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).
Charging by Conduction