Direct visualization of telomeric DNA loops in cells by AFM

Telomeres are an essential nucleoprotein structure at the ends of all eukar yotic chromosomes. Studies on the structure and function of telomeres have received particular attention owing to their possible significance to agein g and cancer. In this work, human telomere from HeLa cells were isolated an d purification by bio-gel P-2 column. Large duplex loops with a tail were f irst observed directly in the purified telomeric DNA species by atomic forc e microscopy (AFM). The lengths of the loop and tail are similar to2.5 +/- 0.5 and 2.0 +/- 1.5 kb, respectively. The AFM images also showed that the c ircular portion of loops is assumed to be a double-stranded structure accor ding to its apparent height, but to be a triplex or tetraplex structure for the loop-tail junction. These results indicate that telomeres in human cel ls may end in large loops, which is an advantage in further studies on the structure and function of telomeres in cells and provides a new procedure f or the study of telomere structure. Copyright (C) 2001 John Wiley & Sons, L td.