What is a nucleosome?
The DNA molecules of eukaryotes are linear chains of nucleotides with lengths in the centimeter range. The size of the nuclei in which this DNA resides is on average only 7 micrometer in diameter. The evolution of such long molecules was possible only because the DNA strings were tightly folded around proteins leading to the compaction of a chromosome. The major protein complex with DNA wrapped around itself is called a nucleosome. Each nucleosome consists of four proteins called histones. Nucleosomes are positively charged at the N-termini of their histones. There are two copies of each histone molecule for every 200 bp of DNA. The histones are called H2A, H2B, H3 and H4. Another, larger single histone molecule, H1 is called a linker histone. It binds to DNA molecules which cross over each other and seals the complex at the exterior of the nucleosome such that the DNA does not unfold. The DNA of a typical eukaryotic genome is about 1m long. The compaction of DNA into nucleosomal chain
