Define a directed graph G=(V,E) where V represents the set of DNA fragments. Let fi, fj be in V. Edge iff fragment fi lies to the left of fj. A consistent ordering can be found by a topological sort.
Recall that a spanning tree is a subset of edges that forms a tree connecting all nodes of the graph. If there are two edge-disjoint spanning trees, color the edges of one tree red and the edges of the other blue. For any two nodes, u and v, there is a red path between them in the first tree. However, there is also a blue path between u and v. Because there are two distinct paths (sharing no edges) between u and v, the nodes are on a cycle in graph G.