Human chromosomes and nucleus, coloured scanning electron micrograph (SEM). Chromosomes are a packaged form of the genetic material DNA (deoxyribonucleic acid). The DNA normally exists in a non-condensed form in the cell nucleus (upper right). It condenses into chromosomes (centre and lower left) during cell replication. In humans, there are 46 chromosomes, consisting of 23 chromosome pairs. Magnification: x4350 when printed at 10 centimetres across.
