7.5: Chromatin Packaging
Each human somatic cell contains 6 billion base-pairs of DNA. Each base-pair is 0.34 nm long, which means that each diploid cell contains a staggering 2 meters of DNA. How is such a long DNA strand packed inside a nucleus measuring only 10 - 20 microns in diameter?
In combination with specialized DNA binding protein called Histones, the DNA double helix forms a compact DNA: protein complex called chromatin. The chromatin itself is further compacted into higher-order structures. The highest level of compaction is achieved during the cell cycle's metaphase, where the chromatin condenses to form the chromatids of a chromosome.
Nucleosomes are the basic functional and repeating unit of chromatin. A nucleosome consists of 8 histone proteins wound around by 147 base pairs of DNA. Under electron microscopy, the chromatin appears as a structure resembling beads on a string due to nucleosomes' presence along its length. This packaging shortens the fiber length by seven-fold.
The nucleosomes are further coiled into 30 nm fibers, so-called because of their diameter of approximately 30 nm. Such a compaction is explained by a widely accepted hypothesis - the solenoid model. A solenoid refers to the structure of a wire coiled on a central axis. This model proposes that nucleosomes are arranged in a left-handed helical conformation with six or more nucleosomes per turn. One of the non-core histone proteins, H1, plays an essential role in nucleosome compaction; in its absence the chromatin fiber turns into irregular clumps of nucleosomes.
Chromatin packaging is an active area of research. The new emerging data has allowed scientists to view chromatin and nucleosomes not as highly defined structures, but rather as a continuum of various inter-convertible conformations at all chromatin packaging stages.