Nucleosome particles are formed of 8 core histones, flexibly interacting with DNA to control the transcriptional activity of that region of the chromosome, as well as enabling the DNA to all fit within the cell.
Nucleosomes can be in either a 10 nm or 30 nm fibre, forming the secondary structure of the DNA. Heterochromatin generally adopts the 30 nm fibre structure, repressing transcription due to the tightly packaged nature of the DNA. Although the 30 nm fibre is not fully understood, the ellipsoid and solenoid models have been suggested as potential structures the nucleosome-DNA complex may adopt. The 10 nm fibre is generally perceived to be more transcriptionally active with the cell controlling the transition between the two states to allow gene expression to occur and be repressed. This ‘beads on a string’ structure, where each histone is visibly separate, enables RNA polymerase to transcribe out the desired regions.
On the 30nm fibre, it is possible to extrude out loops of DNA to transcribe. This is coordinated by CTCF and cohesin. CTCF controls the extent of the extrusion, being a transcription factor enabling the loop to form. It therefore must have one CTCF at each end of the gene loop, forming boundary elements. Cohesin generates the loop of decondensed DNA. This allows selective protein expression on heterochromatic regions.