Meiosis and Fertilisation.

Sex is a complicated way for an organism to reproduce itself. What benefit does it have, compared to simple cell division? The answer lies in the complex process of cell replication known as meiosis, and the way it works holds one of the keys to evolution.

In this discussion, we limit ourselves to the human genome, which has forty six chromosomes, in twenty two pairs, the autosomes, plus two more: either two X chromosomes or an X and a Y.

Most people will have seen a picture similar to the one above, where an extract of the separated chromosomes has been photographed, and the images placed in order . It is only possible to do this during the final stages of cell division. This way of displaying and representing a particular sample of DNA is known as its karyotype.

Under normal conditions, the chromosomes, each being a molecule of DNA are packed together in the nucleus. The total length of chromosomal DNA for a human is about two metres. In the nucleus it is coiled and re-coiled around protein molecules, forming nucleosomes. These are further wound into coils supported on a protein scaffold.

In the first stage of meiosis, the prophase, the chromosomes are copied. They appear as thin threads, each containing the original and copy.

They then arrange themselves with each original chromosome and its copy side by side. Each has a centromere linking its two copies together.

As meiosis continues, they condense further, and it is at this point that they can be viewed as visibly separate chromosomes as in the karyotype above.

Each chromosome splits along its length, except at the centromere.

At this stage, another process called 'crossing over', or recombination, occurs. At one or more points along any, or all, of the chromosomes, equivalent lengths of genetic material may be exchanged.

A number of crossovers may occur, but there has to be at least one on each pair. It should also be added that recombination is not random. In some parts of the DNA it occurs frequently, at others rarely.

The chromatids then separate and form two cells.

The last stage is that the chromosome pairs part company, and each separates into two more cells. Thus there are now four sex cells.

Cells with the normal complement of pairs of chromosome are referred to as diploid.^* The sex cells here are haploid. Bear in mind, at this point, that we are only showing one chromosome pair, when there are twenty three pairs (forty six). Each sex cell receives its twenty three individual chromosomes completely at random, so each sex cell may contain one of over eight million combinations. This is called random assortment.

 
Fertilisation occurs when a sex cell from a male individual in the form of his sperm, containing a random selection of chromosomes from both his mother and father, combines with a female's sex cell, in an ova, containing a random selection of chromosomes from both her mother and father. This has been further randomised by recombination. Thus the individual that develops from this union will have a random selection of alleles from both of his, or her, grandfathers and grandmothers, and from increasing numbers of ancestors from past generations.

With such a complex process, mistakes do occur from time to time. As in mitosis, an error in copying a gene may occur, the difference being that that it will be handed down to succeeding generations. Such a germ line mutation may be harmful, may be beneficial, or simply have no apparent effect at all.

Nevertheless it has implications for genetic engineering. One way that is being investigated for alleviating the symptoms of cystic fibrosis is to introduce altered DNA to the lungs of the sufferer. This altered DNA is not likely to pass on from the individual, but there is something of a consensus that such genetic engineering should not be applied to the germ line, in human beings at least.

Footnote

* Sometimes one or more chromosomes may be lost, or there may be extra. Such cells are referred to as aneuploid

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