The reference-based DNA shotgun assembly problem is studied from an information-theoretic point of view. The task is to assemble the entire genome sequence based on a reference sequence, which is a noisy version of the desired one, and a set of short reads sampled from the desired sequence. Two necessary conditions on the underlying parameters for reconstruction are obtained. A reference-based assembly algorithm is proposed, and it is shown that under these conditions the algorithm can reconstruct the sequence with high probability.