Background DNA Packaging in Eukaryotes

that compact DNA. Histones are small proteins that contain a large percentage of positively charged amino acids, usually lysine and arginine [16]. Although these proteins are fairly small, they are abundant with a weight approximately equaling the weight of DNA in the nucleus [18]. Histones are some of the most conserved proteins across different species with histone H4 being almost the same in many species [7]. In eukaryotes, DNA is compacted into chromatin to allow it to fit into the nucleus. The basic structure of chromatin is the nucleosome, which consists of about 145 base pairs of DNA wrapped around a histone octamer [18]. However, the number of amino acid base pairs in the nucleosome varies depending on species and tissue and cam be anywhere from 150 to 250 base pairs [16]. The histone octamer or histone core consists of a (H3)2(H4)2 tetramer and two H2A-H2B dimmers. The tetramer is at the center of the octomer with a dimmer at each of the ends of where the DNA wraps [7]. DNA wraps around the core histone about 1.75 times creating a 10nm nucleosome with about 14nm or 60 base pairs of linker DNA that tie the nucleosomes together. Histone H1 or the linker histone has an elongated structure, unlike the other histones, with both carbonyl and amino terminal arms extending form its otherwise globular core. H1 binds to DNA in the histone core and extends its arms along the linker DNA between the nucleosomes [18]. The structure that is formed when H1 binds is called a chromatosome and contains an additional 20 base pairs of DNA [3]. Although histone H1 is not needed in the nucleosome to condense DNA into the 30nm fibers that make up chromatin, H1 is required to achieve the maximum compaction of DNA [2]. Nucleosome Mobility Although the compaction of DNA into chromatin organizes DNA allowing it to fit into the nucleus, the chromatin structure also acts as a barrier to DNA replication, transcription and repair [1]. To allow for both adequate compaction of DNA into chromatin and access of DNA to transcription machinery, the nucleosomes structure has to be disrupted in some way