Aims

• To introduce methods for assessing the significance of sequence alignment scores.
• To describe heuristic methods for finding local alignments.
• To explain how PAM and BLOSUM substitution matrices are derived.

Objectives

After this lecture you will:

• understand how z-scores are used in assessing the significance of global alignment scores;
• understand how the BLAST and FASTA programs find local alignments when searching for matches in large databases;
• be aware of e-values and p-values as indicators of the significance of BLAST hits;
• understand how PAM and BLOSUM substitution matrices are derived;
• be aware of which substitution matrices are suitable for different tasks.

Supplementary Material

The lecture handout, featuring some of the lecture slides, is available on-line.
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Tutorial on The Statistics of Sequence Similarity Scores by Stephen Altschul, author of BLAST.

The method for deriving BLOSUM matrices is described in: Henikoff, S. and Henikoff, J.G. (1992) Amino acid substitution matrices from protein blocks. Proc Natl Acad Sci USA, 89, 10915-10919 (PubMed).

There is an overview of BLAST and substitution matrices in: Pertsemlidis, A. and Fondon, J.W. (2001) Having a BLAST with bioinformatics (and avoiding BLASTphemy), Genome Biol., 2, reviews2002.1-reviews2002.10.