5.6:
The Nucleosome Core Particle
Nucleosome core particles are composed of two units each of H2A, H2B, H3, and H4 histone proteins, forming an octamer.
These small, positively charged histone proteins are highly conserved in eukaryotes. For example, only two of the 102 amino acids in the H4 histones of a pea plant and a cow are different.
Each of these four histones contains around 135 amino acids and shares a common structural histone fold motif, making up the nucleosome core particle.
During assembly, histone folds bind to each other in an interaction described as a 'handshake,' forming two H2A-H2B dimers and two H3-H4 dimers.
The H3-H4 dimers tetramerize with the H2A-H2B dimers, forming the octamer core.
One point seven turns of DNA wrap around the octamer which compresses the long DNA molecule to fit inside the nucleus.
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.