Eukaryotic DNA with a typical length of dozens of centimeters should be orderly packed to be readily accessible inside the micrometer-size nucleus. In most eukaryotes, DNA is arranged in the cell nucleus with the help of histones. In this case, the basic level of DNA compaction is the nucleosome, where the double helix is wrapped around the histone octamer containing two copies of each histone H2A, H2B, H3 and H4. Linker histone H1 binds the DNA between nucleosomes and facilitates packaging of the 10 nm "beads on the string" nucleosomal chain into a more condensed 30 nm fiber. Most of the time, between cell divisions, chromatin is optimized to allow easy access of transcription factors to active genes, which are characterized by a less compact structure called euchromatin, and to alleviate protein access in more tightly packed regions called heterochromatin. During the cell division, chromatin compaction increases even more to form chromosomes, which can cope with large mechanical forces dragging them into each of the two daughter cells. Many aspects of transcription are controlled by chemical modification on the histone proteins, known as the histone code.
Chromosome scaffold has important role to hold the chromatin into compact chromosome. Chromosome scaffold is made of proteins including condensin, topoisomerase IIα and kinesin family member 4 (KIF4)
Dinoflagellates are very divergent eukaryotes in terms of how they package their DNA. Their chromosomes are packed in a liquid-crystalline state. They have lost many of the conserved histone genes, using mostly dinoflagellate viral nucleoproteins (DVNPs) or bacteria-derived dinoflagellate histone-like proteins (HLPs) for packaging instead. It is unknown how they control access to genes; those do retain histone have a special histone code.