A human cell usually has 46 molecules of DNA with an average length of 44 mm (total slightly over 2 m). Each molecule is 2 nm in diameter. To put this in perspective, if a DNA molecule were the thickness of a telephone pole (20 cm, or 8 in.), it would reach about 4,400 km (2,700 mi) into space—far higher than the orbits of satellites and space shuttles. Imagine trying to make a pole 20 cm thick and 4,400 km long without breaking it! The problem for a cell is even greater. It has 46 DNA molecules packed together in a single nucleus, and it has to make an exact copy of every one of them and distribute these equally to its two daughter cells when the cell divides. Keeping the DNA organized and intact is a tremendous feat.
Molecular biology and high-resolution electron microscopy have provided some insight into how this task is accomplished. Chromatin looks like a granular thread (fig. 4.1a). The granules, called nucleosomes, consist of a cluster of eight proteins called histones, with the DNA molecule wound around the cluster. Histones serve as spools that protect and organize the DNA. Other nuclear proteins called nonhistones seem to provide structural support for the chromatin and regulate gene activity.
Winding DNA around the nucleosomes makes the chromatin shorter and more compact, but chromatin also has higher orders of structure. The "granular thread," about 10 nm wide, further twists into a coil about 30 nm wide. When a cell prepares to undergo division, the chromatin further supercoils into a fiber about 200 nm wide (fig. 4.1b). Thus, the 2 m of DNA in each cell becomes shortened and compacted in an orderly way that prevents tangling and breakage without interfering with genetic function.
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