A chromosome is a DNA molecule (deoxyribonucleic acid) with an organism of all or part of the genome. Most eukaryotic chromosomes include packaging proteins, aided by chaperone proteins, that bind to and condense the DNA molecule to prevent it from becoming an unmanageable tangle. This three-dimensional genomic structure plays an important role in transcriptional regulation. The chromosomes are the only visible microscope when the cell undergoes the cell division of the metaphase (where all the chromosomes are aligned and their condensed form in the cell). Before this happens, each chromosome is copied once (phase S), and the copy is joined to the original by a centromere, resulting in an X-shaped structure (shown here). The chromosome, or a two-arm structure, is located at one end near the centromere. The original chromosome and the copy are now called sister chromatids. During metaphase, the X-shaped structure is called the metaphase chromosome. In this highly condensed form, the chromosomes are the easiest to distinguish and study