Sequence Similarity Calculators

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Sequence similarity is a quantitative measure of how alike two biological sequences (DNA, RNA, or protein) are at the sequence level. It is expressed as percent identity (identical residues / aligned length × 100) or percent similarity (identical + conserved substitutions / aligned length × 100 for proteins). Sequence similarity is used to infer evolutionary relationships (homology), predict protein structure and function, design PCR primers, identify pathogens, and annotate new genomes. Tools like BLAST, Smith-Waterman, and Clustal Omega quantify sequence similarity and assess its statistical significance.

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Percent Identity vs. Percent Similarity

Percent identity = (identical positions in alignment / total aligned positions, excluding gaps) × 100. Counts only exact matches.

Percent similarity (proteins only) = (identical + conserved substitutions) / aligned length × 100. Conservative substitutions (e.g., Leu↔Val — both nonpolar) are counted as similar. Percent similarity ≥ percent identity always.

BLAST Sequence Similarity

BLAST (Basic Local Alignment Search Tool) finds regions of local similarity between query and database sequences. Key output values:

  • % Identity: Exact matches in the aligned region
  • E-value: Expected number of hits with this score by chance; E < 10⁻⁵ = significant; E < 10⁻¹⁰ = highly significant
  • Bit score: Normalized alignment score; higher = better
  • Query cover: Fraction of query sequence included in the alignment

Similarity Thresholds for Homology

  • Protein identity > 30%: likely homologous (structural similarity probable)
  • 20–30%: 'twilight zone' — may be homologous; verify with structural alignment
  • < 20%: insufficient for reliable homology inference from sequence alone
  • DNA coding sequence identity > 70–80%: usually same gene across related species
  • 16S rRNA > 97%: same bacterial species; > 95%: same genus (approximate thresholds)

Global vs. Local Alignment

Global alignment (Needleman-Wunsch): aligns full length of both sequences; best for similar-length homologs. Local alignment (Smith-Waterman, BLAST): finds best matching sub-regions; better for domain searches and distant homologs.

Glossary

Percent Identity
The fraction of identical residues in an alignment: (identical positions / aligned length) × 100; the primary metric for reporting sequence similarity in BLAST and pairwise alignment results.
E-value (BLAST)
Expected number of database matches with a given alignment score by chance; E < 0.01 is significant; E < 10⁻¹⁰ is highly significant; scales with database size.
Twilight Zone (Sequence)
The 20–30% protein identity range where homology is uncertain from sequence alone; structural comparison or profile-based methods (HMMs) are needed to confirm evolutionary relationship.

Frequently Asked Questions

Percent identity counts only exact sequence matches: identical positions / aligned length × 100. Percent similarity (used for proteins) additionally counts conservative substitutions where the amino acid change preserves physicochemical properties — e.g., Leu→Val (both hydrophobic) or Asp→Glu (both acidic). Both metrics depend on the alignment — different alignment parameters give different scores. Percent identity is more stringent and reproducible; percent similarity requires a substitution matrix (BLOSUM62 is standard) to define 'conservative' changes.

The BLAST E-value (expect value) is the expected number of database matches with a score at least as good as the observed one purely by chance in a database of that size. E-value < 0.01: significant (about 1 false positive per 100 searches); E-value < 10⁻⁵: good evidence of true similarity; E-value < 10⁻¹⁰: very strong evidence. E-value scales with database size — a search of a larger database gives larger (worse) E-values for the same alignment. Always use E-value, not bit score, for cross-database comparisons.

Proteins with >30% identity over a full-length alignment are very likely homologous — sharing common ancestry and usually similar 3D structure. 20–30% identity is the 'twilight zone' — may or may not be homologous; structural comparison is needed to confirm. Below 20% identity (the 'midnight zone'), sequence similarity alone cannot reliably infer homology. Note: homology is binary (either homologous or not) — '30% homologous' is incorrect; the correct expression is '30% identical' or 'shares 30% identity with likely homology.

The 16S rRNA gene (1.5 kb) evolves slowly and is universal to bacteria and archaea. Traditional cutoffs: ≥97% identity = same species; ≥95% = same genus; ≥90% = same family. These are approximate — the modern gold standard for species delineation is Average Nucleotide Identity (ANI) ≥ 95% across the whole genome. However, 16S remains the primary amplicon marker for microbiome profiling (16S metagenomics) due to universal PCR primers and its extensive database coverage in SILVA, Greengenes, and NCBI.