Biopython: Sequence Analysis Toolkit

Overview

Biopython provides a comprehensive suite of modules for sequence-centric bioinformatics: reading and writing every major biological file format (FASTA, FASTQ, GenBank, GFF), querying NCBI databases programmatically, running BLAST searches and parsing results, aligning sequences pairwise or in multiple-sequence alignments, and building and visualizing phylogenetic trees. This skill focuses on analysis workflows — from NCBI data retrieval through alignment to phylogenetic inference.

For PCR primer design, restriction enzyme digestion, cloning simulation, protein structure analysis (Bio.PDB), and molecular weight/Tm calculations, see biopython-molecular-biology.

When to Use

• Download a gene family from NCBI Nucleotide/Protein, align sequences, and construct a phylogenetic tree
• Parse GenBank or GFF3 annotation files and extract CDS sequences for a set of features
• Run a BLAST search against NCBI nt or nr, filter significant hits, and fetch their full sequences
• Compute pairwise sequence identities or score alignments with BLOSUM62/PAM250 matrices
• Index a large multi-FASTA or FASTQ file with SeqIO.index() for random-access retrieval without loading all sequences into RAM
• Convert between sequence formats (FASTA ↔ GenBank ↔ FASTQ ↔ PHYLIP) in a single call
• Traverse, root, prune, and annotate a Newick or Nexus phylogenetic tree programmatically
• Use pysam instead when working with SAM/BAM/CRAM alignment files and mapped reads
• Use scikit-bio instead when you need ecological diversity metrics (UniFrac, beta diversity) or ordination methods such as PCoA
• Use gget instead for quick gene lookups and one-liner NCBI queries without writing an Entrez pipeline

Prerequisites

• Python packages: biopython, numpy, matplotlib
• Optional tools: MUSCLE or ClustalW installed locally (for Bio.Align.Applications wrappers)
• NCBI access: Set Entrez.email before any E-utilities call; obtain a free API key at https://www.ncbi.nlm.nih.gov/account/ for 10 req/s (default is 3 req/s)
• Local BLAST: BLAST+ installed separately (conda install -c bioconda blast) for offline searches

Check before installing: The tool may already be available in the current environment (e.g., inside a pixi / conda env). Run command -v python first and skip the install commands below if it returns a path. When running inside a pixi project, invoke the tool via pixi run python rather than bare python.

pip install biopython numpy matplotlib
conda install -c bioconda blast  # optional, for local BLAST

Quick Start

from Bio import SeqIO, Entrez
from Bio.SeqUtils import gc_fraction

# Fetch a GenBank record and display basic stats
Entrez.email = "[email protected]"
handle = Entrez.efetch(db="nucleotide", id="NM_007294", rettype="gb", retmode="text")
record = SeqIO.read(handle, "genbank")
handle.close()

print(f"ID: {record.id}")
print(f"Length: {len(record.seq)} bp")
print(f"GC content: {gc_fraction(record.seq)*100:.1f}%")
print(f"Features: {len(record.features)}")
print(f"First feature: {record.features[0].type} at {record.features[0].location}")
# ID: NM_007294.4
# Length: 7207 bp
# GC content: 47.3%
# Features: 9

Core API

Module 1: SeqIO — File Parsing and Format Conversion

Bio.SeqIO reads and writes every major sequence format (FASTA, FASTQ, GenBank, EMBL, PHYLIP, Nexus). SeqIO.parse() returns an iterator of SeqRecord objects; SeqIO.index() builds an on-disk or in-memory dictionary for large files.

from Bio import SeqIO

# Parse FASTA and FASTQ
fasta_records = list(SeqIO.parse("sequences.fasta", "fasta"))
print(f"FASTA: {len(fasta_records)} sequences")

# FASTQ: access quality scores
for rec in SeqIO.parse("reads.fastq", "fastq"):
    quals = rec.letter_annotations["phred_quality"]
    avg_q = sum(quals) / len(quals)
    print(f"  {rec.id}: {len(rec.seq)} bp, mean Q={avg_q:.1f}")
    break  # show one example

