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At metaphase, the chromosomes are duplicated and are at their most condensed. In each chromosome. two identical sister chromatids are held together at a constricted region called the centromere.

When a chromosome is condensed, interactions among chromosomal proteins keep loops of DNA tightly packed into a "super-coiled" array.

On closer examination, we can see that the chromosomal proteins and DNA are organized as a cylindrical fiber. If we immerse a chromosome in saltwater. this fiber loosens up to reveal a beads-on-a-string organization.

The string is one molecule of DNA. Each bead is a nucleosome that consists of a DNA molecule looped around a core of histones. Nucleosomes can loosen in controlled ways to give enzymes access to specific DNA regions.

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