Meiosis: Two Divisions That Make Haploid Cells

The doubling problem

A human body cell has 46 chromosomes. Imagine if an egg and a sperm were made by ordinary mitosis: each would have 46, the baby would have 92, the grandchild 184... In just a few generations, cells would be stuffed with chromosomes.

Living things solved this problem long ago. Before fertilization, they use a special division called meiosis that cuts the chromosome number in half, from diploid (2n) to haploid (n). Then fertilization brings it back to 2n. This lesson walks through meiosis one phase at a time.

Before meiosis: interphase

As before mitosis, the cell grows and copies its DNA in the S phase. Every chromosome now has two sister chromatids joined at the centromere. A human cell entering meiosis still has 46 chromosomes, but 92 chromatids.

Meiosis I: separating the pairs

1. Prophase I. Chromosomes condense. Then something new happens: each chromosome finds its homologue and the two line up side by side, gene for gene. This pairing is synapsis, and each joined pair (four chromatids) is a tetrad. While they are paired, chromatids of the two homologues can swap matching segments: crossing over. The nuclear membrane breaks down and the spindle forms. Prophase I is the longest phase of meiosis.
2. Metaphase I. The tetrads line up in pairs across the equator, one homologue on each side. Which side the mother's or the father's chromosome faces is random for each pair.
3. Anaphase I. The homologous chromosomes separate and move to opposite poles. The centromeres do not split: each chromosome still has two chromatids.
4. Telophase I and cytokinesis. Two cells form. Each has one chromosome from each pair, so each is haploid (in humans, 23 chromosomes), but every chromosome is still doubled.

Because it halves the chromosome number, meiosis I is called the reduction division.

Meiosis II: separating the sister chromatids

There is no DNA copying between meiosis I and meiosis II. Each of the two cells now goes through a division that works like mitosis:

1. Prophase II. A new spindle forms in each cell.
2. Metaphase II. Chromosomes line up single file on the equator.
3. Anaphase II. The centromeres split and the sister chromatids separate, moving to opposite poles.
4. Telophase II and cytokinesis. Nuclear membranes re-form. Result: four haploid cells, each with one copy of each chromosome (a single chromatid).

Because of crossing over and the random line-up in metaphase I, the four cells are genetically different from each other and from the parent cell. The next lesson looks closely at why.

Seeing meiosis in the lab

In a school lab you might look at prepared slides of a lily anther (where pollen forms) or a grasshopper testis under a compound microscope at 400×. You look for cells in different stages and sketch them. Clues:

• Pairs side by side in the middle: metaphase I.
• Single file in the middle: metaphase II (or mitosis).
• Two small cells close together, each dividing again: meiosis II.

Good biological drawings use a sharp pencil, clean single lines, no shading, labels with straight ruled lines, a title and the magnification. You will practise with the model in the activity below.