# Parse GenBank with feature access
for rec in SeqIO.parse("chromosome.gb", "genbank"):
    cdss = [f for f in rec.features if f.type == "CDS"]
    print(f"{rec.id}: {len(cdss)} CDS features")
    for cds in cdss[:3]:
        gene = cds.qualifiers.get("gene", ["unknown"])[0]
        print(f"  {gene}: {cds.location}")

from Bio import SeqIO

# SeqIO.index() for random access without loading all records
# Useful for large reference FASTA files (genomes, nr database subsets)
idx = SeqIO.index("large_genome.fasta", "fasta")
print(f"Index contains {len(idx)} sequences")

# Retrieve specific sequences by ID in O(1)
target = idx["chr1"]
print(f"chr1: {len(target.seq):,} bp")
region = target.seq[1_000_000:1_001_000]
print(f"Region [1M-1M+1kb]: {region[:60]}...")

# Format conversion: GenBank → FASTA in one call
n = SeqIO.convert("annotation.gb", "genbank", "sequences.fasta", "fasta")
print(f"Converted {n} records to FASTA")

Module 2: Seq and SeqRecord — Sequence Objects and Feature Annotations

Bio.Seq represents a biological sequence with in-place operations. Bio.SeqRecord wraps a Seq with an ID, description, and a dictionary of feature annotations. Features use FeatureLocation with strand (+1, -1).

from Bio.Seq import Seq
from Bio.SeqRecord import SeqRecord
from Bio.SeqFeature import SeqFeature, FeatureLocation
from Bio.SeqUtils import gc_fraction, MeltingTemp

dna = Seq("ATGAAACCCGGGTTTTAA")
print(f"GC content: {gc_fraction(dna)*100:.1f}%")
print(f"Reverse complement: {dna.reverse_complement()}")
print(f"Transcript: {dna.transcribe()}")
print(f"Translation: {dna.translate()}")
print(f"Translation (to stop): {dna.translate(to_stop=True)}")

# Tm calculation for a short primer
primer = Seq("ATGAAACCCGGG")
tm = MeltingTemp.Tm_Wallace(primer)
print(f"Tm (Wallace): {tm:.1f}°C")

from Bio.Seq import Seq
from Bio.SeqRecord import SeqRecord
from Bio.SeqFeature import SeqFeature, FeatureLocation

# Build a SeqRecord with annotated features
gene_seq = Seq("ATGAAACCCGGGTTTTAAATCGATCG" * 10)
record = SeqRecord(gene_seq, id="MY_GENE_001", description="synthetic example gene")

# Annotate a CDS feature
cds = SeqFeature(
    FeatureLocation(0, 18, strand=+1),
    type="CDS",
    qualifiers={"gene": ["myGene"], "product": ["hypothetical protein"]}
)
record.features.append(cds)

# Extract and translate the feature
cds_seq = cds.location.extract(record.seq)
protein = cds_seq.translate(to_stop=True)
print(f"CDS: {cds_seq}")
print(f"Protein: {protein}")

# Slice record preserving feature annotations
sub = record[0:60]
print(f"Subrecord: {len(sub.seq)} bp, {len(sub.features)} features")

Module 3: Entrez — Programmatic NCBI Database Access

Bio.Entrez wraps the NCBI E-utilities API: esearch finds records matching a query, efetch retrieves full records, elink finds related records across databases, and esummary returns document summaries. Always set Entrez.email and respect the 3 req/s rate limit (10 req/s with API key).

from Bio import Entrez, SeqIO
import time

Entrez.email = "[email protected]"
# Entrez.api_key = "YOUR_API_KEY"  # for 10 req/s

# esearch: find IDs matching a query
handle = Entrez.esearch(db="nucleotide", term="BRCA1[Gene] AND Homo sapiens[Organism] AND mRNA[Filter]", retmax=10)
search_results = Entrez.read(handle)
handle.close()

print(f"Total hits: {search_results['Count']}")
ids = search_results["IdList"]
print(f"Retrieved IDs: {ids}")

