A chromosome is a molecule of DNA (deoxyribonucleic acid), containing some or all of an organism's genetic material (genome). Most eukaryotic chromosomes contain packaging proteins that bind and condense the DNA molecule to prevent it from being an unmanageable tangle helped by chaperone proteins. Chromosomes are usually visible under an electron microscope only when the cell is subjected to a metaphase of cell division (in which all chromosomes are placed in their condensed form in the middle of the cell). Before this occurs, each chromosome is copied once (S phase) and the replication is joined by a centromer to the original, resulting either in an X-shaped structure.If the centeromere is located in the center of the chromosome or a two-arm arrangement, whether it is located at one end. The original chromosome is now called sister chromatids, and the clone.