Nucleosomes are the particles in DNA that are responsible for compaction and transcription, and may also carry hereditary information. Each nucleosome is approximately 10 nm in diameter, and consists of strands of DNA wrapped in a spiral fashion around a core of simple protein called histone. Nucleosomes are located in the nucleus of a cell, and when attached to DNA, form one of the seven forms of chromatin.
When nucleosomes attach to strands of DNA as repeating subunits, the structure resembles a “string of beads.” While in this form, DNA is undergoing active transcription, the process through which DNA is converted into RNA. DNA is not converted directly into proteins in order to avoid mistakes and contamination.
The structure of the nucleosome is centered around the histone protein. Histone is a simple protein with high concentrations of amino acids, which are the basic building blocks of genes. Each histone core contains pairs of each of the four types of histone proteins, which forms the histone octomer. Around the histone octomer wrap 146 base pairs of DNA in its superhelical form, together forming the nucleosome.
Nucleosomes are the “packaging” of the DNA in a cell’s nucleus, and the signature structure is what determines the DNA’s accessibility. Chemicals responsible for transcription cannot connect to the chromatin if a nucleosome is in the way, so the transcription proteins must first completely expel the nucleosome or slide it along the DNA molecule until the chromatin is exposed. Once that portion of DNA is transcribed into RNA, the nucleosomes are allowed to return to their original location.
If stretched out into a straight line, the DNA in each mammalian nucleus would measure approximately two meters long, but the nucleus of a mammalian cell is a mere 10 micrometers in diameter. It is the complex folding action of the nucleosomes that allow the DNA to fit in the nucleus. The “beads-on-a-string” appearance comes from “linker” DNA that connects each nucleosome to form a fiber about 10 nm in diameter. In the presence of the H1 histone, repeating chains of nucleosomes can form chains 30 nm in diameter, with a much denser packing ratio. The presence of H1 in the core of the nucleosome results in a higher packing efficiency, as neighboring proteins react to initiate folding and looping sequences that allow so much information to be contained in such a tiny package. Even today, the exact packing mechanism initiated by nucleosomes is not fully understood.