MEIOSIS

In Meiosis, the resulting daughter cells are different. Why is to so? Let's look at a normal cell first.

A normal cell  is diploid (i.e. it has 2 sets of chromosomes). In rare cases a cell can be polyploid and the number of sets of chromosomes can be more than 2. One set comes from its 'father', the other one from its 'mother'. In normal cell division, mitosis, the two daughter cells have 2 identical sets of chromosomes. Hence it is called Conservative Multiplication.

Meiosis is Non-conservative Multiplication, so the daughter cells are different. It normally occurs in germ or sex cells.

During meiosis, DNA replication (which makes the cell have twice its usual amount of DNA) is followed by two cell divisions. At the end, one parent cell gives rise to four haploid gametes (each with one set of chromosomes). The haploid cell contains only 1 chromosome of each homologous pair (which is only half of the normal amount). There is no replication on the DNA during the short interphase between the two cell divisions. Meiosis consists of 2 divisions, 8 'phases'.

First Division: Prophase I, Metaphase I, Anaphase I, Telophase I

Second Division: Prophase II, Metaphase II, Anaphase II, Telophase II

First Division

The result of this cell division is a reduction of the number of chromosomes per cell. The step preceding the first cell division is the 'interphase'. The DNA is replicated into 2 identical copies, just as in mitosis. During interphase, the DNA is not visible. The only thing that could be seen in a microscope is the nucleolus.

1. Prophase I

This  is quite different to the mitotic prophase, the chromosomes become visible. They look very long, as they are not yet totally condensed. Their ends are linked to one pole of the nucleus. The chromosomes that belong to the same pair come next to each other. The chromosomes become thicker and shorter, as they condense more and more. They are linked to each other in pairs. The 2 chromatids of each chromosome become visible. The chromosomes then start to move away from each other, but remain linked at points called chiasmata. At this stage, some genetic material can be exchanged between chromosomes.

2. Metaphase I

The pairs of chromosomes (tetrads) are situated at the equator of the cell. At this stage, the nuclear membrane has disappeared. The centromeres are orientated toward the poles of the cell, and the chromosomes are fully condensed.

3. Anaphase I

The chromosomes (2 chromatids each) migrate toward opposite poles. There is a separation of chromosomes, and not of chromatids as in mitosis. Each pole will receive one set of chromosomes of 2 chromatids.

4. Telophase I

This is very short, and often mistaken with prophase II. There is no time for the formation of a new cell membrane, nor for duplication of DNA. The next step starts immediately.

Second Division

The second division conserves the number of chromosomes but divides the chromatids.

5. Prophase II

It is very short. Everything is ready. The two centrioles migrate away from each other, and a network of microtubules forms in each daughter cell. The two networks are parallel to each other, perpendicular to the previous one.

6. Metaphase II

The chromosomes are situated on the equator. Therefore we see two equators.

7. Anaphase II

The chromatids, although present at the first cell division, only separate now. They migrate toward the two opposite poles of the cell.

8. Telophase II

The reconstitution of four daughter cells starts and finishes.

Conclusion

In most cases, the division results in spores or gametes. The cell has given rise to four daughter cells, each with a different set of chromosomes. Each has a set of chromosomes, and will meet another set of chromosomes in another cell in order to constitute a fertilised egg which will give a fully working new individual (plant or animal).