[Mitosis and meiosis - introductory video]

Each of us is unique. We each have our own combination of characteristics due to our own unique genotype and the environment in which we grew.

Why are we genetically unique?

The answer lies in the process of sexual reproduction. During fertilisation, one human sperm combines with a human egg cell to produce a zygote. Although all human sperm look identical - as do all human egg cells - each gamete contains a unique combination of genes, so the product of their fusion is also genetically distinct.

In this activity, we'll examine the process of meiosis - the nuclear division that takes place when sperm and eggs are formed.

But how does it give rise to gametes with such a variety of genetic combinations?

Before that, we'll look at mitosis, the nuclear division that occurs when a zygote multiplies and grows in to a new individual.

[Mitosis - plant cell video]

Here's the nucleus of a living plant cell, about to undergo mitosis. The chromosomes become visible and the nuclear membrane breaks down. Watch what happens to the chromosomes.

A new cell wall forms bottom-left to top-right and where we had one cell, we now have two. The two long, thin nuclei can be seen on either side of the new cell wall - one above it and one below it. Here's a close up of the cell with the nucleus at prophase. Each chromosome is a pair of chromatids joined at one point - the centromere. Each chromosome is drawn to the equator of the cell. Once there, the stage of division is metaphase.

Then abruptly, each chromosome splits in two. This is anaphase - where one chromatid moves to each end of the cell. This is now telophase. A new cell wall forms, the chromosomes lose their visibility and two distinct nuclei appear. The cells are now at interphase.

[Mitosis - complete animation]

This cell is about to undergo mitosis. Mitosis is the division of the nucleus, which has to occur before the cell itself can divide. Mitosis is a dynamic and continuous process, but for convenience, biologists identify stages, or phases, of the process. This cell is at interphase. This means that it is not yet dividing but actively growing and replicating the DNA in its chromosomes. The chromosomes themselves are not actually visible. Mitosis proper begins when the DNA molecules in the chromosomes become tightly coiled or condensed. At this point they become visible under the microscope. This is prophase, the first stage of mitosis. As a result of the DNA in each chromosome replicating itself during interphase, each chromosome is made up of two identical strands of DNA. These strands are now known as chromatids, and they are joined at the centromere. Notice that there are only two pairs of chromosomes in this cell - two long and two short.

At the end of prophase, the nuclear membrane breaks down, and contractile fibres, called spindle fibres, form. The spindle fibres attach to the centromere of each chromosome. Here you can see that the individual chromosomes line up along the centre of the cell. This phase is called metaphase.

The next phase, anaphase, happens very quickly. Each centromere splits, and each chromatid is drawn towards one end, or pole, of the cell, as a result of tension in the spindle fibres. Now the chromosomes have reached the poles of the cell. This stage is called telophase. A nuclear membrane reforms around each group of chromosomes. The chromosomes then uncoil, and can no longer be seen.

Now the process of mitosis, or nuclear division, is complete. The cell now divides into two identical progeny cells. Here the cells are at interphase again, and ready to begin the process of mitosis once more. This continuous process of interphase - mitosis - interphase - mitosis is what represents the eukaryotic cell cycle.

[Meiosis - complete animation]

Meiosis is a specialised form of nuclear division. It only happens in those cells which give rise to gametes, that is, sperm and egg cells in animals, or pollen and egg cells in plants. It consists of two nuclear divisions, sometimes in rapid succession. We start with a single diploid cell in interphase. Meiosis begins when the chromosomes become visible.

Here, each chromosome has already replicated, so consists of two chromatids, joined at the centromere. In the cell are pairs of homologous chromosomes or homologues. One chromosome from each pair originated from the maternal parent (here shown in pink), and the other from the paternal parent (here shown in blue).

The early events in meiosis are crucial. Look at the homologous chromosomes and see how they pair up, that is, the maternal and paternal homologues line up together as a pair. At this point the nuclear membrane breaks down, and spindle fibres form. The spindle fibres attach to the centromeres and you can see the pairs of homologous chromosomes lining up along the centre of the cell. This is metaphase I.

The next phase is anaphase I, which happens very quickly. Watch carefully to see how one homologous chromosome from each pair is drawn towards each pole.

Here, at telophase I, the chromosomes have reached the poles of the cell. A nuclear membrane forms around each group of chromosomes. The cell divides to form two progeny cells. Here we come to the end of meiosis I, the first nuclear division of meiosis.

You can see that each cell now has half the original number of chromosomes. The two cells now enter prophase II. As in prophase I the nuclear membrane breaks down, and spindle fibres develop once again. The spindle fibres again attach to the centromeres, and you can again see the chromosomes lining up along the centre of the cell. This is metaphase II. Note that each chromosome still consists of two chromatids.

In this second meiotic division the centromeres divide. The chromatids then separate to each pole of the cell. Each chromatid is now referred to as a chromosome. This is anaphase II.

Here at telophase II, the chromosomes have reached the ends of the cell. A nuclear membrane forms around each group of chromosomes, and both cells divide, to form a total of four progeny cells. You can see that each cell now has one of the four chromosomes that made up the pair of homologous chromosomes at that crucial first stage, prophase I. See how each cell in the gametes produced at the end of this process finishes with a haploid number of chromosomes. Now meiosis is complete.