Sequence Alignment and Reading a Search Result
Two sequences are compared by scoring matches, mismatches and gaps, and the best alignment is the one with the highest total score. A database search returns hits ranked by that score with a statistic saying how often such a match would appear by chance.
What a learner can do afterwards
- Scores a short alignment by hand under a stated scheme, gaps included.
- Reads a search result and separates a strong hit from a chance one using the reported statistic.
- Explains why a short query returns weak statistics even for a perfect match.
1 · Read
Two sequences are compared by scoring matches, mismatches, and gaps. Each column earns or loses points under a stated scheme. The best alignment is the one with the highest total score.
Use plus 2 per match, minus 1 per mismatch, minus 2 per gap. ACGT against AGT aligns with three matches and one gap: 6 minus 2 is 4. A rival alignment scoring 1 loses to it.
A database search returns hits ranked by score, each with a statistic saying how often such a match would appear by chance. A tiny chance value marks a strong hit. A large one marks noise, even with a decent score.
Short queries return weak statistics even for a perfect match. Few letters can match by luck, so the math stays cautious. Lengthen the query before trusting a hit.
High score wins the alignment, and the chance statistic decides whether a hit matters.
2 · Watch
Take it off screen
Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.