Abstract:
We calculate the entropic part of partition function of a bubble embedded in a double stranded DNA (dsDNA) by considering the total weights of possible configurations of a system of two single stranded DNA (ssDNA) of given length which start from a point along the contour of dsDNA and reunite at a position vector {\bf r} measured from the first point and the distribution function of the position vector {\bf r} which separates the two zipper forks of the bubble in dsDNA. For the distribution function of position vector {\bf r} we use the distribution of the end-to-end vector {\bf r} of strands of given length of dsDNA found from the wormlike chain model. We show that when the chains forming the bubble are assumed to be Gaussian the so called loop closure exponent $c$ is 3 and when we made correction by including self avoidence in each chain the value of $c$ becames 3.2.

Abstract:
We study the fluctuational dynamics of a tagged base-pair in double stranded DNA. We calculate the drift force which acts on the tagged base-pair using a potential model that describes interactions at base pairs level and use it to construct a Fokker-Planck equation.The calculated displacement autocorrelation function is found to be in very good agreement with the experimental result of Altan-Bonnet {\it et. al.} Phys. Rev. Lett. {\bf 90}, 138101 (2003) over the entire time range of measurement. We calculate the most probable displacements which predominately contribute to the autocorrelation function and the half-time history of these displacements.

Abstract:
The paper deals with the two-state (opening-closing of base pairs) model used to describe the fluctuation dynamics of a single bubble formation. We present an exact solution for the discrete and finite size version of the model that includes end effects and derive analytic expressions of the correlation function, survival probability and lifetimes for the bubble relaxation dynamics. It is shown that the continuous and semi-infinite limit of the model becomes a good approximation to exact result when a^N << 1, where N is bubble size and a, the ratio of opening to closing rates of base pairs, is the control parameter of DNA melting.

Abstract:
We propose a model for the fluctuation dynamics of the local denaturation zones (bubbles) in double-stranded DNA. In our formulation, the DNA strand is model as a one dimensional Rouse chain confined at both the ends. The bubble is formed when the transverse displacement of the chain attains a critical value. This simple model effectively reproduces the autocorrelation function for the tagged base pair in the DNA strand as measured in the seminal single molecule experiment by Altan-Bonnet et. al (Phys. Rev. Lett. 90, 138101 (2003)). Although our model is mathematically similar to the one proposed by Chatterjee et al. (J. Chem. Phys. 127, 155104 (2007)) it goes beyond a single reaction coordinate description by incorporating the chain dynamics through a confined Rouse chain and thus considers the collective nature of the dynamics. Our model also shows that the autocorrelation function is very sensitive to the relaxation times of the normal modes of the chain, which is obvious since the fluctuation dynamics of the bubble has the contribution from the different normal modes of the chain.

Abstract:
We present a general framework to study the thermodynamic denaturation of double-stranded DNA under superhelical stress. We report calculations of position- and size-dependent opening probabilities for bubbles along the sequence. Our results are obtained from transfer-matrix solutions of the Zimm-Bragg model for unconstrained DNA and of a self-consistent linearization of the Benham model for superhelical DNA. The numerical efficiency of our method allows for the analysis of entire genomes and of random sequences of corresponding length ($10^6-10^9$ base pairs). We show that, at physiological conditions, opening in superhelical DNA is strongly cooperative with average bubble sizes of $10^2-10^3$ base pairs (bp), and orders of magnitude higher than in unconstrained DNA. In heterogeneous sequences, the average degree of base-pair opening is self-averaging, while bubble localization and statistics are dominated by sequence disorder. Compared to random sequences with identical GC-content, genomic DNA has a significantly increased probability to open large bubbles under superhelical stress. These bubbles are frequently located directly upstream of transcription start sites.

Abstract:
We propose a simple nonlinear scaler displacement model to calculate the distribution of effect created by a shear stress on a double stranded DNA (dsDNA) molecule and the value of shear force $F_c$ which is required to separate the two strands of a molecule. It is shown that as long as the force pulls entire strand in the direction of its application the value of $F_c$ depends linearly on the length; the deviation from linear behaviour takes place when part of a strand moves in opposite direction under the influence of force acting on the other strand. The calculated values of $F_c$ as a function of length of dsDNA molecules are in very good agreement with the experimental values of Hatch et al (Phys. Rev. E $\bf 78$, 011920 (2008)).

Abstract:
We calculate the spectrum of torsional vibrations of a double-stranded structure that models the double helix of the DNA. We come to the conclusion that within the framework of the model elementary excitations may display an asymmetry as regards their winding and direction of the propagation, depending on initial polarization. The asymmetry could have a bearing on processes that take place in molecules of the DNA.

Abstract:
We study numerically the mechanical stability and elasticity properties of duplex DNA molecules within the frame of a network model incorporating microscopic degrees of freedom related with the arrangement of the base pairs. We pay special attention to the opening-closing dynamics of double-stranded DNA molecules which are forced into non-equilibrium conformations. Mechanical stress imposed at one terminal end of the DNA molecule brings it into a partially opened configuration. We examine the subsequent relaxation dynamics connected with energy exchange processes between the various degrees of freedom and structural rearrangements leading to complete recombination to the double-stranded conformation. The similarities and differences between the relaxation dynamics for a planar ladder-like DNA molecule and a twisted one are discussed in detail. In this way we show that the attainment of a quasi-equilibrium regime proceeds faster in the case of the twisted DNA form than for its thus less flexible ladder counterpart. Furthermore we find that the velocity of the complete recombination of the DNA molecule is lower than the velocity imposed by the forcing unit which is in compliance with the experimental observations for the opening-closing cycle of DNA molecules.

Abstract:
Using the Langevin Dynamics simulation, we have studied the effects of the shear force on the rupture of short double stranded DNA at different temperatures. We show that the rupture force increases linearly with the chain length and approaches to the asymptotic value in accordance with the experiment. The qualitative nature of these curves almost remains same for different temperatures but with a shift in the force. We observe three different regimes in the extension of covalent bonds (back bone) under the shear force.

Abstract:
It had been proposed that overstretching double stranded DNA from the 5-5 ends will produce a new fiber form of dsDNA that is narrower than the S form that has been suggested to result when the dsDNA is overstretched from the 3-3 ends. We present the first comparison of the structures that result when dsDNA is overstretched from the 5-5, 3-3, and 3-5 ends and show that the stability of the overstretched form depends on the ends to which the force is applied. The stability also depends strongly on the ionic environment as well as the presence of reagents that bind to ssDNA.