Chromosome
A chromosome is a package of DNA with part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells the most important of these proteins are the histones. These proteins, aided by chaperone proteins, bind to and condense the DNA molecule to maintain its integrity.[1][2] These chromosomes display a complex three-dimensional structure, which plays a significant role in transcriptional regulation.[3]
Chromosomes are normally visible under a light microscope only during the metaphase of cell division (where all chromosomes are aligned in the center of the cell in their condensed form).[4] Before this happens, each chromosome is duplicated (S phase), and both copies are joined by a centromere, resulting either in an X-shaped structure (pictured above), if the centromere is located equatorially, or a two-arm structure, if the centromere is located distally. The joined copies are now called sister chromatids. During metaphase, the X-shaped structure is called a metaphase chromosome, which is highly condensed and thus easiest to distinguish and study.[5] In animal cells, chromosomes reach their highest compaction level in anaphase during chromosome segregation.[6]
Chromosomal recombination during meiosis and subsequent sexual reproduction play a significant role in genetic diversity. If these structures are manipulated incorrectly, through processes known as chromosomal instability and translocation, the cell may undergo mitotic catastrophe. Usually, this will make the cell initiate apoptosis leading to its own death, but sometimes mutations in the cell hamper this process and thus cause progression of cancer.
Some use the term chromosome in a wider sense, to refer to the individualized portions of chromatin in cells, either visible or not under light microscopy. Others use the concept in a narrower sense, to refer to the individualized portions of chromatin during cell division, visible under light microscopy due to high condensation.
The word chromosome (/ˈkroʊməˌsoʊm, -ˌzoʊm/[7][8]) comes from the Greek χρῶμα (chroma, "colour") and σῶμα (soma, "body"), describing their strong staining by particular dyes.[9] The term was coined by the German anatomist Heinrich Wilhelm Waldeyer,[10] referring to the term chromatin, which was introduced by Walther Flemming.
Some of the early karyological terms have become outdated.[11][12] For example, Chromatin (Flemming 1880) and Chromosom (Waldeyer 1888), both ascribe color to a non-colored state.[13]
