Q.Meiosis-I is often considered a complete cell division. What is the necessity of occurrence of Meiosis-II in cell cycle?

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.

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.

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. …