Chromosome

A chromosome is a structure that packages DNA, containing some or all of an organism's genetic material. In most cases, long, thin DNA fibers are wrapped around proteins that form nucleosomes; in eukaryotic cells, histones are the key packaging proteins. With help from chaperone proteins, histones bind to and condense the DNA, preserving its integrity. These eukaryotic chromosomes have a complex three-dimensional shape that significantly influences how genes are regulated.

Chromosomes are typically only visible with a light microscope during metaphase of cell division, when they are condensed and aligned at the cell's center. Before this, each chromosome is duplicated during S phase, and the two copies—called sister chromatids—are joined at a centromere. This creates either an X-shaped structure if the centromere is in the middle, or a two-armed shape if it is near an end. During metaphase, this duplicated structure, known as a metaphase chromosome, is highly condensed and easiest to study. In animal cells, chromosomes reach their tightest packing during anaphase when they are being separated.

Recombination between chromosomes during meiosis, followed by sexual reproduction, is essential for creating genetic diversity. If chromosomes are mishandled—through events like chromosomal instability or translocation—the cell may undergo mitotic catastrophe. This usually triggers apoptosis, causing the cell to die, but sometimes mutations prevent this, potentially leading to cancer.

The term "chromosome" is sometimes used broadly to refer to individual pieces of chromatin in a cell, whether or not they are visible under a microscope. More narrowly, it refers to those pieces during cell division, when they are condensed enough to be seen.

**Etymology** The word comes from the Ancient Greek *khrôma* ("colour") and *sôma* ("body"), referring to the strong staining from certain dyes. German anatomist Heinrich Wilhelm Waldeyer coined the term, building on "chromatin," introduced by Walther Flemming. Some early terms are now outdated: both "chromatin" (Flemming, 1880) and "chromosom" (Waldeyer, 1888) imply color for something that is not inherently colored.

**History of discovery** Otto Bütschli first recognized these structures as chromosomes. Starting in the mid-1880s, Theodor Boveri provided key evidence that chromosomes carry hereditary information, developing ideas of "chromosome continuity" and "chromosome individuality." Wilhelm Roux proposed that each chromosome has a distinct genetic makeup, and Boveri tested and confirmed this. After Gregor Mendel’s work was rediscovered in the early 1900s, Boveri linked inheritance rules to chromosome behavior. His influence shaped two generations of American cytologists, including Edmund Beecher Wilson, Nettie Stevens, Walter Sutton, and Theophilus Painter. In his textbook *The Cell in Development and Heredity*, Wilson combined the work of Boveri and Sutton (both around 1902) into the Boveri–Sutton chromosome theory. Ernst Mayr noted that some prominent geneticists—William Bateson, Wilhelm Johannsen, Richard Goldschmidt, and T.H. Morgan—strongly contested it, but definitive proof came from chromosome maps in Morgan’s own lab. In 1923, Painter reported 24 pairs of human chromosomes (48 total), an error others repeated until Joe Hin Tjio determined the correct count of 46 in 1956.

**Prokaryotes** Prokaryotes—bacteria and archaea—usually have a single circular chromosome. Bacterial chromosomes (also called genophores) vary greatly in size, from 130,000 base pairs in the endosymbionts *Candidatus Hodgkinia cicadicola* and *Candidatus Tremblaya princeps* to over 14,000,000 base pairs in *Sorangium cellulosum*. Some bacteria have more than one chromosome: *Borrelia burgdorferi* (causing Lyme disease) has a single linear chromosome, vibrios often carry two chromosomes of very different sizes, and *Burkholderia* species can have one, two, or three chromosomes.

**Structure in sequences** Prokaryotic chromosomes have less sequence-based structure than eukaryotic ones. Bacteria usually have a single origin of replication, while some archaea have multiple. Prokaryotic genes are often arranged in operons and typically lack introns, unlike eukaryotes.

**DNA packaging** Prokaryotes lack a nucleus; their DNA is organized into a nucleoid, a distinct region within the cell. This structure is dynamic and maintained by various histone-like proteins that associate with the bacterial chromosome. In archaea, DNA packaging is more organized, resembling eukaryotic nucleosomes. Some bacteria also contain plasmids.