Most human somatic cells are diploid (2n46), contain two
copies of the genome and divide by mitosis. Germline oocytes
and spermatocytes divide by meiosis to produce haploid
gametes (n23). Some human somatic cells, for example giant
megakaryocytes, are polyploid and others, for example muscle
cells, contain multiple diploid nuclei as a result of cell fusion.
During cell division the DNA of the chromosomes becomes
highly condensed and they become visible under the light
microscope as structures containing two chromatids joined
together by a single centromere. This structure is essential for
segregation of the chromosomes during cell division and
chromosomes without centromeres are lost from the cell.
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