In mammalian cells about two metres of linear DNA have to be packed into a nucleus of roughly 10 Â m diameter . Nucleosomes form the fundamental repeating units of eukaryotic chromatin , which is used to pack the large eukaryotic genomes into the nucleus while still ensuring appropriate access to it ( in mammalian cells approximately 2 m of linear DNA have to be packed into a nucleus of roughly 10 Â m diameter ) .