Direct visualization of telomeric DNA loops in cells by AFM

Telomeres are an essential nucleoprotein structure at the ends of all eukaryotic chromosomes. Studies on the structure and function of telomeres have received particular attention owing to their possible significance to ageing and cancer. In this work, human telomere from HeLa cells were isolated and purification by bio-gel P-2 column. Large duplex loops with a tail were first observed directly in the purified telomeric DNA species by atomic force 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 circular portion of loops is assumed to be a double-stranded structure according to its apparent height, but to be a triplex or tetraplex structure for the loop-tail junction. These results indicate that telomeres in human cells 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 for the study of telomere structure. Copyright (C) 2001 John Wiley & Sons, Ltd.