A nucleosome consists of a DNA left-handed double helix that is wrapped around core proteins called histones. Nucleosomes are the basic unit of DNA packaging used to store the genetic information within a eukaryotic cell's nucleus.
Within the nucleus of eukaryotic cells, an organism's genome is contained within the organism's chromosomes. Chromosomes consist of repeating units referred to as nucleosomes. Each nucleosome consists of 147 base pairs of DNA arranged in a left handed double helix. The strand of DNA winds around a histone octomer protein. Each nucleosome core particle is separated by about 80 base pairs of linker DNA.
The histone protein is composed of eight proteins. There are two copies each of four histone proteins: Histone 2A, Histone 2B, Histone 3, and Histone 4. Histone 1 exists as a histone linker protein and is associated at the DNA entrance and exit binding to the linker DNA. Histone 1 serves to help bind nucleosomes in order to further compact the chromatin. The tails of histone proteins stick out and are available to be modified by different histone modifying enzymes as signals for transcription, replication, and DNA repair functions. Methylation, acetylation, and phosphorylation are the most common types of modifications.