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Nucleosomes are the basic structures of chromatin and constitute a general repressor in Eukaryote due to the compaction of DNA which limits its accessibility to DNA-binding factors. The first level of compaction consists in wrapping the DNA 147 bp long fragments around a histone octamer, which makes this DNA less accessible to the DNA binding factors than the linker DNA. The unwrapped linker DNA is 2050 bp long. Nucleosomes further condense by linker histones H1 to form a 30 nm fiber. Differential compaction of the interphase chromatin is important for proper functioning of the genome. Density of nucleosomes along DNA varies between organisms and depends on the functional properties of chromatin. In transcriptionally active euchromatin the density is 6 nucleosomes/11 nm while in inactive heterochromatin as much as 1215 nucleosomes/11 nm. Genomic regions that strongly exclude a nucleosome are often found near gene promoter. Nucleosomes have some DNA sequence preferences. DNA fragments containing poly(dAT:dT) elements are poor in nucleosomes. Several factors control nucleosome positioning they are, among others, structures in DNA and in chromatin which depend on the chromatin remodelers and epigenetic modifications of DNA and histones, such as DNA methylation, histone posttranslational modifications and histone variants. N-terminal tails of core histones perform several independent functions. The precise positioning of nucleosomes plays an important role in the regulation of gene expression. The core histone tail domains are molecular determinants responsible for positioning, as it has been demonstrated by the results of experiments in which the core histone tails were removed. In the condensation of nucleosome arrays into higher order chromatin structures, core histone tails take part. The 30 nm fiber constitutes the first order of folding of a nucleosome array. It represents only a minority of chromatin. In the higher order structures the contact between adjacent nucleosomes depend on core histone N-terminal tails and an interaction between them. The degree of compaction depends on the type of histone modifications. It is well known that core histone acetylation results in the decondensation of chromatin and is characteristic of transcriptionally active chromatin. Trimethylation of some lysine residues, e.g. lysine 20 in H4, marks transcriptionally inactive chromatin, and is typical of highly compacted transcriptionally inactive heterochromatin. Variants of the core histone H2A: H2A.Z, H2A.v, H2A.Bbdb and H2A.Bdb can participate in the chromatin compaction and proper regulation of gene expression. In Saccharomyces cerevisiae loss of H2A.Z is tolerated, but the regulation of gene expression is affected. H2A.Z is excluded from constitutive heterochromatin, but present in so-called facultative heterochromatin. H2A.v is present both in eu- and heterochromatin. Its role in the chromatin stabilization seems to consist in the formation of condensed chromatin. New insight into how the folding process is regulated concerns the role of a cluster of seven amino acid residues (the acidic path) present mainly in H2A. This acidic path can interact with a basic N-terminal tail of histone H4 (residues 14 19) from one nucleosome with an adjacent nucleosome when nucleosomal arrays fold into the 30 nm fiber. The acidic path in H2A can be modified in some H2A variants. H2A.Bbd has three acidic residues within the acidic path. This variant inhibits the formation of 30 nm chromatin fiber. As a result of inhibiting the formation of 30 nm fiber, H2A.Bbd enhances transcription. On the other hand, H2A.Z promotes the formation of 30 nm fiber on account of the presence of two additional amino acid residues which extend acidic path of H2A.Z compared to that of H2A. As H2A.Z is present in the facultative heterochromatin (condensed euchromatin), it is possible that this histone variant participates in the condensation of euchromatin.The compacteness of chromatin is modified by ATP-dependent chromatin-remodeling complex, ATPases. They use the energy of ATP hydrolysis to move nucleosomes to different localizations along the DNA. Chromatin remodelers can act both as chromatin condensing and decondensing factors. Chromatin remodeling ATPases play an important role in physiological and developemental processes in eukaryotes.

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The Editorial Board
Andrzej Łukaszyk - przewodniczący, Zofia Bielańska-Osuchowska, Szczepan Biliński, Mieczysław Chorąży, Aleksander Koj, Włodzimierz Korochoda, Leszek Kuźnicki, Aleksandra Stojałowska, Lech Wojtczak

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Katedra i Zakład Histologii i Embriologii Uniwersytetu Medycznego w Poznaniu, ul. Święcickiego 6, 60-781 Poznań, tel. +48 61 8546453, fax. +48 61 8546440, email:

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