Q.An anther has 1200 pollen grains. How many pollen mother cells must have been there to produce them?

Meiosis Overview

Think about what happens when a sperm meets an egg. Each one carries half the number of chromosomes of a normal body cell. When they fuse, the full number is restored. If sperm and egg were made by ordinary cell division (mitosis), each would have the full set — and fusion would double it every generation. That would be a disaster.

So nature invented a special division that cuts the chromosome number in half. That is meiosis.

The core idea in one sentence

Meiosis is a two-step division process that converts one diploid cell (2n) into four haploid cells (n), each genetically different from the parent and from each other.

The "two-step" part is crucial. Unlike mitosis, which is a single division, meiosis happens in two consecutive rounds: Meiosis I and Meiosis II.

Why two rounds?

The first round, Meiosis I, is the reductional division. Here, homologous chromosomes (one from each parent) pair up and then separate. The chromosome number drops from diploid to haploid. Each daughter cell after Meiosis I has one copy of each chromosome — but each chromosome still consists of two sister chromatids.

The second round, Meiosis II, is the equational division. It looks very much like mitosis. The sister chromatids finally separate. Each of the two cells from Meiosis I divides again, giving four haploid cells in total.

What happens to the chromosomes?

Start with a diploid cell. For a human, that means 46 chromosomes — 23 pairs. Each pair consists of one chromosome from your mother and one from your father. These are homologous chromosomes.

Before meiosis begins, DNA replication happens once, just like before mitosis. So each chromosome now has two identical sister chromatids. But the chromosome number is still 46 (each chromosome, even with two chromatids, counts as one chromosome).

Meiosis I — the homologous pairs line up together, exchange segments (crossing over), and then get pulled to opposite poles. The cell divides. Now you have two cells, each with 23 chromosomes. But each of those 23 chromosomes still has two chromatids. The number has been halved: from 46 to 23.

Meiosis II — no further DNA replication. The chromosomes line up again, and this time the sister chromatids separate. Each of the two cells divides, giving four cells. Each has 23 chromosomes, and each chromosome is now a single chromatid. …