Reverse Complement Calculator

Double-stranded nucleic acid is antiparallel: where one strand runs 5-prime to 3-prime left to right, its partner runs 3-prime to 5-prime in the same direction. Since sequences are written 5-prime to 3-prime by universal convention, obtaining the partner strand requires two operations rather than one. Complementing swaps each base for its Watson-Crick pair, A with T (two hydrogen bonds) and G with C (three), or A with U in RNA; reversing then restores the 5-prime to 3-prime reading order. Doing only one of the two produces a string that corresponds to no real molecule, which is a common source of silent error in primer design. IUPAC ambiguity codes complement in pairs, with R and Y exchanging, K and M exchanging, B and V exchanging, D and H exchanging, while S, W and N map to themselves. The operation is required constantly in practice. A reverse PCR primer is the reverse complement of the 3-prime end of the target region on the top strand. The R2 read of an Illumina paired-end library is sequenced from the opposite strand and must be reverse complemented before comparison with R1. Open reading frames on the minus strand are found by scanning the reverse complement of the assembly. The antisense strand of an siRNA duplex and the sequence of a hybridisation probe are both defined this way. A sequence that equals its own reverse complement is a true palindrome, which is why most Type II restriction sites such as GAATTC read identically on both strands. Two conventions deserve care: lowercase letters usually mark repeat-masked or soft-masked regions and should survive the transformation, and RNA input must be complemented to U rather than T.

The requirement for this transformation follows directly from the structure published by James Watson and Francis Crick in Nature in April 1953. Their model depended on the X-ray diffraction images produced by Rosalind Franklin and Raymond Gosling at King's College London, particularly Photograph 51, and on the base composition rules Erwin Chargaff had established around 1950, which showed adenine equimolar with thymine and guanine with cytosine across species. The antiparallel arrangement of the two chains was an explicit feature of the model, and it immediately implied that either strand specifies the other. Matthew Meselson and Franklin Stahl confirmed the copying mechanism experimentally in 1958 with their density-gradient demonstration of semiconservative replication. The reverse complement became a routine computation only once sequences were long enough to require software. Frederick Sanger's dideoxy chain-termination method, published in 1977, produced reads from both strands that had to be brought into a common orientation before assembly, and Rodger Staden's programs at the MRC Laboratory of Molecular Biology in Cambridge, developed from the late 1970s, built the operation into the first widely used sequence handling package. The GCG Wisconsin Package from the University of Wisconsin in the 1980s supplied the same function commercially, and the open-source EMBOSS suite reimplemented it as revseq around 2000. It is now a one-line primitive in nearly every bioinformatics toolkit, from Biopython and BioPerl to seqkit and samtools.