Sequence similarity searching helps researchers compare a DNA, RNA or protein sequence against a database to find related sequences. BLAST and FASTA are widely used tools for this purpose. They identify promising local similarities efficiently, but a hit alone does not prove that two sequences have the same function.
What does BLAST do?
BLAST (Basic Local Alignment Search Tool) uses heuristic methods to find local alignments between a query sequence and database sequences. Different BLAST programs suit different query and database types, such as nucleotide-to-nucleotide or translated nucleotide-to-protein searches. Choose the program that matches the biological question and the sequence types.
What does FASTA do?
FASTA is both a sequence format name and a family of sequence-search software tools. The FASTA search programs use heuristic strategies to find similarity between a query and database entries. Do not confuse a FASTA-formatted text file—which stores sequence identifiers and residues—with the FASTA search software itself.
How to read a search result
- Percent identity: the proportion of aligned positions that match exactly.
- Query coverage: how much of the query is included in the alignment; high identity over a tiny region may not answer the full question.
- E-value: the expected number of alignments of similar score that might occur by chance in a database search under the model used. Lower values generally indicate a more statistically significant match.
- Alignment length and gaps: help show whether the similarity spans a meaningful region and whether insertions or deletions are present.
A sensible workflow
Check that the sequence is in the correct format and has no accidental characters. Select the correct molecule type and search program, choose an appropriate database, and record the parameters. Review the top hits alongside identity, coverage, E-value, alignment length and annotation quality. If the result matters for a publication, save the search date and settings so the analysis can be reproduced.
Common mistakes include treating the top hit as definitive proof of function, ignoring query coverage, comparing unrelated molecule types or overlooking low-complexity regions. Similarity is evidence to investigate, not a substitute for biological interpretation. Explore additional study topics through SciMantra Study Resources.
Leave a Reply