Each human somatic cell contains 6 billion base pairs of DNA. Each base pair is 0.34 nm long, meaning each diploid cell contains a staggering 2 meters of DNA. This long DNA strand is packed inside a nucleus measuring only 10-20 microns in diameter with the help of specialized DNA-binding proteins called histones. Together they form a compact DNA-protein complex called chromatin. The chromatin 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
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 resembles beads on a string due to the presence of nucleosomes along its length. The packaging of DNA into nucleosomes shortens its length by sevenfold.
Solenoid model
The nucleosomes are further coiled into 30 nm fibers. Such 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.
Compaction also makes DNA less accessible to the replication and transcription enzymes. As a result, less condensed chromatin, called euchromatin, is more accessible to transcription enzymes as compared to densely packed heterochromatin.
Chromatin packaging is an active area of research. The emerging data has allowed scientists to view chromatin and nucleosomes not as highly defined structures but as a continuum of various inter-convertible conformations at all chromatin packaging stages.
This text is adapted from Openstax, Anatomy and Physiology 2e, Section 3.3: Nucleus and DNA replication and Openstax, Biology, Section 10.1: Cell division.
