Chromosome. Coloured scanning electron micrograph (SEM) of a chromosome with a chromatid break (left). Chromosomes consist of two identical chromatids joined at a centromere. Each chromatid is comprised of deoxyribonucleic acid (DNA) coiled around proteins. A break in a chromatid can happen spontaneously or because of environmental factors such as radiation or physical stress. The chromatid may be rejoined by enzymes, with no further consequences. However, the fragment not attached to the centromere can be lost, deleting part of the chromosome. Depending upon the length and genes lost this may be lethal, especially in the sex cells, which have only 1 copy of each chromosome. Magnification: x8,800 at 10cm wide.