DNA Complement Strand Tool

One input box takes a DNA sequence — the loaded example is ATGCGTACGTAC — and the script walks it letter by letter through a four-entry table: A becomes T, T becomes A, G becomes C and C becomes G. The example returns TACGCATGCATG, refreshed on every keystroke. Input is uppercased first, so lowercase sequence works untouched, which matters because reference genomes soft-mask repetitive regions in lowercase. Base pairing is all the page does: no GC content, no melting temperature, no alignment, no restriction site search.

Orientation is the catch. Bases come out in the same left-to-right positions they went in, but the two DNA strands are antiparallel. If your input is written 5' to 3', as convention demands, the output read left to right runs 3' to 5'. It is the correct partner base for base, stacked under the input the way a double helix is drawn, and that is exactly what you want when checking pairing, an annealing site or a point mutation on both strands. It is not what you order as an oligo. To write the partner strand in the conventional 5' to 3' form, reverse the output: TACGCATGCATG read backwards is GTACGTACGCAT, which is precisely what the reverse complement tool returns in one step.

Any character outside the four bases is replaced by an empty string, silently. ATGNCGT returns TACGCA — six letters from a seven-character input, because the N dropped out and the neighbours closed the gap. IUPAC ambiguity codes (R, Y, S, W, K, M), alignment gap dashes and position numbers all disappear the same way. U is not in the table either, so pasting RNA deletes every uracil: AUGC comes back as TCG. An input with no valid base at all leaves the result box blank, with no error message. One habit avoids the whole trap: compare the length of what you pasted with the length of what came out, and treat any difference as a warning.