Latest update: 29/06/2017

How chromosomes are formed

Chromosomes consist of the same succession of billions of nucleotides as our 'tangled DNA', but it has been 'cut' and arranged into two groups of 23 pieces. This is because at a particular moment in cell division – the metaphase - the DNAorganises itself into 46 chromosomes. 

We know this picture as our karyogram.

In each pair of chromosomes, one comes from the mother and one from the father.
All human chromosomes are made up of two arms, a short arm (p for petit) and a long arm (q), separated from each other by the centromere. At the end of the chromosomes are the telomeres.

In some chromosomes the p arm is so short that it can barely be seen. These are known as acrocentric chromosomes 13, 14, 15, 21 and 22.


Sex chromosomes: male versus female

In a man's somatic cells there is always an X and a Y chromosome and in awoman's somatic cells there are always two X chromosomes.

In women oneof the two X chromosomes is deactivated at an early stage of embryonicdevelopment. Which one is deactivated seems to be chosen at random. Inany case it is not the same chromosome that is deactivated in everycell. Women therefore express some of the characteristics whose genesare situated on the X chromosome to varying degrees.

In a man the two sex chromosomes are not identical but we still consider them as a pair. There is little genetic information on the Y chromosome, but it plays a crucial role in the development of the embryo. The Y chromosome initiates production of male hormones and therefore the development of male sexual characteristics.

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