Generate reverse complements and complements of DNA/RNA sequences using Biopython, including IUPAC ambiguity codes, gapped alignments, and minus-strand features. Use when working with the opposite strand, building reverse primers, normalizing strand orientation before alignment, or extracting a coding sequence from a minus-strand feature.
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Generate reverse complements and complements of DNA/RNA sequences using Biopython, including IUPAC ambiguity codes, gapped alignments, and minus-strand features. Use when working with the opposite strand, building reverse primers, normalizing strand orientation before alignment, or extracting a coding sequence from a minus-strand feature.
tool_type
python
primary_tool
Bio.Seq
Version Compatibility
Reference examples tested with: BioPython 1.83+
Before using code patterns, verify installed versions match. If versions differ:
Python: pip show <package> then help(module.function) to check signatures
If code throws ImportError, AttributeError, or TypeError, introspect the installed
package and adapt the example to match the actual API rather than retrying.
Reverse Complement
Generate complementary and reverse complementary sequences using Biopython.
"Get the reverse complement" -> Produce the 5'-to-3' sequence of the opposite strand.
Python: seq.reverse_complement() (BioPython Seq)
CLI: samtools faidx ref.fa region --reverse-complement (extracts and RCs a region)
The Governing Principle
Never hand-roll the complement table. Biopython's reverse_complement() already encodes the full IUPAC mapping correctly, case-insensitively, and on minus-strand features it is applied for the analyst automatically by . Every silent corruption in this domain comes from reimplementing what Biopython already does right: swapping ambiguity codes, forgetting that S/W/N are self-complementary, complementing the wrong molecule type, or reverse-complementing a second time after already did it. Reach for the library method; reach for a guard () before it; never reach for a custom dictionary.
SeqFeature.extract()
extract()
molecule_type
Required Import
from Bio.Seq import Seq
Which Method for Which Question
Question
Method
Output strand/direction
Opposite strand, conventional 5'->3'
reverse_complement()
5'->3' of the complementary strand (the usual answer)
Base-paired sequence, same direction
complement()
3'->5' of the complementary strand
Opposite strand of RNA, keep U
reverse_complement_rna()
5'->3', emits U
Complement of RNA, keep U
complement_rna()
3'->5', emits U
Coding strand from template (or vice versa)
reverse_complement()
the other strand, 5'->3'
mRNA sequence from the coding strand
transcribe() (NOT a complement)
same strand, T->U
reverse_complement()
Returns the reverse complement (5'->3' of the opposite strand). This is the most commonly used operation.
reverse_complement() complements all 15 IUPAC codes plus X correctly. The mapping is non-obvious for ambiguity codes - this is exactly why hand-rolling corrupts silently.
Code
Bases
Complement
Code
Bases
Complement
A
A
T
M
A/C
K
T
T
A
B
C/G/T
V
G
G
C
V
A/C/G
B
C
C
G
D
A/G/T
H
R
A/G
Y
H
A/C/T
D
Y
C/T
R
S
G/C
S (self)
K
G/T
M
W
A/T
W (self)
N
any
N (self)
S, W, N, and X are SELF-complementary. The pairs that get swapped wrong by hand are B<->V and D<->H. The table is built for upper and lower case, so complementation is case-insensitive (Seq('atRY').reverse_complement() works).
DNA vs RNA: the U handling rule
reverse_complement() runs in DNA mode: it treats any U as a T and EMITS T (docstring: "Any U in the sequence is treated as a T"). It does not raise and does not leave U.
Seq('ACGU').reverse_complement() # Returns Seq('ACGT') -- U mapped to A, emitted as T
Seq('ACGU').reverse_complement_rna() # Returns Seq('ACGU') -- stays RNA
transcribe() does NOT complement. It swaps T->U on the SAME strand. Confusing "complement the template" with "transcribe the coding strand" is silent corruption. True biological transcription from the template strand is template_dna.reverse_complement().transcribe().
Gaps and Non-Table Characters
complement and reverse_complement do NOT validate the alphabet (unlike translate()). A gap - is not a table key, so it passes through unchanged and reversal preserves gap columns - the desired behavior for aligned sequences. Any other non-table character (?, *) also passes through silently.
Goal: Produce a new FASTA file with all sequences reverse-complemented.
