Extension of a human telomeric primer using C-rich DNA nanocircles. A radioactive primer composed of three repeats of the G-rich human telomeric sequence (GGGTTA)3 was incubated with (A) Klenow Fragment of E.coli DNA polymerase I, lacking 3′–5′-exonuclease activity; (B) calf thymus polymerase alpha; (C) human polymerase beta; and (D) HeLa nuclear cell extracts. Reactions further contained 100 nM DNA nanocircle, traces of 5′-32P-labeled primer (GGGTTA)3, and 1 mM dNTPs, and were incubated at 37°C for 1 h. Lane 1, control reaction lacking DNA nanocircle; lane 2, 36 nt circle; lane 3, 42 nt circle; lane 4, 48 nt circle; lane 5, 54 nt circle; lane 6, 60 nt circle; and M, size marker.