Chromosomal rainbow. Fluorescent light micrograph of probes bound to human chromosome 10. Chromosomes are composed of deoxyribonucleic acid (DNA) coiled around proteins. DNA contains sections, called genes, which encode the body's genetic information. In this technique each block of colour is a probe that binds to a specific DNA sequence (locus). During meiosis, the formation of the sex cells, pairs of chromosomes line up together and may overlap and swap segments (recombination). This technique allows the rapid mapping of these chromosomal rearrangements as well as abnormalities such as deletions and insertions of blocks of DNA.