# efetch: download full GenBank records
for acc_id in ids[:3]:
    handle = Entrez.efetch(db="nucleotide", id=acc_id, rettype="gb", retmode="text")
    record = SeqIO.read(handle, "genbank")
    handle.close()
    print(f"  {record.id}: {len(record.seq)} bp — {record.description[:60]}")
    time.sleep(0.4)  # stay within rate limit

from Bio import Entrez
import time

Entrez.email = "[email protected]"

# elink: cross-database links (e.g., PubMed article → related nucleotide sequences)
handle = Entrez.elink(dbfrom="pubmed", db="nucleotide", id="29087512")
link_results = Entrez.read(handle)
handle.close()

linked_ids = []
for linkset in link_results:
    for db_links in linkset.get("LinkSetDb", []):
        if db_links["DbTo"] == "nucleotide":
            linked_ids = [lnk["Id"] for lnk in db_links["Link"]]
            break

print(f"Nucleotide sequences linked to PubMed 29087512: {len(linked_ids)}")
print(f"First IDs: {linked_ids[:5]}")

# esummary: lightweight metadata without downloading full records
if linked_ids:
    handle = Entrez.esummary(db="nucleotide", id=",".join(linked_ids[:5]))
    summaries = Entrez.read(handle)
    handle.close()
    for doc in summaries:
        print(f"  {doc['AccessionVersion']}: {doc['Title'][:70]}")

Module 4: BLAST — Remote and Local Sequence Similarity Search

Bio.Blast.NCBIWWW.qblast() submits queries to NCBI BLAST servers and returns XML handles. Bio.Blast.NCBIXML.parse() (or .read() for single queries) yields Blast objects with alignments and hsps. For large-scale searches, use local BLAST+ via subprocess.

from Bio.Blast import NCBIWWW, NCBIXML
from Bio.Seq import Seq

# Remote BLASTP against Swiss-Prot (small, reviewed database)
query = Seq("MTEYKLVVVGAGGVGKSALTIQLIQNHFVDEYDPTIEDSY")
result_handle = NCBIWWW.qblast("blastp", "swissprot", str(query), hitlist_size=10)

# Parse results
blast_record = NCBIXML.read(result_handle)
print(f"Query: {blast_record.query}")
print(f"Database: {blast_record.database}")
print(f"Hits: {len(blast_record.alignments)}")

E_VALUE_THRESH = 1e-5
for alignment in blast_record.alignments[:5]:
    for hsp in alignment.hsps:
        if hsp.expect < E_VALUE_THRESH:
            identity_pct = hsp.identities / hsp.align_length * 100
            print(f"\n  Hit: {alignment.title[:70]}")
            print(f"  E-value: {hsp.expect:.2e}, Identity: {identity_pct:.1f}%, Score: {hsp.score}")
            print(f"  Query:  {hsp.query[:60]}")
            print(f"  Match:  {hsp.match[:60]}")
            print(f"  Sbjct:  {hsp.sbjct[:60]}")

import subprocess
from Bio.Blast import NCBIXML
from Bio import SeqIO

# Local BLAST+ via subprocess (faster for large batches)
# Requires: makeblastdb and blastp installed (conda install -c bioconda blast)

# Step 1: Build a local database from a FASTA file
subprocess.run(
    ["makeblastdb", "-in", "ref_proteins.fasta", "-dbtype", "prot", "-out", "ref_db"],
    check=True
)

# Step 2: Run blastp against local database
result = subprocess.run(
    ["blastp", "-query", "query.fasta", "-db", "ref_db",
     "-outfmt", "5",           # XML output for NCBIXML parsing
     "-evalue", "1e-5",
     "-num_threads", "4",
     "-out", "blast_results.xml"],
    check=True
)

# Step 3: Parse XML results
with open("blast_results.xml") as fh:
    for blast_rec in NCBIXML.parse(fh):
        print(f"Query: {blast_rec.query_id}")
        for aln in blast_rec.alignments[:3]:
            hsp = aln.hsps[0]
            print(f"  Hit: {aln.hit_id}, E={hsp.expect:.2e}, Id={hsp.identities}/{hsp.align_length}")

