The Nucleosome

Proteins play a key role in determining the physical structure of a chromosome. The most abundant of these are small, positively charged proteins called histones. This positive charge allows them to tightly associate with the negatively charged DNA. During certain stages of the cell cycle, DNA is wound tightly around specific types of histones, forming structures called nucleosomes. Nucleosomes are often described as ‘beads’ on a ‘string’ of DNA. A nucleosome consists of a few key elements.  The first of these is an octamer of histone proteins, two molecules each of H2A, H2B, H3, and H4. Next, a nucleosome also has a 145 -147 bp length of DNA wrapped around the protein octamer nearly two times.  Together, the histone octamer and the DNA wound around it are known as a nucleosome core particle.  Each of the histones in the nucleosome core particle has a small, positively charged tail consisting of 11-27 amino acids.  The tails extend out from the nucleosome core particle and aid in keeping the negatively charged DNA and the histones associated. Furthermore, the histone tails can interact with tails from neighboring core particles, which facilitates DNA packaging.   A fifth type of histone, H1, plays a key role in nucleosome structure, though it is not part of the nucleosome core particle. H1 binds to the DNA where it joins and then leaves the octamer, acting as a clamp and keeping the DNA in place. Finally, the nucleosome also encompasses the stretch of linker DNA adjacent to the nucleosome core particle. The linker DNA that separates each core particle can vary in length, from about 30 to 40 base pairs, between cell types.  While the terms nucleosome and nucleosome core particle are often used interchangeably, the nucleosome actually refers to the nucleosome core particle and the adjacent linker DNA.  Altogether, nucleosomes are capable of reducing a long DNA molecule into a chromatin thread that is about one-third of its original length.