The aim of the present paper is to study the entropic elasticityof the dsDNA molecule, having a crystallographic lengthL of the order of 10–30persistence lengths A, when it is subject to spatial obstructions. We have not tried to obtain the single-moleculepartition function by solving a Schödringer-like equation. We prefer to restrictour considerations to a discretized version of the worm-like chain model with anadded one-monomer potential, simulating the spatial constraints. We derived thetransfer matrix connecting the partition functions relative to adjacent ‘effectivemonomers’ directly from the discretized Boltzmann formula. We have plugged Diracδ-functions in the functional integral that are adequate to ensure that the monomercoordinate and the tangent vector are independent variables. The partition functionis, then, given by an iterative process which is both numerically efficient andphysically transparent. As a test of our discretized approach, we have studied twoconfigurations involving a dsDNA molecule confined between a pair of parallel plates. Onemolecule end is anchored to one plate by a biochemical bond. A stretching forceF, normal to the plates, is pulling the other end away. In the first case, the crystallographic lengthL is smaller than thetwo-plate distance L0. The molecule feels, then, only the anchoring barrier effect. The predicted elongationversus force curve is pushed upward with respect to the WLC (worm-like chain) modelresult. This effect is most spectacular in the low force regime. For large forces, say,Fhigh = 5 kB T/A, the elongationversus L is very well fitted by a straight line with a slope given by the standard WLC model and aconstant term . In the second case, L takes values up to Lmax = 1.5 L0. With astretching force still equal to Fhigh, the standard WLC model predicts that the molecule cannot fit within the plates whenL > L* = 1.29L0. We have studied the evolution of the elongation derivative with respect toL,together with the mean square free-end fluctuations along the force. They both exhibit a sharp decreasewhen L ≥ L0. We present a semiquantitative argument suggesting that the terminal segment involving20% of the internal monomers flattens against the repulsive barrier when . In conclusion, we suggest extensions of the present work, relevant to the analysisof micromanipulation experiments. Finally, we have gathered in an appendixformal developments, leading to a precise relation between the transfer matrixand the Hamiltonian methods for the study of spatially constrained dsDNA.