Module 5: Phylo — Tree Parsing, Manipulation, and Visualization

Bio.Phylo reads Newick, Nexus, PhyloXML, and NeXML formats. Trees are represented as Clade objects with nested children. Key methods: find_clades() (generator over nodes), common_ancestor(), distance(), root_with_outgroup(), and prune(). Visualization uses matplotlib.

from Bio import Phylo
import io

# Parse a Newick tree from a string
newick = "((Homo_sapiens:0.01, Pan_troglodytes:0.012):0.08, (Mus_musculus:0.15, Rattus_norvegicus:0.14):0.12, Drosophila_melanogaster:0.85);"
tree = Phylo.read(io.StringIO(newick), "newick")

print(f"Number of terminals: {tree.count_terminals()}")
print(f"Total branch length: {tree.total_branch_length():.3f}")
print(f"Terminals: {[t.name for t in tree.get_terminals()]}")

# Root with outgroup and compute distances
tree.root_with_outgroup("Drosophila_melanogaster")
human = tree.find_any("Homo_sapiens")
chimp = tree.find_any("Pan_troglodytes")
mouse = tree.find_any("Mus_musculus")
print(f"\nDistance Human–Chimp: {tree.distance(human, chimp):.4f}")
print(f"Distance Human–Mouse: {tree.distance(human, mouse):.4f}")

# Traverse all internal nodes
for clade in tree.find_clades(order="level"):
    if not clade.is_terminal():
        children = [c.name or "internal" for c in clade.clades]
        print(f"  Internal node → {children}")

from Bio import Phylo
import matplotlib.pyplot as plt
import io

newick = "((Homo_sapiens:0.01, Pan_troglodytes:0.012):0.08, (Mus_musculus:0.15, Rattus_norvegicus:0.14):0.12, Drosophila_melanogaster:0.85);"
tree = Phylo.read(io.StringIO(newick), "newick")
tree.root_with_outgroup("Drosophila_melanogaster")
tree.ladderize()   # sort clades by size for a clean layout

# Annotate bootstrap support (if present in node labels)
for clade in tree.find_clades():
    if clade.confidence is not None:
        clade.name = f"{clade.confidence:.0f}"

fig, ax = plt.subplots(figsize=(8, 5))
Phylo.draw(tree, axes=ax, do_show=False, show_confidence=True)
ax.set_title("Primate + Outgroup Phylogeny")
plt.tight_layout()
plt.savefig("phylogeny.png", dpi=150, bbox_inches="tight")
print("Saved phylogeny.png")

Module 6: PairwiseAligner — Pairwise Sequence Alignment

Bio.Align.PairwiseAligner replaces the legacy pairwise2 module (deprecated since Biopython 1.80). It supports local and global alignment, gap open/extend penalties, and any substitution matrix from Bio.Align.substitution_matrices.

from Bio.Align import PairwiseAligner, substitution_matrices

# Global protein alignment with BLOSUM62
aligner = PairwiseAligner()
aligner.mode = "global"
aligner.substitution_matrix = substitution_matrices.load("BLOSUM62")
aligner.open_gap_score = -11
aligner.extend_gap_score = -1

seq1 = "MTEYKLVVVGAGGVGKSALTIQLIQNHFVDEYDPTIEDSY"
seq2 = "MTEYKLVVVGAVGVGKSALTIQLIQNHFVDEYDPTIEDSY"  # G12V variant

alignments = aligner.align(seq1, seq2)
best = alignments[0]
print(f"Score: {best.score:.1f}")
print(f"Number of alignments: {aligner.score(seq1, seq2):.0f} (score only, faster)")
print(best)   # formatted alignment string

identity = sum(a == b for a, b in zip(*best) if a != "-") / best.shape[1]
print(f"Identity: {identity*100:.1f}%")

from Bio.Align import PairwiseAligner, substitution_matrices

# Local DNA alignment
aligner = PairwiseAligner()
aligner.mode = "local"
aligner.match_score = 2
aligner.mismatch_score = -1
aligner.open_gap_score = -5
aligner.extend_gap_score = -0.5

query = "ACGTACGTACGT"
subject = "TTTTACGTACGTACGTTTTT"
alignments = list(aligner.align(query, subject))
print(f"Local alignments found: {len(alignments)}")
best = alignments[0]
print(f"Score: {best.score}")
print(f"Aligned region in subject: [{best.coordinates[1][0]}:{best.coordinates[1][-1]}]")
print(best)

