What are nucleosomes? What are they made up of?

* Under electron microscope, chromatin appears as beads on string. These beads are called nucleosomes.
* Nucleosome contain 200 bp of DNA molecule wrapped around a core of histone octamer.
* Histone octamer having two copies of each four types of histone proteins namely H₂A, H₂B, H₃ and H₄.
* H₁ histone molecule lies outside the nucleosome core and seals the two turns of DNA.
* The DNA that continues between two successive nucleosomes is called linker DNA.