A normal cell has 2 sets of chromosomes; it is diploid, (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, known as 'mitosis', the two daughter cells also have two sets of chromosomes. That's why it is called Conservative Multiplication.
In the testis of a mammal , or the anthers of a flower, or any organ containing germ cells, one can observe a type of cell division called meiosis. 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 one 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 is a succession of events:
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 two identical copies, just as in mitosis. For illustration purposes, I have drawn the DNA in different colours. During interphase, the DNA is not visible. The only thing you could see in a microscope is the nucleolus, an area of intense RNA synthesis.|
|During 'prophase', which 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. This is illustrated
in the drawing on the right. 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. At this stage, 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 : it is now that crossing-over may take place.
|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.|
|Anaphase I : the chromosomes (two 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.|
|The telophase 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 straight away.|
The second division conserves the number of chromosomes but divides the chromatids.
|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.|
|Metaphase II : the chromosomes are situated on the equator. We therefore see two equators.|
|Anaphase II : the chromatids, although present at the first cell division, only separate now. They migrate toward the two opposite poles of the cell.|
|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). Maybe you could think of all the different processes that led to variation in the meiosis... But I have prepared something for you!|