Chromosomes. Coloured scanning electron micrograph (SEM) of two chromosomes. The SEM is overlaid on a DNA autoradiogram background. Chromosomes contain the genetic material DNA (deoxyribonucleic acid). These chromosomes are at a stage where they have replicated. The replicate (copy) and the original are termed chromatids and align parallel to each other, joined at an area called a centromere. The 46 human chromosomes hold the DNA that carries the genetic code, unique for every individual. DNA controls the development and functioning of every living organism. The code consists of the sequence of DNA nucleotide bases, seen as the bands (black) on an autoradiogram. Magnification unknown.