Hemicatenated DNA loops. A DNA fragment containing a 62 bp tract of poly(CA)·poly(TG) flanked by non-repetitive sequences from plasmid pUC19 is used. After thermal denaturation, pairing of the opposite strands can occur with a shift. In this case, a transient structure is produced in which the terminal non-repetitive regions remain single-stranded. In the presence of HMGB1 this structure can fold and form a loop, with one of the single strands at one end inserting inside the fork formed by the two single strands at the opposite end. Subsequently, complete pairing of the terminal non-repetitive regions leads to the formation of a hemicatenated junction, in which two duplexes cross with one strand on each duplex passing between the two strands of the opposite duplex, forming a DNA hemicatenane .