There are two types of eukaryotic chromatin that are euchromatin, which can be transcriptionally active, and the more densely packed heterochromatin, which is transcriptionally inactive.
Chromatin consists of DNA and proteins, the majority of which are highly conserved histone. Chromatin is structurally organized in a hierarchical manner. In the first level of chromatin organization, ~200 bp of DNA are doubly wrapped around a histone octamer to form a nucleosome.
Each nucleosome is associated with one molecule of histone H1. The passage of transcribing RNA polymerase causes nucleosome to dissociate from the DNA and then to rebind it in a process that appears to be driven by supercoiling.
DNA replication causes the parental nucleosomes to be randomly distributed between the daughter duplexes. The assembly of nucleosomes from their components is mediated by the molecular chaperone nucleoplasmin.
In the second level of chromatin organization, the nucleosome filaments coil into 30 nm-thick filaments that probably contain six nucleosomes per turn. Then, in the third and final level of chromatin organization, the 30 nm-thick filaments form 15- to 30-µm-long radial loops that project from the axis of the metaphase chromosome.
This accounts for DNA's packing ratio of >8000 in the metaphase chromosome. The larvae of certain dipteran flies, including Drosophila, contain banded polytene chromosomes, which consist of up to 1024 identical DNA strands in parallel register.
Chromatin consists of DNA and proteins, the majority of which are highly conserved histone. Chromatin is structurally organized in a hierarchical manner. In the first level of chromatin organization, ~200 bp of DNA are doubly wrapped around a histone octamer to form a nucleosome.
Each nucleosome is associated with one molecule of histone H1. The passage of transcribing RNA polymerase causes nucleosome to dissociate from the DNA and then to rebind it in a process that appears to be driven by supercoiling.
DNA replication causes the parental nucleosomes to be randomly distributed between the daughter duplexes. The assembly of nucleosomes from their components is mediated by the molecular chaperone nucleoplasmin.
In the second level of chromatin organization, the nucleosome filaments coil into 30 nm-thick filaments that probably contain six nucleosomes per turn. Then, in the third and final level of chromatin organization, the 30 nm-thick filaments form 15- to 30-µm-long radial loops that project from the axis of the metaphase chromosome.
This accounts for DNA's packing ratio of >8000 in the metaphase chromosome. The larvae of certain dipteran flies, including Drosophila, contain banded polytene chromosomes, which consist of up to 1024 identical DNA strands in parallel register.
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