Genome rearrangements and sorting by reversals

Sequence comparison in molecular biology is in the beginning of a major paradigm shift-a shift from gene comparison based on local mutations to chromosome comparison based on global rearrangements. In the simplest form the problem of gene rearrangements corresponds to sorting by reversals, i.e. sorting of an array using reversals of arbitrary fragments. Kececioglu and Sankoff gave the first approximation algorithm for sorting by reversals with guaranteed error bound and identified open problems related to chromosome rearrangements. One of these problems is Gollan´s conjecture on the reversal diameter of the symmetric group. We prove this conjecture and further study the problem of expected reversal distance between two random permutations. We demonstrate that the expected reversal distance is very close to the reversal diameter thereby indicating that reversal distance provides a good separation between related and non-related sequences. The gene rearrangement problem forces us to consider reversals of signed permutations, as the genes in DNA are oriented. Our approximation algorithm for signed permutation provides a `performance guarantee´ of 3/2. Finally, we devise an approximation algorithm for sorting by reversals with a performance ratio of 7/4