y zes ▁the ▁trans ient ▁breaking ▁and ▁rejo ining ▁of ▁a ▁single ▁str and ▁of ▁DNA ▁which ▁lets ▁the ▁broken ▁str and ▁rotate ▁around ▁the ▁int act ▁str and , ▁thus ▁alter ing ▁the ▁topology ▁of ▁DNA . ▁This ▁gene ▁is ▁local ized ▁to ▁chrom os ome ▁ 2 0 ▁and ▁has ▁pseud ogen es ▁which ▁res ide ▁on ▁chrom os om es ▁ 1 ▁and ▁ 2 2 . ▁ ▁Mechan ism ▁ ▁As ▁review ed ▁by ▁Champ oux , ▁the ▁type ▁IB ▁to pois omer ases , ▁including ▁T OP 1 , ▁form ▁a ▁c oval ent ▁intermediate ▁in ▁which ▁the ▁active ▁site ▁ty ros ine ▁becomes ▁attached ▁to ▁the ▁ 3 ' ▁ph osph ate ▁end ▁of ▁the ▁cle aved ▁str and ▁rather ▁than ▁the ▁ 5 ' ▁ph osph ate ▁end . ▁ ▁The ▁e uk ary otic ▁to pois omer ases ▁I ▁were ▁found ▁to ▁nick ▁the ▁DNA ▁with ▁a ▁preference ▁for ▁a ▁sequence ▁of ▁nucle ot ides ▁that ▁extends ▁from ▁positions ▁- 4 ▁to ▁- 1 ▁from ▁the ▁nick . ▁ ▁The ▁preferred ▁nucle ot ides ▁in ▁the ▁str and ▁to ▁be ▁cut ▁are ▁ 5 ' -( A / T )( G / C )( A / T ) T - 3 ' ▁with ▁the ▁en zym e ▁c oval ently ▁attached ▁to ▁the ▁- 1 ▁T ▁resid ue , ▁though ▁sometimes ▁a ▁C ▁resid ue ▁is ▁found ▁at ▁the ▁- 1 ▁position . ▁ ▁The ▁T OP 1 ▁protein ▁of ▁humans ▁has ▁been ▁sub div