The number and structure of the chromosomes in the nucleus of a cell. The karyotype is identical in all the diploid cells of an organism. The procedure of karyotyping involves culturing a sample of tissue cells from the individual concerned, then arresting the cell cycle in metaphase, when the chromosomes are in their most condensed state, by addition of the poison colchicine. The cells are immersed in a hypotonic solution, which causes them to burst; the nuclei are then fixed and stained to reveal the structural features of the chromosomes. The addition of Giemsa dye causes the appearance of the characteristic banding patterns of each chromosome pair. By viewing through a microscope fitted with a digital camera and suitable software, the homologous pairs of chromosomes can be arranged in numerical order. See chromosome map; karyogram; spectral karyotyping.