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8/19/2019 Clase 2.b BLAST
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David Requena Anicama
Laboratorio de Bioinformática y Biología molecular –
UPCHLaboratorio de Bioinformática – FARVET
BLAST: Basic Local AnalysisSearch Tool
8/19/2019 Clase 2.b BLAST
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BLAST
• Atschul et al.
• Alignment heuristic method
• Hosted by NCBI
• Downloadable
8/19/2019 Clase 2.b BLAST
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Steps:
• Filter LCR or SR: N(nt), X(aa).
• SEG, DUST
• XNU
• K-letter words:
11(nt) or 3(aa).
High scores Confusion
8/19/2019 Clase 2.b BLAST
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8/19/2019 Clase 2.b BLAST
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Steps:
•
Organizing the selected words:Searching tree
•Extend matches to HSP:
R P P Q G L F
D P P E G V V
HSP
Database
-2 7 7 2 6 1 -1
Query
8/19/2019 Clase 2.b BLAST
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Steps:
•
HSP extension:
• Keep HSP > S: Random sequences
8/19/2019 Clase 2.b BLAST
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Steps:
• HSP statistical significance:•
SW follows EVD (Gumbel Extreme Value Distribution)
• λ y k Statistical parameters:
• Susbtitution matrix
• Penalties
• Sequence composition
• ′ ≈ ln
• ′ ≈ ln
≥ = 1 ()
=log(′′)
m’ = Query effective sequence
n’ = Database effective sequence
H = Expected score of aleatory alignment
8/19/2019 Clase 2.b BLAST
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E-value:
Número de veces que una secuencia de la base dedatos “no relacionada” puede obtener un score (s)
mayor a x por azar.
D = Number of database sequences.
= 1 − > ≈
8/19/2019 Clase 2.b BLAST
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