5.6: The Nucleosome Core Particle
Nucleosomes are the DNA-histone complex, where the DNA strand is wound around the histone core. The histone core is an octamer containing two copies of H2A, H2B, H3, and H4 histone proteins.
Nucleosomes, paradoxically, perform two opposite functions simultaneously. On the one hand, their primary aim is to protect the delicate DNA strands from physical damage and help achieve a higher compaction ratio. On the other hand, they must allow polymerase enzymes to access histone-bound DNA during replication and transcription. Nucleosomes solve both problems by partially unfolding the DNA as needed while keeping the majority of the DNA wrapped around the histones.
The histone core proteins have a mobile extended tail region and share a common structurally conserved motif called the "histone fold." The histone fold is made up of alpha helices and loops. During the histone dimerization, loops of two histone proteins align together, forming a dimer.
Each histone binds to the three consecutive minor grooves of DNA. Their alpha helix and N-terminal tail are crucial in binding to the DNA. As a result, any chemical modifications to the histone tail can modify the chromatin assembly and function. The most common histone modifications include acetylation, methylation, and phosphorylation.
Histone proteins have various isoforms or variants like H2A.1, H2A.2, H2A.X, H3.3, or CENP-A. These variants differ in their amino acid sequences and perform distinct functions. The nucleosomes with histone variants are significantly more mobile than ordinary nucleosomes. For example, the incorporation of H2A.Z into the nucleosome is shown to activate transcription.
