Human chromosome 21. Colored transmission electron micrograph (TEM) of human chromosome number 21. There are 46 chromosomes in the nuclei of all cells apart from the sex cells. The chromosomes carry DNA (deoxyribonucleic acid) genetic information in the form of genes. They can make exact copies of themselves while the cell is resting between successive cell divisions. Normally parents pass two chromosomes numbered 21 onto their son or daughter. However sometimes three of these chromosomes are inherited resulting in Down's syndrome, a form of mental handicap. Negatively stained. Magnification: x20,000 at 5x7cm, x22,600 at 6x9cm size. x10.000 at 35mm