Approach: Parse records as a stream, build new SeqRecords from .reverse_complement(), write to output.
Reference (BioPython 1.83+):
from Bio import SeqIO
from Bio.SeqRecord import SeqRecord
defreverse_complement_records(records):
for record in records:
yield SeqRecord(record.seq.reverse_complement(), id=record.id + '_rc', description=record.description + ' reverse complement')
records = SeqIO.parse('sequences.fasta', 'fasta')
SeqIO.write(reverse_complement_records(records), 'sequences_rc.fasta', 'fasta')
Extract a Coding Sequence from a Minus-Strand Feature
Goal: Get the correct 5'->3' coding sequence for a gene annotated on the minus strand.
Approach: Call feature.extract(parent.seq). For strand == -1, extract() ALREADY reverse-complements the slice and returns the coding sequence. Do NOT reverse-complement again.
Reference (BioPython 1.83+):
from Bio.Seq import Seq
from Bio.SeqFeature import SeqFeature, SimpleLocation
parent = Seq('AAATGGGCCCTTTAAA')
feature = SeqFeature(SimpleLocation(3, 12, strand=-1), type='CDS')
cds = feature.extract(parent) # Already reverse-complemented; this is the coding sequence# cds.reverse_complement() # WRONG -- double-RC bug, valid-looking but wrong strand
Search Both Strands for a Motif
Goal: Find a motif on both strands and report forward-strand coordinates.
Approach: Search the forward sequence, then search its reverse complement, mapping minus-strand hits back to forward coordinates.
inplace defaults to False (standardized in 1.79). On an immutable Seq, inplace=True raises TypeError: Sequence is immutable (a loud, useful error). In-place mutation works only on MutableSeq.
from Bio.Seq import MutableSeq
m = MutableSeq('ATGC')
m.reverse_complement(inplace=True) # m is now MutableSeq('GCAT')
The Protein Trap
Since the 1.78 alphabet removal there is no molecule-type checking. Reverse-complementing a protein produces SILENT GARBAGE with no warning: residues that are also nucleotide codes get complemented (Seq('MAIVMGR').reverse_complement() -> Seq('YCKBITK'); M->K, V->B), while protein-only letters E, F, I, L, P, Q, Z and * pass through unchanged. The old IUPAC.protein ValueError guard is gone. Guard on the molecule type, not the Seq:
if record.annotations.get('molecule_type') notin ('DNA', 'RNA'):
raise ValueError('reverse_complement is only valid for nucleotide sequences')
Common Errors
Symptom
Cause
Fix
U replaced by T in result
reverse_complement() runs in DNA mode (U treated as T)
Use reverse_complement_rna() to keep RNA
Result is meaningless letters, no error
Reverse-complemented a protein (silent since 1.78)
Guard on molecule_type, not the Seq
Coding sequence is the wrong strand
Called .reverse_complement() after extract() on a minus-strand feature
extract() already RC'd it; do not RC again
TypeError: Sequence is immutable
inplace=True on a Seq
Use a MutableSeq, or take the returned value
Ambiguity codes complement wrongly
Hand-rolled complement table (B/V, D/H swapped; S/W/N not self-complementary)
Use Biopython's reverse_complement(); never reinvent the table
Same strand returned instead of complement
Used transcribe() thinking it complements
transcribe() only swaps T->U; use reverse_complement() for the other strand
TypeError on a plain string
Passed a str instead of a Seq
Wrap input in Seq() first
References
Cornish-Bowden A (1985) "Nomenclature for incompletely specified bases in nucleic acid sequences: recommendations 1984." Nucleic Acids Res 13(9):3021-3030 (PMID 2582368). Defines the IUPAC ambiguity codes (R, Y, S, W, K, M, B, D, H, V, N) that Biopython's complement table implements.
Related Skills
seq-objects - Create and mutate Seq/MutableSeq objects to complement
transcription-translation - transcribe() vs complement(); six-frame translation uses the reverse complement
motif-search - Search both strands by reverse-complementing the query or sequence
sequence-io/read-sequences - Parse FASTA/GenBank records before reverse-complementing
primer-design/primer-basics - Reverse primers are the reverse complement of the target 3' end
restriction-analysis/restriction-sites - Restriction sites are often palindromic (self-complementary)
