Solenoid (DNA)

The solenoid structure of chromatin is a model for the structure of the 30 nm fibre. It is a secondary chromatin structure which helps to package eukaryotic DNA into the nucleus. The solenoid structure of chromatin is a model for the structure of the 30 nm fibre. It is a secondary chromatin structure which helps to package eukaryotic DNA into the nucleus. Chromatin was first discovered by Walther Flemming by using aniline dyes to stain it. In 1974, it was first proposed by Roger Kornberg that chromatin was based on a repeating unit of a histone octamer and around 200 base pairs of DNA. The solenoid model was first proposed by John Finch and Aaron Klug in 1976. They used electron microscopy images and X-ray diffraction patterns to determine their model of the structure. This was the first model to be proposed for the structure of the 30 nm fibre. DNA in the nucleus is wrapped around nucleosomes, which are histone octamers formed of core histone proteins; two histone H2A-H2B dimers, two histone H3 proteins, and two histone H4 proteins. The primary chromatin structure, the least packed form, is the 11 nm, or “beads on a string” form, where DNA is wrapped around nucleosomes at relatively regular intervals, as Roger Kornberg proposed. Histone H1 protein binds to the site where DNA enters and exits the nucleosome, wrapping 147 base pairs around the histone core and stabilising the nucleosome, this structure is a chromatosome. In the solenoid structure, the nucleosomes fold up and are stacked, forming a helix. They are connected by bent linker DNA which positions sequential nucleosomes adjacent to one another in the helix. The nucleosomes are positioned with the histone H1 proteins facing toward the centre where they form a polymer. Finch and Klug determined that the helical structure had only one-start point because they mostly observed small pitch angles of 11 nm, which is about the same diameter as a nucleosome. There are approximately 6 nucleosomes in each turn of the helix. Finch and Klug actually observed a wide range of nucleosomes per turn but they put this down to flattening. Finch and Klug's electron microscopy images had a lack of visible detail so they were unable to determine helical parameters other than the pitch. More recent electron microscopy images have been able to define the dimensions of solenoid structures and identified it as a left-handed helix. The structure of solenoids are insensitive to changes in the length of the linker DNA. The solenoid structure's most obvious function is to help package the DNA so that it is small enough to fit into the nucleus. This is a big task as the nucleus of a mammalian cell has a diameter of approximately 6 µm, whilst the DNA in one human cell would stretch to just over 2 metres long if it were unwound. The 'beads on a string' structure can compact DNA to 7 times smaller. The solenoid structure can increase this to be 40 times smaller. When DNA is compacted into the solenoid structure can still be transcriptionally active in certain areas. It is the secondary chromatin structure that is important for this transcriptional repression as in vivo active genes are assembled in large tertiary chromatin structures.