# Batch pairwise identity matrix
seqs = ["ACGTACGT", "ACGTATGT", "TTGTACGT", "ACGTACGG"]
n = len(seqs)
aligner2 = PairwiseAligner(mode="global", match_score=1, mismatch_score=-1)
print("\nPairwise identity matrix:")
for i in range(n):
    row = []
    for j in range(n):
        score = aligner2.score(seqs[i], seqs[j])
        max_len = max(len(seqs[i]), len(seqs[j]))
        row.append(f"{score/max_len:.2f}")
    print("  " + "  ".join(row))

Key Concepts

SeqRecord and Feature Coordinates

SeqRecord stores sequence metadata and a list of SeqFeature objects. Features use zero-based, half-open FeatureLocation(start, end, strand) — the same convention as Python slicing. Use feature.location.extract(record.seq) to obtain the strand-correct subsequence. Compound locations (e.g., spliced exons) use CompoundLocation.

from Bio import SeqIO

# Extract all CDS protein translations from a GenBank record
for rec in SeqIO.parse("gene.gb", "genbank"):
    for feat in rec.features:
        if feat.type == "CDS":
            gene = feat.qualifiers.get("gene", ["?"])[0]
            product = feat.qualifiers.get("product", ["unknown"])[0]
            cds_nt = feat.location.extract(rec.seq)
            protein = cds_nt.translate(to_stop=True)
            print(f"{gene} ({product}): {len(protein)} aa — {protein[:10]}...")

Phylo Clade Objects

A Clade is both a node and the subtree rooted at that node. Terminal clades (leaves) have .name set; internal clades may have .confidence (bootstrap) and .branch_length. Use tree.find_clades() with terminal=True/False to filter. The root is tree.root (also a Clade). Phylo.read() returns a Tree wrapper; tree.root gives the root Clade.

Common Workflows

Workflow 1: Gene Family Phylogeny — NCBI Download → MUSCLE Alignment → Tree

Goal: Download all BRCA1 orthologs from Vertebrata, align with MUSCLE, and build a neighbor-joining tree.

import subprocess
import time
from Bio import Entrez, SeqIO, Phylo, AlignIO
from Bio.Align import MultipleSeqAlignment
import matplotlib.pyplot as plt

Entrez.email = "[email protected]"

# Step 1: Search NCBI Protein for BRCA1 orthologs
handle = Entrez.esearch(
    db="protein",
    term="BRCA1[Gene] AND Vertebrata[Organism] AND RefSeq[Filter]",
    retmax=20
)
results = Entrez.read(handle); handle.close()
ids = results["IdList"]
print(f"Found {results['Count']} sequences, using {len(ids)}")

# Step 2: Fetch sequences in FASTA format
handle = Entrez.efetch(db="protein", id=",".join(ids), rettype="fasta", retmode="text")
with open("brca1_orthologs.fasta", "w") as fh:
    fh.write(handle.read())
handle.close()
print(f"Saved {len(ids)} sequences to brca1_orthologs.fasta")
time.sleep(1)

# Step 3: Align with MUSCLE (must be installed: conda install -c bioconda muscle)
subprocess.run(
    ["muscle", "-align", "brca1_orthologs.fasta", "-output", "brca1_aligned.fasta"],
    check=True
)
alignment = AlignIO.read("brca1_aligned.fasta", "fasta")
print(f"Alignment: {len(alignment)} sequences × {alignment.get_alignment_length()} columns")

# Step 4: Build a neighbor-joining tree using Bio.Phylo + distance matrix
from Bio.Phylo.TreeConstruction import DistanceCalculator, DistanceTreeConstructor

calculator = DistanceCalculator("identity")
dm = calculator.get_distance(alignment)
constructor = DistanceTreeConstructor(calculator, method="nj")
tree = constructor.build_tree(alignment)

