division
It is easy to blur replication and division together, but they are distinct processes with different jobs. Replication copies DNA; division distributes it. In meiosis, there is exactly one round of replication and two rounds of division.
| Feature | DNA replication | Meiotic division |
|---|---|---|
| Number of rounds | One | Two (meiosis I and II) |
| When it occurs | S phase of interphase | After interphase, sequentially |
| Main outcome | Duplicated chromosomes (sister chromatids) | Four haploid cells |
| Genetic material per cell | Doubled | Halved, then halved again |
The mismatch between one replication and two divisions is what produces four haploid cells instead of two diploid ones. Each division step sorts the already-copied DNA, and because there is no intervening replication, the chromosome number drops appropriately; Khan Academy offers a helpful walkthrough of the phases.
One consequence worth noting is genetic variation. Because homologous chromosomes pair and can exchange segments during meiosis I — a process called crossing over — the DNA copies made in S phase can be reshuffled before they are distributed. That variation depends on replication having already produced the sister chromatids that participate in these events.
Key Takeaways
- DNA is replicated during the S phase of interphase, before meiosis begins — not during meiosis itself.
- The S phase is part of interphase (G1, S, G2), which precedes the two meiotic divisions.
- One round of replication supports two rounds of division (meiosis I and meiosis II).
- Each of the four haploid daughter cells receives a complete genome because the DNA was duplicated first.
- No replication occurs between meiosis I and meiosis II.
- Replication copies DNA; meiotic division distributes it — a single duplication followed by two divisions yields four haploid cells.
This content is for general educational purposes and is not a substitute for professional advice. For questions about genetics or a specific medical concern, consult a qualified healthcare professional.
