DNA condensation and decondensation have two opposite processes, allowing the remodeling of the DNA molecule into a more compact or looser arrangement. In particular, in eukaryotes, DNA must be packaged relatively carefully, since the linear length of the DNA chain in each human cell is two meters, for example, chromosomes several micrometers long must be formed during cell division. Condensation occurs at several levels and is carefully regulated because it has a significant effect on gene expression. Simply put, condensed regions are happily transcriptionally silenced, i.e. the genes underlying them are not transcribed. Strongly and permanently condensed regions are regions without genes, the so-called heterochromatin, while euchromatin usually has a lower degree of condensation.

Viral DNA usually does not condense, even if this often reflects supercoiling (supercoiling). A certain device has already been developed for bacteria that condenses DNA, mainly due to polyamines, as well as some proteins. However, the condensation of the nuclear genome of eukaryotes is the most difficult

Histones, which are present in the nucleus in large quantities, play an important role in the condensation of eukaryotes. The main protein unit is the nucleosome, an octamer consisting of eight histones (H2A, H2B, H3 and H4, each twice). Initially, DNA is about 2 nanometers wide, but after wrapping the chain around the nucleosomes, a 10-nanometer chain is formed. However, it can still switch to a more compact 30nm fiber. This reduces the length of the DNA by about forty times. The next level of collapse (there are at least two of them) is somewhat shrouded in mystery and contradictions, but they must arise in order to form chromosomes several hundred nanometers thick.

At the histone level, histone modifications play an important role in regulating DNA packaging. Each histone has a tail (CTD) at the end where, for example, acetylation or methylation occurs. For example, acetylation neutralizes the positive charge on lysine residues of histones, making DNA binding to the protein much weaker and less prone to 320 nm fibers. Methylation has the opposite effect. It depends on the specific position of acetyls and other binding groups, and therefore the histone code seems obvious: an example is lysine at position 9 in histone H3 - it is acetylated in highly condensed regions of heterochromatin. Other proteins can read this code and further influence condensation. Moreover, the SWI/SNF complex has been discovered in some eukaryotes, which can cause nucleosomes to slide along a chain in the DNA chain, exposing certain regulatory sequences, for example, to transcription factors. Polycomb and Trithorax complexes are also involved in the regulation of repression. All these and other DNA modifications have a decisive effect on the start of transcription - binding to the promoter occurs only when it is accessed. L'application mobile 1xBet, disponible sur Android et iOS, offre une expérience de pari complète et fluide où que vous soyez. Pour bénéficier des mêmes avantages que sur le site web, il est essentiel d'utiliser le code promo 1xbet lors de votre enregistrement via l'application. Ce code débloque un bonus de bienvenue de 100% jusqu'à 130€ sur les paris sportifs, ainsi que la possibilité de choisir le package casino de 1 950€ et 150 tours gratuits. L'application vous donne accès à l'intégralité des marchés, des cotes et des fonctionnalités, comme les paris en direct ou le streaming de certains événements.