DNA / RNAトランスレーター

From DNA to protein in three steps

This tool walks the central dogma of molecular biology in miniature: DNA is copied into messenger RNA (transcription), and that mRNA is read three letters at a time to build a protein (translation). You paste a DNA sequence using the four bases A, T, G, and C, and the translator does three things. It pairs each base with its complement to show the opposite strand; it transcribes the sequence into mRNA; and it reads the mRNA in codons — groups of three bases — looking up which amino acid each codon specifies. The result is the strands, the GC content, and, where a start codon exists, the resulting protein chain. All of this runs entirely in your browser; the sequence you enter is never uploaded.

The complementary strand and a subtlety about the transcript

Base pairing is fixed: A pairs with T, and G pairs with C. So the tool builds the complementary strand by swapping each base for its partner. Transcription, though, has a detail worth getting right. In this tool the mRNA transcript is your input strand with every T replaced by U (uracil, RNA's stand-in for thymine) — that is, it treats your typed sequence as the coding (sense) strand and simply swaps T for U. It does not transcribe the complementary strand. This matches the common textbook convention where you read off the coding strand directly. Keep that in mind: the mRNA shown will look like your input with U's where the T's were, not like the reverse complement.

How the protein is read — the open reading frame

Translation doesn't start at the beginning of the sequence; it starts at the first AUG codon, which codes for methionine and acts as the universal "start." The tool scans the mRNA, finds the first AUG, and from there reads forward in non-overlapping triplets. Each codon is matched to one of 20 amino acids using the standard genetic code, and translation halts at the first stop codon — UAA, UAG, or UGA — which specifies no amino acid and ends the protein. This stretch from start to stop is the open reading frame. If no AUG is present, the tool reports that no start codon was found and produces no protein, because there is nothing to translate.

A worked example

Enter ATGGCCTGA. Transcribed to mRNA it becomes AUGGCCUGA. Reading in triplets from the AUG: the first codon AUG is methionine (Met, the start), the second GCC is alanine (Ala), and the third UGA is a stop. The protein is therefore Met–Ala, two amino acids long, and translation cleanly terminates. The tool color-codes the start codon green and the stop codon red in the codon table, and reports the protein length. GC content here is the fraction of G and C bases: this sequence has 5 of 9 bases as G or C, so GC content is about 55.6%.

Why GC content matters

GC content is simply the percentage of bases that are G or C: (G + C) ÷ total length × 100. It's a small number with real biological weight. G–C pairs are held together by three hydrogen bonds versus two for A–T, so GC-rich DNA is more thermally stable and melts at a higher temperature. That's why GC content is a routine consideration when designing PCR primers and estimating melting temperatures — sequences that are too GC-rich or too GC-poor behave awkwardly in the lab.

Common mistakes and pitfalls

• Expecting the reverse complement as mRNA. As noted, this tool transcribes the coding strand (T→U). If you need the transcript of the other strand, paste that strand instead.
• Frame shifts. The reading frame is set by the first AUG. Adding or removing a single base before it shifts every downstream codon and produces a completely different protein — a real biological phenomenon worth remembering.
• Non-ATGC characters. The tool strips anything that isn't A, T, G, or C (and ignores case), so spaces and line breaks are fine, but ambiguity codes like N won't be interpreted.
• No stop codon. If translation runs off the end without hitting a stop, the tool warns you the protein may be incomplete — your reading frame likely ran past the available sequence.