Reverse Complement Tool
Paste a DNA or RNA sequence and get the reverse complement, complement, or reverse strand instantly. Handles FASTA input, IUPAC ambiguity codes, mixed case, and RNA output, with one-click copy and FASTA download.
Transformation
Output alphabet
The complementary strand written 5' to 3' (the most common need).
Reverse complement (DNA)
63 basesTTAAAGCTTG CATGCCTGCA GGTCGACTCT AGAGGATCCC CGGGTACCGA GCTCGAATTC CAT
All three transformations
TTAAAGCTTG CATGCCTGCA GGTCGACTCT AGAGGATCCC CGGGTACCGA GCTCGAATTC CAT
TACCTTAAGC TCGAGCCATG GGCCCCTAGG AGATCTCAGC TGGACGTCCG TACGTTCGAA ATT
AATTTCGAAC GTACGGACGT CCAGCTGAGA TCTCCTAGGG GCCCATGGCT CGAGCTTAAG GTA
Double-stranded view
Top strand is your input (5' to 3'); the bottom strand is its complement read 3' to 5', the way the two strands pair in the double helix.
Restriction sites (10 enzymes cut)
| Enzyme | Recognition site | Cuts | Cut after base |
|---|---|---|---|
| EcoRI | G▾AATTC | 1 | 4 |
| SacI | GAGCT▾C | 1 | 14 |
| KpnI | GGTAC▾C | 1 | 20 |
| SmaI | CCC▾GGG | 1 | 22 |
| BamHI | G▾GATCC | 1 | 25 |
| XbaI | T▾CTAGA | 1 | 31 |
| SalI | G▾TCGAC | 1 | 37 |
| PstI | CTGCA▾G | 1 | 47 |
| SphI | GCATG▾C | 1 | 53 |
| HindIII | A▾AGCTT | 1 | 55 |
Open reading frames (1 found, six frames)
| Frame | Start | End | Length | Protein |
|---|---|---|---|---|
| +1 | 1 | 63 | 20 aa | MEFELGTRGSSRVDLQACKL |
Where the Reverse Complement Shows Up in Molecular Biology
The reverse complement is one of the most frequently repeated operations in a wet-lab and a bioinformatics workflow, because DNA is double-stranded and antiparallel. Watson and Crick (1953) described the two strands running in opposite directions, which is exactly why reconstructing the opposite strand requires both a base swap and a reversal. Every time you read a feature annotated on the minus strand of a genome, the sequence you see in a browser is the reverse complement of the plus-strand coordinates.
In primer and probe design the reverse primer is the reverse complement of the 3' end of the target region, so the two primers face each other and amplify the amplicon between them. A single mistake, complementing without reversing, produces a primer that will never anneal correctly. Pairing this tool with the primer melting temperature calculator lets you design a reverse primer and check its melting temperature in the same session.
In cloning and synthetic biology, inserting a fragment in the correct orientation, designing complementary overhangs for Gibson assembly, or building an antisense oligonucleotide all depend on an accurate reverse complement. In sequencing analysis, reads that map to the reverse strand are stored as their reverse complement, and tools such as SAMtools flag them accordingly. The antisense (template) strand that RNA polymerase reads is simply the reverse complement of the sense strand.
Beyond the transformation itself, sequence composition drives downstream behaviour. The tool reports the GC content and base counts of your input, which predict duplex stability and inform whether a region is prone to secondary structure. For a fuller composition breakdown, including GC skew and per-window analysis, use the GC content calculator, and to read a coding sequence in protein space, the DNA to protein translation tool handles all six reading frames.
