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A circular cell-cycle diagram shows the degree to which chromatin is condensed inside a cell during the five stages of mitosis. Each stage is labeled and numbered beside an illustration of a cell. At

Cells package their DNA not only to protect it, but also to regulate which genes are accessed and when. Cellular genes are therefore similar to valuable files stored in a file cabinet — but in this case, the cabinet's drawers are constantly opening and closing; various files are continually being located, pulled, and copied; and the original files are always returned to the correct location.

View Terms of UseView Full-Size ImageAn illustration shows two nucleosomes. Each nucleosome is composed of double-helical DNA wrapped around eight histone subunits. The histones are represented by blue cylinders, and, in the nucleosome, the histones are arranged in two layers; four cylinders are aligned two by two to form a square top layer, and the four remaining cylinders are also aligned two by two to form the same square shape as a bottom layer. DNA is coiled twice around the circumference of each nucleosome. There is a short stretch of DNA between the two nucleosomes, and DNA also extends off to the right of the nucleosome on the right and off to the left</p>
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of the nucleosome on the left. Small red spheres, representing methylated cytosines, are present on the DNA associated the nucleosomes." />

What Are Chromosomes?

Why Is Complex Packing Critical for Eukaryotic Chromosomes?

Although nucleosomes may look like extended "beads on a string" under an electron microscope, they appear differently in living cells. In such cells, nucleosomes stack up against one another in organized arrays with multiple levels of packing. The first level of packing is thought to produce a fiber about 30 nanometers (nm) wide. These 30 nm fibers then form a series of loops, which fold back on themselves for additional compacting (Figure 5).

Chromatin packing also offers an additional mechanism for controlling gene expression. Specifically, cells can control access to their DNA by modifying the structure of their chromatin. Highly compacted chromatin simply isn't accessible to the enzymes involved in DNA transcription. replication. or repair. Thus, regions of chromatin where active transcription is taking place (called euchromatin ) are less condensed than regions where transcription is inactive or is being actively inhibited or repressed (called heterochromatin ) (Figure 6).

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