Nucleosomes are responsible for DNA compaction and transcription, and consist of DNA strands coiled around a histone protein core. They form chromatin and allow DNA to fit into the nucleus. Transcription proteins must eject or slide nucleosomes to access DNA. The exact mechanism of nucleosome-initiated packaging is not fully understood.
Nucleosomes are the DNA particles responsible for compaction and transcription and can also carry hereditary information. Each nucleosome is about 10 nm in diameter and consists of DNA strands coiled around a core of simple protein called a histone. Nucleosomes are found in the nucleus of a cell and, when attached to DNA, form one of seven forms of chromatin.
When nucleosomes attach to DNA strands as repeating subunits, the structure resembles a “string of beads.” While in this form, the DNA is undergoing active transcription, the process by which DNA is converted into RNA. DNA is not converted directly into proteins to avoid errors and contamination.
The nucleosome structure 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 form the histone octomer. Around the histone octomer wrap 146 base pairs of DNA in its superhelix form, together forming the nucleosome.
Nucleosomes are the ‘packaging’ of DNA in the nucleus of a cell and the characteristic structure is what determines the accessibility of the DNA. The chemicals responsible for transcription cannot connect to chromatin if a nucleosome is in the way, so transcription proteins must first eject the nucleosome completely or slide it along the DNA molecule until the chromatin is exposed. Once that portion of DNA is transcribed into RNA, the nucleosomes can return to their original position.
If stretched in a straight line, the DNA in each mammalian nucleus would measure about two meters in length, but the nucleus of a mammalian cell is only 10 micrometers in diameter. It is the complex folding action of nucleosomes that allows DNA to fit into the nucleus. The “beads on a strand” appearance comes from the “linker” DNA that connects each nucleosome to form a fiber about 10 nm in diameter. In the presence of histone H1, 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 increased packing efficiency, as nearby proteins react to initiate folding and looping sequences that allow so much information to be packed into such a small package. Even today, the exact mechanism of nucleosome-initiated packaging is not fully understood.
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