In the 1860s, animal cell nuclei were observed to contain linear bodies that were named chromosomes (Greek: chromos, color; soma, body) because they are strongly stained by certain basic dyes.

Most genes are located in these, under microscope visible structures, chromosomes. There are normally two copies of each chromosome (homologous pairs) present in virtually every cell.

The number of unique chromosomes (N) in a cell is known as its haploid number (46 in Humans) and the total number of chromosomes (2N) is its diploid number (23 in Humans). Different species differ in their haploid number of chromosomes.

A cell during mitosis showing its chromosomes
(in dark blue) being pulled to opposite poles of the cell.

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