, ▁H 4 , ▁H 2 A , ▁H 2 B ) ▁in ▁nucle os om es ▁in ▁e uk ary otes ▁and ▁often ▁confer ▁specific ▁struct ural ▁and ▁functional ▁features . ▁The ▁term ▁might ▁also ▁include ▁a ▁set ▁of ▁linker ▁hist one ▁( H 1 ) ▁variants , ▁which ▁lack ▁a ▁distinct ▁canonical ▁is o form . ▁The ▁differences ▁between ▁the ▁core ▁canonical ▁hist ones ▁and ▁their ▁variants ▁can ▁be ▁summar ized ▁as ▁follows : ▁( 1 ) ▁canonical ▁hist ones ▁are ▁rep lication - dependent ▁and ▁are ▁expressed ▁during ▁the ▁S - phase ▁of ▁cell ▁cycle ▁whereas ▁hist one ▁variants ▁are ▁rep lication - in dependent ▁and ▁are ▁expressed ▁during ▁the ▁whole ▁cell ▁cycle ; ▁( 2 ) ▁in ▁animals , ▁the ▁gen es ▁encoding ▁canonical ▁hist ones ▁are ▁typically ▁cluster ed ▁along ▁the ▁chrom os ome , ▁are ▁present ▁in ▁multiple ▁copies ▁and ▁are ▁among ▁the ▁most ▁conser ved ▁prote ins ▁known , ▁whereas ▁hist one ▁variants ▁are ▁often ▁single - copy ▁gen es ▁and ▁show ▁high ▁degree ▁of ▁variation ▁among ▁species ; ▁( 3 ) ▁canonical ▁hist one ▁gen es ▁lack ▁intr ons ▁and ▁use ▁a ▁stem ▁loop ▁structure ▁at ▁the ▁ 3 ’ ▁end ▁of ▁their ▁m R NA , ▁whereas ▁hist one ▁variant ▁gen es ▁may ▁have ▁intr ons ▁and ▁their ▁m R NA ▁tail ▁is ▁usually ▁poly aden yl ated . ▁Complex ▁mult ic ell ular ▁organ isms ▁typically ▁have ▁a ▁large ▁number ▁of ▁hist one ▁variants ▁providing ▁a ▁variety ▁of ▁different ▁functions . ▁Rec ent