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Key concepts, Eukaryotic chromatin
The genomes of all organisms are compacted by interaction with specific proteins. These often
play roles in gene regulation.
In bacteria, a large circular chromosome is complexed by proteins in a dynamic manner that
compacts it and yet makes it accessible for transcription.
Eukaryotes utilize a set of structural proteins, the histones, to organize the chromatin into a
repeating structure.
The repeating elements of eukaryotic chromatin are celled nucleosomes. Nucleosomes consist
of core particle and linker DNA. In each core particle, about 147 bp of DNA is wrapped in 1.67
left-handed turns about an octamer of histones, two each of four types: H2A, H2B, H3 and H4.
H2A and H2B form heterodimers, while H3 and H4 form a tetramer.
Some of the histones exist in variant forms, and all are subject to a number of possible posttranslational modifications. This allows an enormous range of variation in nucleosomes.
Assembly of nucleosomes is facilitated by proteins called histone chaperones.
The nucleosome is a dynamic structure and may exhibit a number of aberrant forms under
special conditions.
The chromatin fiber exhibits much specific positioning of nucleosomes, but their spacing is
erratic and species-dependent.
The linker DNA between nucleosomes is frequently occupied by members of the H1 class of
histones.
Chromatin fibers tend to fold into a compact 30-nm fiber at physiological salt concentrations.
The fine structure of the 30-nm fiber remains a matter or controversy.
Individual chromosomes appear to inhabit unique territories in the interphase nucleus.
In mitosis, sister chromatids condense into the long-recognized mitotic chromosomes. These
are highly condensed and held together by specific proteins. The manner of folding of mitotic
chromosomes is still under debate.
Chromosomes contain certain specialized regions. Centromeres are regions for attachment to
the microtubular apparatus of the mitotic spindle for chromosome segregation. Telomeres are
regions at the ends of chromosomes with special structure to protect DNA from degradation of
unscheduled DNA repair. Each of these regions has its own specific proteins that stabilize the
structure.