For 33 years scientists confidently miscounted human chromosomes as 48
In 1923 Theophilus Painter peered through his microscope and counted 24 pairs of human chromosomes, 48 in all. Others repeated his figure for decades. Only in 1956 did the Indonesian-born cytogeneticist Joe Hin Tjio establish the true number, 46, a reminder that even basic facts about ourselves can hide in plain sight.
A chromosome is a package of DNA holding some or all of an organism's genetic material. In complex cells, the long, thin DNA strands wrap around histone proteins that condense and protect them, and the resulting three-dimensional folding helps control which genes are read. Chromosomes usually become visible under a light microscope only during metaphase of cell division, when each duplicated chromosome, two sister chromatids joined at a centromere, is tightly coiled and lined up at the cell's centre.
The name, coined by the German anatomist Heinrich Wilhelm Waldeyer in 1888, means coloured body, after the strong stain these structures take up; ironically they have no colour of their own. Otto Butschli first recognised them. From the mid-1880s Theodor Boveri showed they carry heredity, testing Wilhelm Roux's idea that each chromosome holds different genetic information. Once Gregor Mendel's work was rediscovered around 1900, Boveri and Walter Sutton independently tied inheritance to chromosome behaviour, and Edmund Beecher Wilson named the Boveri-Sutton chromosome theory. Famous geneticists such as William Bateson and T. H. Morgan resisted it, until chromosome maps from Morgan's own laboratory settled the matter.
Chromosome errors matter. Recombination during meiosis shuffles genes and fuels genetic diversity, but instability or misplaced segments can trigger mitotic catastrophe. Cells usually respond by self-destructing, yet mutations sometimes block that response and allow cancer to progress.
Bacteria and archaea generally have one circular chromosome, coiled into a region called the nucleoid rather than a nucleus. Sizes vary enormously, from about 130,000 base pairs in certain bacteria living inside insects to over 14 million in the soil bacterium Sorangium cellulosum. There are exceptions: the Lyme disease bacterium has a linear chromosome, and Vibrio species typically carry two of very different sizes. Many bacteria also hold plasmids, small DNA circles that help genes jump between cells.
Source: Chromosome
