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From Greek: khroma meaning "color" and soma meaning "body". Based on appearance of chromosomes which demonstrate bands if stained during the mitotic process.

Chromosomes are long strings of DNA, bound around supporting protein structures, which normally reside within the nucleus of each cell. For each cell type, at a particular time in the cell cycle the three dimensional position of the chromosome can be predicted fairly accurately relative to other chromosomes and nuclear structures[1]. Indeed in somatic cells there is claimed to be a relation between where gene expression takes place, chromosome position and higher organism structure[2] as well as sperm[3]. The DNA on the chromosomes encodes 20,000-25,000 protein encoding genes[4] and many more regulator genes, such as for miRNA which are still being characterised. This makes up our unique genetic blueprint.

The collection of all genetic material of an organism is referred to as its genome. That of Homo sapiens is visualised here. There are 23 pairs of chromosomes in most human cells, 22 are autosomes, while the remaining pair are the sex chromosomes (X or Y).

Various parts of chromosome are identified with names. Telomeres are the end sections. Centromeres are the part of the chromosome which hold the two identical copies of DNA together during mitosis. Shorter arm named p (for petit) and the longer arm q (etymology: either 'q' as next letter after p or French 'queue' meaning tail).

Within the respective p or q regions, each arm is divided into between 1 and 3 regions primarily based on Giemsa staining of chromosomes, numbered from the centromere outwards towards the telomere, e.g. 7p1 means the first region of the short arm of chromosome 7. Further subdivisions are added depending on alternating light and dark bands. With increased resolution, further subdivisions can be added, e.g. 7q11.22 means the long arm of chromosome 7 in region 1, band 1, sub-band 2, sub-sub-band 2. The correct way to read the notation is "seven q one one dot two two" and not "seven q eleven dot twenty-two".


For some time, until 1955 the human chromosome number was thought to be 48 although the counts were not consistent[5]. During the Christmas/New Year period 1955/56 two researchers (Albert Levan and Joe Hin Tjio)[6] got it all sorted out at the University of Lund, Sweden[7].