Semiglobal Alignment solved by 230

Feb. 8, 2013, 3 p.m. by Rosalind Team

Topics: Alignment, Dynamic Programming

Gaps on the Ends are Free

We have covered both global and local alignments. However, sometimes we need a hybrid approach that avoids the weaknesses of these two methods. One such alternate approach is that of fitting alignments outlined in “Finding a Motif with Modifications”.

Another tactic is to allow ourselves to trim off gaps appearing on the ends of a global alignment for free; this is relevant if one of our strings to be aligned happens to contain additional symbols on the ends that are not relevant for the particular alignment at hand.


A semiglobal alignment of strings $s$ and $t$ is an alignment in which any gaps appearing as prefixes or suffixes of $s$ and $t$ do not contribute to the alignment score.

Semiglobal alignment has sometimes also been called "overlap alignment". Rosalind defines overlap alignment differently (see “Overlap Alignment”).

Given: Two DNA strings $s$ and $t$ in FASTA format, each having length at most 10 kbp.

Return: The maximum semiglobal alignment score of $s$ and $t$, followed by an alignment of $s$ and $t$ achieving this maximum score. Use an alignment score in which matching symbols count +1, substitutions count -1, and there is a linear gap penalty of 1. If multiple optimal alignments exist, then you may return any one.

Sample Dataset


Sample Output



This problem follows Jones & Pevzner, An Introduction to Bioinformatics Algorithms, Problem 6.24.

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