A Cell Preparing to Undergo Meiosis Duplicates Its Chromosomes During
A cell preparing to undergo meiosis duplicates its chromosomes during the S phase of interphase — the DNA synthesis stage that occurs before meiosis I begins. Meiosis is the specialized cell division that produces gametes (sperm and egg cells), each with half the normal chromosome number. By copying its DNA first, the cell ensures every resulting gamete receives a complete set of genetic instructions.
When Chromosome Duplication Occurs
Interphase is the growth and preparation period before division, and it has three stages: G1 (cell growth), S (DNA synthesis), and G2 (final checks before division). Duplication happens in S phase, when each chromosome is replicated to form two identical sister chromatids joined at a region called the centromere. According to the National Center for Biotechnology Information, this replication is tightly regulated so the genome is copied once — and only once — per division cycle.
At the end of S phase, the cell still has the same chromosome number as before (for example, 46 in a human cell), but each chromosome now consists of two chromatids. A diploid human cell entering meiosis therefore contains 46 chromosomes made of 92 chromatids. Duplication is complete before any meiotic division starts.
Duplication vs. Meiosis I
It helps to separate two events that are easy to confuse. Duplication is one round of DNA replication; meiosis is two consecutive divisions, meiosis I and meiosis II. The cell copies its DNA only once, then divides twice — a pattern Nature Education - Scitable describes as “one replication, two divisions.”
During meiosis I, homologous chromosomes (matching pairs, one from each parent) pair up and separate, reducing the chromosome number by half. During meiosis II, the sister chromatids created back in S phase are finally split apart, much like in mitosis. The result is four haploid cells, each with a single set of chromosomes.
A quick example: a human cell with 46 chromosomes replicates its DNA in S phase, making 46 duplicated chromosomes. After meiosis I, each daughter cell has 23 duplicated chromosomes; after meiosis II, four cells each carry 23 single chromosomes.
One more detail matters here. Crossing over occurs after replication, during prophase I, so it acts on already-duplicated chromosomes. Each pair of homologs forms a structure containing four chromatids. When homologs separate in anaphase I, the sister chromatids remain attached; only in anaphase II are they pulled apart.
Key Takeaways
- Chromosome duplication happens in the S phase of interphase, before meiosis I begins.
- DNA is copied once, but the cell divides twice, producing four haploid cells.
- After S phase, chromosome number stays the same while chromatid number doubles.
- Meiosis I separates homologous chromosomes; meiosis II separates the sister chromatids made during S phase.
- Sister chromatids are identical copies of one chromosome; homologs are the maternal and paternal versions.
