The length of DNA in a eukaryotic cell is N 2.2 m How can such a huge DNA be packaged in a nucleus of micrometer in diameter.
In eukaryotes, the DNA is wrapped around a positively charged histone octamer into a structure called a nucleosome. An atypical nucleosome consists of 200bp of DNA helix. The repeating units of nucleosomes form chromatin fibers. These chromatin fibers condense at the metaphase stage of cell division to form chromosomes. The packaging of chromatin at a higher level requires an additional set of proteins called non-histone chromosomal proteins thus in the nucleus, certain regions of the chromatin are loosely packed & they Stain lighter than the other region, these are called euchromatin. The other regions are lightly packed & they stain darker & is called heterochromatin.
