ON SUPERCOILING INSTABILITY IN CLOSED DNA

Under the increase of its linking number, a closed DNA molecule can <<buckle>> and change to a supercoiled state. We show here that thermal fluctuations do not suppress this mechanical instability, and in fact can effectively lower its threshold. However, contrary to recent suggestions, we argue that such a fluctuation-driven instability (in thermal equilibrium) can neither take place in nicked DNA loops with one strand broken, nor in open DNA chains.