Diagram of a replicated and condensed
eukaryotic chromosome. (1)
– one of the two identical parts of the chromosome after
– the point where the two chromatids touch. (3) Short (p) arm. (4) Long (q) arm.
A chromosome (from
ancient Greek: χρωμόσωμα, chromosoma, chroma means color, soma means body) is a
DNA molecule with part or all of the genetic material (
genome) of an organism.
Chromosomes are normally visible under a
light microscope only when the cell is undergoing the
cell division. Before this happens, every chromosome is copied once (
S phase), and the copy is joined to the original by a
centromere, resulting in an X-shaped structure. The original chromosome and the copy are now called
sister chromatids. During metaphase, when a chromosome is in its most condensed state, the X-shape structure is called a metaphase chromosome. In this highly condensed form chromosomes are easiest to distinguish and study.
Chromosomes vary widely between different
organisms. Some species such as certain
bacteria, which lack
histones, also contain
plasmids or other
. These are circular structures in the
cytoplasm that contain cellular DNA and play a role in
horizontal gene transfer.
 In prokaryotes (see
 the DNA is often densely packed and organized; in the case of
archaea, by homology to eukaryotic histones, and in the case of bacteria, by
DNA condensation of the duplicated chromosomes during
cell division (
meiosis) results either in a four-arm structure (pictured to the right) if the
centromere is located in the middle of the chromosome or a two-arm structure if the centromere is located near one of the ends. 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 and die, or it may unexpectedly evade
apoptosis, leading to the progression of
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. However, 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.