<P> The nucleosome core particle consists of approximately 146 base pairs (bp) of DNA wrapped in 1.67 left - handed superhelical turns around a histone octamer consisting of 2 copies each of the core histones H2A, H2B, H3, and H4 . Core particles are connected by stretches of "linker DNA", which can be up to about 80 bp long . Technically, a nucleosome is defined as the core particle plus one of these linker regions; however the word is often synonymous with the core particle . Genome - wide nucleosome positioning maps are now available for many model organisms including mouse liver and brain . </P> <P> Linker histones such as H1 and its isoforms are involved in chromatin compaction and sit at the base of the nucleosome near the DNA entry and exit binding to the linker region of the DNA . Non-condensed nucleosomes without the linker histone resemble "beads on a string of DNA" under an electron microscope . </P> <P> In contrast to most eukaryotic cells, mature sperm cells largely use protamines to package their genomic DNA, most likely to achieve an even higher packaging ratio . Histone equivalents and a simplified chromatin structure have also been found in Archea, suggesting that eukaryotes are not the only organisms that use nucleosomes . </P> <P> Pioneering structural studies in the 1980s by Aaron Klug's group provided the first evidence that an octamer of histone proteins wraps DNA around itself in about 1.7 turns of a left - handed superhelix . In 1997 the first near atomic resolution crystal structure of the nucleosome was solved by the Richmond group, showing the most important details of the particle . The human alpha - satellite palindromic DNA critical to achieving the 1997 nucleosome crystal structure was developed by the Bunick group at Oak Ridge National Laboratory in Tennessee . The structures of over 20 different nucleosome core particles have been solved to date, including those containing histone variants and histones from different species . The structure of the nucleosome core particle is remarkably conserved, and even a change of over 100 residues between frog and yeast histones results in electron density maps with an overall root mean square deviation of only 1.6 Å . </P>

What is the total number of nucleosomes in all fibers