A Worked Example: Designing a Reverse Primer
Suppose the 3' end of your amplicon on the forward (sense) strand is 5'-GGATCCTACGATGCAAGTC-3'. The reverse primer has to bind the opposite strand, so it is the reverse complement of this region. The two steps are shown below, base by base: first complement each base in place, then read the complemented string from right to left so the primer is written 5' to 3'. The complement alone, CCTAGGATGCTACGTTCAG, is the opposite strand written 3' to 5' and would order the wrong bases at the primer's 5' end; only after reversing do you get the usable reverse primer GACTTGCATCGTAGGATCC.
| Step | Sequence (read left to right) | Orientation |
|---|---|---|
| Input (sense strand) | GGATCCTACGATGCAAGTC | 5' to 3' |
| 1. Complement each base | CCTAGGATGCTACGTTCAG | 3' to 5' |
| 2. Reverse the complement | GACTTGCATCGTAGGATCC | 5' to 3' |
The final row is the reverse primer. Its base composition is 10 of 19 GC (roughly 53% GC content), which sits in the healthy 40 to 60% range for a stable primer. Drop the reverse complement into the primer melting temperature calculator to confirm its melting temperature matches the forward primer before ordering.
Common Mistakes When Taking a Reverse Complement
- Complementing without reversing. Swapping the bases but leaving them in the original order gives the opposite strand written 3' to 5'. A primer built from it will not anneal in the expected orientation. Always reverse after complementing.
- Reversing without complementing. Flipping the sequence order alone just reads the same strand backwards; it is not the opposite strand at all. Both operations are required, and the tool applies them together by default.
- Forgetting that RNA uses uracil. When the output should be RNA, thymine must become uracil. Leaving thymines in an RNA result, or complementing A to T instead of A to U, produces an invalid RNA sequence. Switch the output alphabet to RNA rather than editing bases by hand.
- Ignoring the annotated strand. A coding sequence on the minus strand of a genome must be reverse-complemented before it reads as a valid open reading frame. Reading the plus-strand coordinates directly yields a frameshifted or nonsense translation.
- Mishandling IUPAC ambiguity codes. Codes such as R, Y, S, and W have specific complements (R to Y, S and W self-complementary). Treating every non-ACGT character as N discards real information; this tool complements each ambiguity code correctly.
How to Use This Tool
Paste your sequence
DNA or RNA. FASTA headers, spaces, numbers, and line breaks are ignored, so paste straight from a sequence file.
Choose the transformation
Reverse complement (default), complement only, or reverse only. Each shows a different view of the opposite strand.
Pick DNA or RNA output
Keep DNA output, or switch to RNA so thymines in the result are written as uracils.
Copy or download
Copy the result or download a FASTA file ready for primer design, cloning, or the next analysis step.
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Base Pairing Reference
A ⇄ T
Adenine pairs with thymine (two hydrogen bonds)
G ⇄ C
Guanine pairs with cytosine (three hydrogen bonds)
A ⇄ U
In RNA, adenine pairs with uracil
R ⇄ Y
Purine (A/G) complements pyrimidine (C/T)
S ⇄ S
Strong (G/C) is self-complementary
W ⇄ W
Weak (A/T) is self-complementary
K ⇄ M
Keto (G/T) complements amino (A/C)
N ⇄ N
Any base stays any base
Frequently Asked Questions
What is the reverse complement?
The reverse complement of a nucleotide sequence is the sequence of its complementary strand read in the 5' to 3' direction. Because the two strands of DNA are antiparallel, the complementary strand runs in the opposite direction, so you both complement each base (A with T, G with C) and reverse the order. For example, the reverse complement of 5'-ATGC-3' is 5'-GCAT-3'. This is the sequence you would actually read on the opposite strand.
Is reverse complement 5 to 3?
Yes. By convention the reverse complement is reported 5' to 3', the same direction in which sequences are written and oligonucleotides are synthesized. The complementary strand physically runs antiparallel to the input, so its 5' end sits opposite the input's 3' end; reversing the complemented bases re-expresses that strand in the standard 5' to 3' reading. If you complement the bases but forget to reverse, you get the opposite strand written 3' to 5', which is almost never what you want.
