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The spatial arrangement of the chromatin within the nucleus is not random - specific regions of the chromatin can be found in certain territories. Territories are, for example, the lamina-associated domains (LADs), and the topologically associating domains (TADs), which are bound together by protein complexes. [16]
The organization of chromosomes into distinct regions within the nucleus was first proposed in 1885 by Carl Rabl.Later in 1909, with the help of the microscopy technology at the time, Theodor Boveri coined the termed chromosome territories after observing that chromosomes occupy individually distinct nuclear regions. [6]
Diagram of the nucleus showing the ribosome-studded outer nuclear membrane, nuclear pores, DNA (complexed as chromatin), and the nucleolus. The nucleus contains nearly all of the cell's DNA , surrounded by a network of fibrous intermediate filaments called the nuclear matrix , and is enveloped in a double membrane called the nuclear envelope .
Chromatin can form a tertiary chromatin structure and be compacted even further than the solenoid structure by forming supercoils which have a diameter of around 700 nm. [12] This supercoil is formed by regions of DNA called scaffold/matrix attachment regions (SMARs) attaching to a central scaffolding matrix in the nucleus creating loops of ...
As architectural DNA components that organize the genome of eukaryotes into functional units within the cell nucleus, S/MARs mediate structural organization of the chromatin within the nucleus. These elements constitute anchor points of the DNA for the chromatin scaffold and serve to organize the chromatin into structural domains.
Pyknosis is also characterized by eventual fragmentation (karyorrhexis) of the dense nuclear chromatin, resulting in dark, round, and dense nuclear fragments. [5] Karyorrhexis is the fragmentation of a pyknotic cell’s nucleus and the cleavage and condensing of chromatin. [5]
The current chromatin compaction model. The organization of the DNA that is achieved by the nucleosome cannot fully explain the packaging of DNA observed in the cell nucleus. Further compaction of chromatin into the cell nucleus is necessary, but it is not yet well understood.
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 .