Histones

The eukaryotic chromosome is certainly much more complex than that of a prokaryote. For start, there is a complex between DNA and proteins and the latter are specialised for organising euykaryotic DNA into packets for further management by the cell.

The proteins involved in complexing with DNA are either histones or non-histone chromosomal proteins. 

The histones themselves are responsible for the basic level of chromosomal packing and together with DNA form the nucleosome. This is the basic protein-DNA complex. In the nucleosome, DNA is wound round a histone core protein to make a nucleosome core particle which is often called a histone core. These core particles are linked by strands of bare DNA called ‘linker DNA’. The linker DNA is only 54 base pairs.   

The histone core is an octamer of 4 histomne proteins called H2A, H2B, H3 and H4. Each histone protein has a structured domain called a ‘histone’ fold and an unstructured ‘N-terminal tail’. These histone tails are involved in the 30-nm fibre formation by forming hydrogen bonds with adjacent nucleosome tails. These tails are used for covalent modifications which involve acetylation, methylation and phosphorylation. Such modifications determine the interaction of histone with other proteins, which may in turn regulate chromatin structure, and transcription.