Meiosis and its importance

1Why not make gametes by mitosis?

Suppose human sperm and eggs were made by mitosis. Each would carry 46 chromosomes, and the zygote made at fertilisation would have 92. The next generation would have 184, the one after that 368, and within a few generations a nucleus could not hold them all. That never happens: every human generation has 46. The reason is a second kind of nuclear division, meiosis, which makes gametes with half the chromosome number. Fertilisation then joins two half sets to make one full set again.

2Where meiosis happens

Meiosis happens only in the organs that make gametes. In humans these are the testes, which make sperm, and the ovaries, which make eggs (you met both in Senior 1). In a flowering plant it happens in the anthers, where pollen grains form, and in the ovules, where the egg cells form. Every other cell in the body divides by mitosis.

3Two divisions after one copy

Before meiosis the cell goes through interphase and copies its DNA once, in the S phase, just as before mitosis. It then divides twice, with no copying in between:

• Meiosis I separates the homologous chromosomes of each pair. This is the division that halves the chromosome number.
• Meiosis II separates the sister chromatids, much as mitosis does.

Figure 1 follows the same kind of cell you saw divide by mitosis, with $2n=42n\; =\; 4$: a long pair and a short pair, blue from the father and orange from the mother.

Figure 1: Meiosis in a cell with 2n = 4. The + signs are the spindle poles and the dashed lines are equators. Crossing over is drawn once, in the long pair: the swapped end pieces show as a patch of the other colour.

Count the chromosomes stage by stage, using Figure 1:

4Why meiosis matters

1. It keeps the chromosome number constant from one generation to the next. Gametes are haploid, so fertilisation restores the diploid number: $23+23=4623\; +\; 23\; =\; 46$.

2. It makes every gamete genetically different, in two ways:

• Crossing over in prophase I makes new mixtures of the father's and mother's genes on the same chromosome. In Figure 1, two of the four cells carry a long chromosome that is part blue and part orange: neither parent had a chromosome like that.
• Independent assortment: at metaphase I each pair lines up on the equator facing either way, independently of every other pair. In Figure 1 the long pair has blue on top but the short pair has orange on top; it could just as well have been the other way. With $nn$ pairs, independent assortment alone can give $2^{n}2^n$ different combinations of chromosomes.

On top of this, fertilisation is random: any one of millions of sperm may fertilise the egg. Together, these explain why brothers and sisters with the same parents are not identical, unless they are identical twins from one zygote.

Why variation is useful: a population whose members differ is more likely to include some that survive a new disease, a drought or a change in climate. Compare the field of cloned potato plants in the last lesson, where one disease could destroy every plant.