# Step 5: Root and save
tree.root_at_midpoint()
tree.ladderize()
Phylo.write(tree, "brca1_nj.nwk", "newick")
print("Saved NJ tree to brca1_nj.nwk")

# Step 6: Visualize
fig, ax = plt.subplots(figsize=(10, 8))
Phylo.draw(tree, axes=ax, do_show=False)
ax.set_title("BRCA1 Ortholog NJ Tree")
plt.tight_layout()
plt.savefig("brca1_tree.png", dpi=150, bbox_inches="tight")
print("Saved brca1_tree.png")

Workflow 2: BLAST Pipeline — FASTA Query → Remote BLAST → Fetch Top Hits

Goal: Take a query FASTA, BLAST against NCBI nr, filter significant hits, retrieve full GenBank records for the top matches, and write a summary CSV.

import csv
import time
from Bio import SeqIO, Entrez
from Bio.Blast import NCBIWWW, NCBIXML

Entrez.email = "[email protected]"

# Step 1: Load query sequence
query_record = SeqIO.read("query.fasta", "fasta")
print(f"Query: {query_record.id} ({len(query_record.seq)} aa)")

# Step 2: Remote BLAST against nr protein database
print("Submitting BLAST job (may take 1-5 minutes)...")
result_handle = NCBIWWW.qblast(
    "blastp", "nr", str(query_record.seq),
    hitlist_size=20,
    expect=1e-5,
    word_size=6,
    matrix_name="BLOSUM62"
)

# Step 3: Parse results and filter
E_THRESH = 1e-5
MIN_IDENTITY = 50.0
top_hits = []

blast_record = NCBIXML.read(result_handle)
for alignment in blast_record.alignments:
    for hsp in alignment.hsps:
        if hsp.expect > E_THRESH:
            continue
        identity_pct = hsp.identities / hsp.align_length * 100
        if identity_pct < MIN_IDENTITY:
            continue
        accession = alignment.accession
        top_hits.append({
            "accession": accession,
            "title": alignment.title[:80],
            "length": alignment.length,
            "score": hsp.score,
            "evalue": hsp.expect,
            "identity_pct": round(identity_pct, 1),
            "coverage": round(hsp.align_length / blast_record.query_length * 100, 1),
        })

print(f"Filtered to {len(top_hits)} significant hits")

# Step 4: Fetch full GenBank records for top 5 hits
accessions = [h["accession"] for h in top_hits[:5]]
time.sleep(1)
handle = Entrez.efetch(db="protein", id=",".join(accessions), rettype="gb", retmode="text")
gb_records = list(SeqIO.parse(handle, "genbank"))
handle.close()
SeqIO.write(gb_records, "top_blast_hits.gb", "genbank")
print(f"Saved {len(gb_records)} full GenBank records to top_blast_hits.gb")

# Step 5: Write CSV summary
with open("blast_summary.csv", "w", newline="") as fh:
    writer = csv.DictWriter(fh, fieldnames=top_hits[0].keys())
    writer.writeheader()
    writer.writerows(top_hits)
print(f"Saved blast_summary.csv ({len(top_hits)} hits)")

Workflow 3: FASTQ Quality Filter and Format Pipeline

Goal: Read a FASTQ file, filter reads by mean quality and length, and output a FASTA for downstream alignment.

from Bio import SeqIO
from Bio.SeqUtils import gc_fraction

input_fastq = "raw_reads.fastq"
output_fasta = "filtered_reads.fasta"
MIN_QUAL = 20     # mean Phred quality
MIN_LEN = 100     # minimum read length
MAX_LEN = 300     # maximum read length

def passes_qc(record, min_q=MIN_QUAL, min_l=MIN_LEN, max_l=MAX_LEN):
    quals = record.letter_annotations["phred_quality"]
    mean_q = sum(quals) / len(quals)
    length = len(record.seq)
    return mean_q >= min_q and min_l <= length <= max_l

kept = 0
total = 0
with open(output_fasta, "w") as out_fh:
    for record in SeqIO.parse(input_fastq, "fastq"):
        total += 1
        if passes_qc(record):
            # Convert FASTQ → FASTA (drops quality scores)
            SeqIO.write(record, out_fh, "fasta")
            kept += 1

print(f"Total reads: {total:,}")
print(f"Passed QC: {kept:,} ({kept/total*100:.1f}%)")
print(f"Saved to: {output_fasta}")

