Histone proteins are basic proteins and are used in packing of eukaryotic (absent in prokaryotes) DNA. DNA and histones together comprise chromatin, forming bulk of the eukaryotic chromosomes. Histones are of five major kinds $H_1, H_{2 A}, H_{2 B}, H_3$ and $H_4 . H_1$ histones link neighbouring nucleosomes (fundamental packing units of an eukaryotic chromosome), while other are elements of nucleosome structure. During S-phase of cell cycle synthesis of histone proteins take place because at this stage number of chromosomes become double to that of somatic number.