Energy flow considerations and thermal fluctuational opening of DNA base pairs at a replicating fork: unwinding consistent with observed replication rates.

The effect of an open loop of various sizes on the thermal stability of the adjoining intact base pairs in a duplex DNA chain is studied in a lattice model of Poly(dG).Poly(dC). We find that for a Y-shaped fork configuration the thermal fluctuation at the fork is so enhanced that the life time of the adjoining base pair is much smaller than the 1 millisecond time scale associated with helicase separation of a base pair in some systems. Our analysis indicates that thermal fluctuational base pair opening may be of importance in facilitating the enzyme unwinding process during chain elongation of a replicating DNA. It is most likely that the thermal fluctuational opening of the base pair at the junction of a replicating fork is fast enough so that a DNA unwinding enzyme can encounter an unstacked base pair with reasonable probability. This conclusion can explain several experimental observations regarding the temporal relationship between ATP hydrolysis by accessory proteins and primer elongation by a holoenzyme complex in ssDNA. We also discuss a mechanism by which the energy associated with ATP hydrolysis may enhance the thermal driven base opening mechanism.