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.


Autosomes

The first 22 pairs of chromosomes are homologous: they have the same structure, but that does not mean that they contain the same information. We call these body-defining or autosomal and they are numbered from largest to smallest.
The 23rd pair determines our gender: XY for a man, XX for a woman.