Key Parameters

Best Practices

1. Always set `Entrez.email` and respect rate limits. NCBI blocks IPs that exceed 3 req/s without a key. Add time.sleep(0.4) between requests in loops, or use Entrez.api_key for 10 req/s.

   from Bio import Entrez
   import time
   Entrez.email = "[email protected]"
   Entrez.api_key = "YOUR_API_KEY"    # from https://www.ncbi.nlm.nih.gov/account/
   # In any batch loop: time.sleep(0.11)  # ~9 req/s with key

1. Use `SeqIO.index()` for large FASTA files instead of list(SeqIO.parse(...)). Loading all records into a list consumes O(N) memory; index() reads only the byte offsets and fetches records on demand.

   # Bad for large files: loads everything into RAM
   # records = {r.id: r for r in SeqIO.parse("genome.fasta", "fasta")}

   # Good: on-disk index, O(1) access
   from Bio import SeqIO
   idx = SeqIO.index("genome.fasta", "fasta")
   rec = idx["chr22"]          # fetches only this record

1. Use `PairwiseAligner` not the legacy `pairwise2`. Bio.pairwise2 is deprecated since Biopython 1.80 and will be removed. PairwiseAligner is faster, supports substitution matrices directly, and returns Alignment objects with coordinate arrays.

1. Close Entrez handles immediately after reading. Handles are HTTP connections; leaving them open risks timeouts and resource exhaustion.

   handle = Entrez.efetch(db="nucleotide", id="NM_007294", rettype="gb", retmode="text")
   record = SeqIO.read(handle, "genbank")
   handle.close()    # do not skip this

1. Prefer local BLAST for batch searches. Remote NCBIWWW.qblast() is suitable for ad hoc queries but can queue for minutes on NCBI servers. For screening >100 sequences, build a local BLAST+ database with makeblastdb and call blastp/blastn via subprocess with -outfmt 5 (XML) for NCBIXML parsing.

1. Root trees before measuring distances. Phylo.distance() measures the sum of branch lengths along the path between two nodes. On an unrooted tree, the path is still unique, but midpoint-rooting or outgroup-rooting makes biological sense for visualizations and clade assertions.

1. Strip alignment gaps before building SeqRecord collections. BLAST and alignment results may include gap characters (-). Seq operations like .translate() will raise errors on gapped sequences; strip with seq.replace("-", "") or use ungap().

Common Recipes

Recipe: Batch Entrez Fetch with History Server

When to use: Downloading more than 500 records from NCBI — avoids URL length limits and keeps search results server-side.

from Bio import Entrez, SeqIO
import time

Entrez.email = "[email protected]"

# Search and store results on NCBI history server
handle = Entrez.esearch(db="nucleotide", term="16S rRNA[Gene] AND Bacteria[Organism]",
                        retmax=500, usehistory="y")
results = Entrez.read(handle); handle.close()
webenv = results["WebEnv"]
query_key = results["QueryKey"]
count = int(results["Count"])
print(f"Total: {count} sequences")

# Fetch in batches of 200
batch_size = 200
all_records = []
for start in range(0, min(count, 1000), batch_size):
    handle = Entrez.efetch(
        db="nucleotide", rettype="fasta", retmode="text",
        retstart=start, retmax=batch_size,
        webenv=webenv, query_key=query_key
    )
    batch = list(SeqIO.parse(handle, "fasta"))
    handle.close()
    all_records.extend(batch)
    print(f"  Fetched {len(all_records)}/{min(count, 1000)}")
    time.sleep(0.5)

SeqIO.write(all_records, "16S_sequences.fasta", "fasta")
print(f"Saved {len(all_records)} sequences")

Recipe: Parse GFF3 with Sequence Features

When to use: Extract gene/CDS sequences from a GFF3 annotation paired with a reference FASTA.

