the absence of the putative S-phase. However, we stress that if experiments are to conclusively rule for or against either picture of overstretching, then the consequences of both must be better understood. This paper is an attempt to clarify the physical consequences of assuming the existence of an elongated, hybridized phase of DNA. We thank Gijs Wuite, Erwin Peterman and John Marko for correspondence. Fig. 2 was adapted from Ref. [11]. This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02–05CH11231. IV. ACKNOWLEDGEMENTS 5