Light micrograph of six chromosomes from an onion (Allium cepa) cell showing different degrees of harlequin banding. Chromosomes are a packaged form of a cell's genetic material DNA (deoxyribonucleic acid). The DNA condenses into chromosomes during cell replication. Each chromosome consists of two identical strands (chromatids), aligned parallel to each other and joined at an area called the centromere. The harlequin staining technique shows which chromatid was formed first (dark purple) and which second (light purple). Occasionally sister chromatids may exchange segments, particularly in the presence of genotoxic substances, resulting in the banded patterns seen in this micrograph.