DNA Reverse Complement Tool

The tool takes one DNA field — the example is ATGCGTACGTAC — and chains two operations: it splits the sequence into characters, reverses their order, then swaps each base for its partner using the A with T, G with C table. Reversing before or after complementing makes no difference to the answer, and the example returns GTACGTACGCAT. Check it by hand: the complement of ATGCGTACGTAC is TACGCATGCATG, and read backwards that is GTACGTACGCAT. The output already comes out 5' to 3', the convention in which primers and oligos are written, quoted and ordered.

This is the everyday PCR operation. The forward primer copies the start of the amplicon on the top strand; the reverse primer is the reverse complement of the end of that same amplicon. If the region finishes with GGTCAGTTCACG on the top strand, your reverse primer is CGTGAACTGACC — paste the first and compare. There is a quick sanity check too: palindromic restriction sites return themselves. GAATTC, the EcoRI site, has GAATTC as its reverse complement, so if input and output differ there, you mistyped something. The same transformation flips a BLAST hit that landed on the minus strand back to plus-strand orientation before you line it up against the reference, or reorients a read that came off the opposite sequencing primer.

Two kinds of limit apply. The first is degenerate code: the table has four keys, so every other letter maps to nothing. A degenerate primer like ACGTRYN comes back as ACGT — four bases instead of seven, with no warning — whereas a proper reverse complement would map R to Y, Y to R and N to N and preserve the length. Ordering an oligo from that output is an expensive mistake. U is missing from the table as well, so RNA returns mangled. The second limit is scope: the page performs the sequence transformation and nothing else. No melting temperature, no GC content, no hairpin or primer-dimer screen. And because the reversal runs over the whole pasted text, a FASTA with a header or several records comes back scrambled, header letters folded in among the bases.