from Bio import SeqIO

# Load genome FASTA into indexed dict
genome = SeqIO.to_dict(SeqIO.parse("genome.fasta", "fasta"))

# Parse GFF3 manually (Biopython does not have a native GFF3 parser;
# use the gffutils package for complex queries, or parse directly for simple cases)
genes = []
with open("annotation.gff3") as fh:
    for line in fh:
        if line.startswith("#") or not line.strip():
            continue
        fields = line.strip().split("\t")
        if len(fields) < 9 or fields[2] != "CDS":
            continue
        chrom, _, feat_type, start, end, _, strand, _, attrs = fields
        start, end = int(start) - 1, int(end)   # GFF3 is 1-based
        if chrom not in genome:
            continue
        seq = genome[chrom].seq[start:end]
        if strand == "-":
            seq = seq.reverse_complement()
        attr_dict = dict(kv.split("=") for kv in attrs.strip().split(";") if "=" in kv)
        gene_id = attr_dict.get("gene_id", attr_dict.get("ID", "unknown"))
        genes.append((gene_id, seq, strand))

print(f"Extracted {len(genes)} CDS features")
for gid, seq, strand in genes[:3]:
    print(f"  {gid} ({strand}): {len(seq)} bp — protein: {seq.translate(to_stop=True)[:8]}...")

Recipe: Compute a Pairwise Identity Matrix from a Multiple Alignment

When to use: Quickly assess sequence diversity within an alignment file (FASTA, PHYLIP, Clustal) and identify outlier sequences.

from Bio import AlignIO
import numpy as np

alignment = AlignIO.read("aligned_sequences.fasta", "fasta")
n = len(alignment)
names = [rec.id for rec in alignment]
length = alignment.get_alignment_length()

# Build identity matrix
identity_matrix = np.zeros((n, n))
for i in range(n):
    for j in range(n):
        if i == j:
            identity_matrix[i, j] = 1.0
            continue
        matches = sum(
            a == b and a != "-"
            for a, b in zip(str(alignment[i].seq), str(alignment[j].seq))
        )
        aligned_cols = sum(a != "-" and b != "-"
                           for a, b in zip(str(alignment[i].seq), str(alignment[j].seq)))
        identity_matrix[i, j] = matches / aligned_cols if aligned_cols else 0.0

print("Pairwise identity matrix:")
print(f"{'':20s} " + "  ".join(f"{n[:8]:>8s}" for n in names))
for i, name in enumerate(names):
    row = "  ".join(f"{identity_matrix[i,j]*100:8.1f}" for j in range(n))
    print(f"{name[:20]:20s} {row}")

# Find most divergent pair
min_id = np.min(identity_matrix[identity_matrix > 0])
idx = np.unravel_index(np.argmin(np.where(identity_matrix > 0, identity_matrix, 1)), identity_matrix.shape)
print(f"\nMost divergent pair: {names[idx[0]]} vs {names[idx[1]]} ({min_id*100:.1f}%)")

Troubleshooting

Related Skills

• biopython-molecular-biology — restriction digestion, PCR primer design, protein structure analysis (Bio.PDB), molecular weight and Tm calculations; complement to this skill
• pysam-genomic-files — SAM/BAM/CRAM alignment file access, pileup, and region queries; use when working with read alignments
• scikit-bio — ecological diversity statistics (UniFrac, Bray-Curtis), ordination (PCoA), and microbiome analysis; more specialized than Biopython's phylogenetics for diversity analyses
• gget-genomic-databases — one-liner gene lookups, BLAST, and database queries without setting up an Entrez pipeline
• etetoolkit — advanced phylogenetic tree visualization, annotation with NCBI taxonomy, and publication-quality tree rendering

References

• Biopython Tutorial and Cookbook — comprehensive official tutorial (Chapters 2, 5, 6, 7, 9, 11)
• Biopython API Reference — complete module and class documentation
• GitHub: biopython/biopython — source, changelog, and issue tracker
• NCBI E-utilities Documentation — Entrez API reference for esearch, efetch, elink, esummary
• PairwiseAligner Migration Guide — replacing the deprecated Bio.pairwise2 module