Is antisense the same as reverse complement?
For a given coding (sense) strand, its reverse complement is the antisense (template) strand written 5' to 3', so in practice they describe the same physical strand. The antisense strand is the template that RNA polymerase reads, and computing the reverse complement of the sense strand reconstructs it. The distinction matters when you design antisense oligonucleotides or interpret which strand a gene is annotated on, because the annotation direction tells you whether to reverse-complement before reading a coding sequence.
Is reverse complement always necessary?
Not always. You need the reverse complement only when you are working with the opposite strand: designing a reverse PCR primer, reading a feature annotated on the minus strand, building a complementary overhang for cloning, or interpreting a read that mapped to the reverse strand. When you stay on the same strand in the same 5' to 3' orientation, no transformation is needed. The complement-only and reverse-only modes in this tool exist because occasionally you want just one of the two steps rather than both.
How to get reverse complement?
Take each base and swap it for its Watson-Crick partner (A becomes T, T becomes A, G becomes C, C becomes G), then reverse the entire string so it reads 5' to 3'. Doing the two steps in the correct order matters: complementing without reversing gives the wrong strand orientation. Paste your sequence into the box above and the tool performs both steps at once, and also shows the complement-only and reverse-only results so you can see each transformation.
What is the difference between complement and reverse complement?
The complement swaps each base for its pair but keeps the same left-to-right order, which represents the opposite strand read 3' to 5'. The reverse complement additionally flips the order so the opposite strand reads 5' to 3', the standard convention for reporting a sequence. In almost every molecular biology application, such as primer design or cloning, the reverse complement is the one you want because sequences are written and synthesized 5' to 3'.
What is the reverse complement of an RNA sequence?
For RNA you complement using A with U (instead of T) and G with C, then reverse. This tool has an RNA output mode that substitutes uracil for thymine in the result. When you provide a DNA template and want the messenger RNA-equivalent reverse complement, switch the output alphabet to RNA and the thymines in the result become uracils.
How do you reverse complement a sequence in Python?
In Biopython, Seq('ATGC').reverse_complement() returns the result directly. Without Biopython, you can build a translation table with str.maketrans('ACGT', 'TGCA'), apply it, then reverse the string with slicing ([::-1]). This browser tool gives the same answer without any setup and handles IUPAC ambiguity codes, mixed case, FASTA headers, and RNA output, which a one-line script often does not.
Does the reverse complement tool handle ambiguous IUPAC codes?
Yes. IUPAC ambiguity codes are complemented to their correct base-set partners: R (A or G) becomes Y (C or T), S and W are self-complementary, K becomes M, B becomes V, D becomes H, and N stays N. Case is preserved, so lowercase soft-masked bases remain lowercase in the output. Whitespace, numbers, and FASTA header lines are ignored automatically.
Related Sequence Tools
To break down base composition, GC skew, and melting behaviour, use the GC content calculator. To read a coding sequence across all six reading frames, the DNA to protein translation tool returns the protein for each frame. When designing amplification primers, the primer melting temperature calculator computes nearest-neighbor melting temperatures. For the full analysis of biological data, the bioinformatics analysis service covers differential expression, survival modelling, and publication-ready figures.
Reviewed by
Dr. Sarah Mitchell
Dr. Sarah Mitchell holds a PhD in Biostatistics from Johns Hopkins Bloomberg School of Public Health and has over 15 years of experience in systematic review methodology and meta-analysis. She has authored or co-authored 40+ peer-reviewed publications in journals including the Journal of Clinical Epidemiology, BMC Medical Research Methodology, and Research Synthesis Methods. A former Cochrane Review Group statistician and current editorial board member of Systematic Reviews, Dr. Mitchell has supervised 200+ evidence synthesis projects across clinical medicine, public health, and social sciences. She reviews all Research Gold tools to ensure statistical accuracy and compliance with Cochrane Handbook and PRISMA 2020 standards.
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