In prokaryote (bacteria) the genetic material is single circular double stranded DNA molecule. It is not separated from the cytosol due to absence of nuclear membrane. This DNA does not contain histone proteins. However chromosomes of some species are found to contain small quantity of heat-stable protein analogous to eukaryotic histone. In eukaryote the DNA binds to histone protein and forms nucleosome. They are arranged in a sequence and form chromosome. DNA and histones are absent in plant virus ie, they contain the genetic material as RNA.