Because of the intimate connection between the nucleosome's core histones and the DNA, scientists were under the impression for a considerable amount of time that once a nucleosome was created, it would maintain its original location on the DNA. In this scenario, the operation of genetic readout mechanisms would be challenging because, in theory, they require uncomplicated access to a huge number of distinct DNA sequences in order to function properly. In addition to this, the rapid movement of the equipment responsible for DNA transcription and replication across chromatin would be slowed down by it. However, the DNA in an isolated nucleosome unwinds from each end at a rate of around four times per second. The DNA is exposed for a period ranging from ten to fifty milliseconds before the partially unwrapped structure recloses. The results of kinetic experiments point to this conclusion. Therefore, the bulk of the DNA in a nucleosome that has been detached is theoretically free to attach to other proteins. This is the case since the DNA is no longer enclosed by the nucleosome.

It is obvious that additional loosening of DNA-histone interactions is required for the chromatin in a cell because eukaryotic cells contain a wide range of ATP-dependent chromatin remodeling complexes. This is the case even though eukaryotic cells have their own unique set of chromatin remodeling complexes. These complexes each have a component that is responsible for the breakdown of ATP. This subunit binds to both the double-stranded DNA and the single-stranded DNA that wind around the protein core of the nucleosome. Using the energy released from the hydrolysis of ATP, the protein complex briefly alters the shape of a nucleosome. This is accomplished by moving the DNA in question in relation to the core. As a result, the DNA's hold on the histone core is loosened. The remodeling complexes have the ability to catalyze nucleosome sliding by causing many rounds of ATP hydrolysis, which in turn move the nucleosome core along the DNA double helix. They are able to shift nucleosomes in this manner in order to expose specific DNA sequences, so rendering those sequences accessible to other proteins within the cell. Working in conjunction with a wide variety of other proteins that bind to histones and serve as histone chaperones, some remodeling complexes have the ability to disassemble nucleosomes and remove all or a portion of the nucleosome core from a nucleosome. Either initiating an exchange of the H2A-H2B histones or removing the complete octameric core from the DNA can accomplish this goal. Both methods have their advantages and disadvantages. The measurements that are taken as a result of these processes demonstrate that a normal nucleosome is replaced on the DNA inside the cell once every one to two hours.

There are many chromatin remodeling complexes present in cells. These complexes are dependent on ATP and engage in a wide variety of actions. The vast majority are large protein aggregates that have ten or more subunits, some of which bind to specific modifications on histones. The activity of these complexes is subject to the stringent control of the cell. Chromatin remodeling complexes are moved to specific DNA sites, where they then exert their influence in a localized manner to alter the structure of chromatin in response to the activation and deactivation of genes. Although some DNA sequences attach to the nucleosome core more tenaciously than others, it appears that other highly bound proteins on the DNA have a more significant impact on the positioning of nucleosomes. When there are bound proteins nearby, the formation of a nucleosome is more likely to occur. Some people erect obstructions that force the nucleosomes to move in the opposite direction. Because of this, the precise locations of nucleosomes along a length of DNA are determined by the presence of extra proteins that are attached to the DNA as well as the type of proteins that are associated to the DNA. In addition, due to the presence of ATP-dependent chromatin remodeling complexes, the arrangement of nucleosomes on DNA is capable of being extremely dynamic and can alter in a very short amount of time in response to the requirements of a cell.

In spite of the fact that chromosomal DNA can form extraordinarily long strings of nucleosomes, it is likely that the extended "beads-on-a-string" form of chromatin in a living cell only occasionally assumes this configuration. Instead, the nucleosomes stack one on top of the other to generate arrays that include DNA that is packed even more closely together. When nuclei are very gently disrupted onto an electron microscope grid, the majority of the chromatin is seen to be in the shape of a fiber with a diameter of approximately 30 nm. This is significantly wider than the chromatin in the "beads-on-a-string" form, which was previously thought to be the only form of chromatin.

The process by which nucleosomes are packed into arrays is not completely understood. The structure of a tetranucleosome, which is a complex of four nucleosomes and was determined by x-ray crystallography and high-resolution electron microscopy of reconstituted chromatin, lends support to a zigzag model for the stacking of nucleosomes in a 30-nm fiber. This model predicts that nucleosomes will be stacked in a zigzag pattern. Cryoelectron imaging of carefully crafted nuclei, on the other hand, reveals that the bulk of chromatin regions have a less regular structural structure.

Why do nucleosomes bind to each other with such a strong affinity? Histone tails play an important role in nucleosome-to-nucleosome interactions, in particular the H4 tail. This is one of the most important aspects. Histone H1, a distinct histone that is often found in a ratio of 1 to 1 with nucleosome cores, is yet another essential component. This so-called linker histone has undergone far less conservation during the process of evolution in comparison to the individual core histones. One molecule of histone H1 is attached to each nucleosome. This molecule makes contact with both the DNA and the protein, and it modifies the path that the DNA takes as it exits the nucleosome. It is hypothesized that this change in the exit route of the DNA contributes to the compactness of the nucleosomal DNA.

The vast majority of eukaryotic organisms manufacture many histone H1 proteins, all of which have sequences of amino acids that are fairly close to one another but yet quite distinct. Any collection of nucleosomes will, without a doubt, acquire helpful additional properties as a result of the existence of countless other DNA-binding proteins as well as proteins that bind directly to histones. This is because of the fact that nucleosomes are modular structures.

A nucleotide sequence in DNA that functions as a functional component in the manufacture of a protein, a structural RNA molecule, or a catalytic or regulatory RNA molecule is referred to as a gene. Genes can be found in all living organisms. In eukaryotic cells, the genes that code for proteins are normally organized as a series of alternating introns and exons that are linked to regulatory DNA sequences. A single molecule of DNA that is unusually lengthy, which is related to a large number of proteins and comprises a linear array of many genes, is the component that makes up a chromosome. There are 3.2 109 DNA nucleotide pairs in the human genome. These nucleotide pairs are distributed as two copies of each of the 22 distinct autosomes as well as the two sex chromosomes. Only a small percentage of this DNA contains the instructions necessary to make RNA molecules or proteins that are active. Other types of important nucleotide sequences found in chromosomal DNA include replication origins, telomeres, and centromeres. These nucleotide sequences attach sister DNA molecules to the mitotic spindle and ensure their accurate segregation to daughter cells during the M phase of the cell cycle. Replication origins and centromeres are found at the ends of chromosomes.

In eukaryotic cells, the DNA molecule is firmly attached to an equal number of histone proteins, producing nucleosomes, which are DNA-protein particles that are stacked in a repetitive pattern. The nucleosome is comprised of an octameric core of histone proteins, which is wrapped around the DNA double helix. There are around 200 nucleotide pairs that separate each nucleosome, and these nucleotide pairs are normally arranged in quasi-regular arrays with the assistance of histone H1 molecules to produce a chromatin fiber that is 30 nm in diameter. In spite of the fact that chromatin is dense, in order to access the DNA, the chromatin structure must be highly active. Although the nucleosome itself goes through some spontaneous DNA unwrapping and rewrapping, the predominant mechanism for reversibly modifying the local chromatin structure is ATP-driven chromatin remodeling complexes. These complexes are dispersed throughout the cell and activated at precisely the right moment to target specific regions of chromatin. As a result of the collaboration between remodeling complexes and histone chaperones, the nucleosome cores can be relocated, reconstructed with other histones, or totally destroyed in order to expose the DNA that lies behind.