Meiosis II: Separating the Copies
You already know that in Meiosis I, homologous chromosomes pair up, swap segments (crossing over), and then get pulled apart into two different cells. Each of those two cells now has one copy of each chromosome — but here's the catch: that copy is still made of two identical sister chromatids joined at the centromere. So each cell has half the number of chromosomes, but each chromosome is still doubled.
Meiosis II is the clean-up act. It takes those two cells and splits the sister chromatids apart, just like in mitosis. The result? Four cells, each with a single, undoubled copy of every chromosome.
Meiosis I reduces the chromosome number from diploid to haploid. Meiosis II does not change the chromosome number — it just separates the chromatids. That's why Meiosis II is called the equational division.
What actually happens in Meiosis II?
There is no DNA replication before Meiosis II. The two cells from Meiosis I go straight into a second division. The stages are the same as mitosis: prophase II, metaphase II, anaphase II, telophase II.
In prophase II, the nuclear envelope breaks down again, and spindle fibres form. In metaphase II, the chromosomes line up at the equator — but now each chromosome is a single unit (two chromatids still joined), not a pair of homologues. In anaphase II, the centromere splits, and the sister chromatids are pulled to opposite poles. Each chromatid is now a full-fledged chromosome. In telophase II, nuclear envelopes reform, and you get four haploid cells.
The key difference from mitosis: in mitosis, you start with a diploid cell and end with two identical diploid cells. In Meiosis II, you start with two haploid cells (each with doubled chromosomes) and end with four haploid cells (each with single chromosomes).
Why does Meiosis II matter?
Two big reasons.
First, it keeps the chromosome number constant across generations. If meiosis only did the reduction division (Meiosis I) and stopped, you'd have haploid gametes with doubled chromosomes. When they fused at fertilisation, the zygote would have twice the normal number. By splitting the chromatids in Meiosis II, each gamete ends up with exactly one copy of each chromosome. When two gametes fuse, the diploid number is restored — no more, no less. …