http://digfir-published.macmillanusa.com/lodish_animation/a0404_nucleotide_polym.html\tChapter 4. Nucleotide Polymerization by DNA Polymerase
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http://digfir-published.macmillanusa.com/lodish_animation/a0404_nucleotide_polym_dlap.xml\tDLAP questions\t519bca41b11c0f1368000056