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Cell Division: Eukaryotes
Meiosis
Preparation

Where?

Meiosis is a form of cell division that occurs in specialized eukaryotic cells that have two (or more) copies of all the chromosomes, DNA and genes necessary for survival. As a consequence of a single cell undergoing meiotic cell division, 1 to 4 new cells are produced that only contain single copies of all the chromosomes, DNA and genes necessary for survival.

Sexually reproducing eukaryotic organisms cannot have less than a single copy of all their genes on their chromosomes. This genetic state is called haploid, and it is often represented by the single letter n.

Many multicellular organisms, (mosses, for example), are haploid and only have a single set of chromosomes and genes in all the cells of their body. These "body cells" are called somatic cells, from the Latin word "soma" meaning "body".

The somatic cells of most other multicelluar organisms (such as humans) have a double set of chromosomes and genes. This genetic state is called diploid, and it is often represented by the number/letter combination 2n.

Meiosis occurs in diploid cells (2n) as they get ready to reduce the amount of genetic information they carry down to the haploid state (n). This is vital preparation for many kinds of sexual reproduction, and in diploid multicellular organisms it occurs in specialized, gamete producing parts of the body called gonads.


preparation

Depending on the organism, and when and where meiosis occurs, cells prepare for this kind of drastic cell division in much the same way that they prepare for mitotic (asexual) cell division.

Cells destined to change their genetic state from diploid (2n) to haploid (n), start by passing through a typical period of interphase, in which proteins and other macromolecules are produced in the usual way. In S-phase, which is typically much longer in these pre-meiotic cells, all the DNA molecules are duplicated by semiconservative DNA replication. Technically, therefore, each diploid cell has 4 copies of all the related DNA molecules and genes as it passes into G2 phase of the cell cycle.

Thus:

G1 [2 copies] ---> S-phase ---> G2 [4 copies]


separation

In a typical mitotic cell division these 4 copies (of the DNA and genes) would be reduced down to 2 copies (the diploid number) by the end of M-phase (mitosis).

However, in the two periods of a typical meiotic division, these 4 copies are first reduced to 2 copies by separating the pairs of homologous chromosomes (reduction division), and then the doubled DNA molecules are further reduced to single copies in the second period of division.

Thus:

[2 copies]+[2 copies] ----> [2 copies]
---> [1 copy]


genes go along
with the DNA

Genes are encoded messages carried on the copies of the DNA molecules. As the DNA is copied and then distributed to the haploid cells, so to are the genes carried on these DNA molecules.

Genetic engineers can now put special "tags" on genes, so that it is possible to follow their progress through the process of meiosis, reduction division, and haploid cell production. In this way it is possible to follow not only what is happening to the DNA molecules but also what is happening to the information that these DNA molecules are carrying.

Thus:



the difference

There is a subtle, but important difference between what happens to the DNA molecules and what happens to the biological information they carry during meiosis.

The duplication (synthesis) of the DNA molecules is completely mechanical, and their separation into separate nuclei, and thus into separate cells, is also mechanical. When it goes right, and there are no mistakes, all the haploid cells produced at the end all have exactly one copy of each DNA molecule they need.

However, the information these DNA molecules carry is not always the same, even if the "same" gene is involved.

Genetic engineers have shown that the biological information carried by apparently the same DNA gene sequence on two different DNA molecules is not always identical. For example the sequence -ATTGC- might be found in one copy of a gene, whereas the slightly different sequence -ATTGG- might be found on a different copy of that same gene.

Both genes "work" in that they code for the production of the same protein that has exactly the same function and role in the cell, but this genetic "tag" makes it possible to distinguish between them. This is a powerful way of following the progress of the various genes themselves as they pass through meiosis.

When a "tagged" and a "non-tagged" gene pair are thus followed, it is seen that the DNA and genes are both doubled in S-phase. But, during the first period of meiosis, the doubled "tagged" gene and the doubled "non-tagged" gene are separated from each other. This is called reduction division.

It is only in the second period of meiosis that the two copies of the "tagged" gene are separated, and the two copies of the "non-tagged" gene are separated. Each of these individual copies of the DNA and the genes they carry ends up in one of the final, haploid products.


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© 2002, Professor John Blamire