DSBs ( a ) and SSBs ( b – d ) generated in presence of Top2 ( a ); ETO ( b ); Top1 ( c ); and <t>DNA-damaging</t> agents that modify the DNA termini ( d ). Red arrow represents successful nick translation. Stop sign represents unsuccessful nick translation. Nick translation by DNA <t>polymerase</t> I necessitates a 3'-OH, which is not reconstituted in case of Top1 cleavage or when the DNA termini is damaged (shown by asterisk). In these cases the principal enzymes involved in processing and repair of the ends are listed below the black arrow. TDP1, tyrosyl-DNA phosphodiesterase 1, PNKP, polynucleotide kinase 3'-phosphatase, APE1, AP endonuclease I [ 17 ]
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