What this calculator does
Transcription copies a DNA template strand into messenger RNA by complementary base pairing, with one substitution: RNA uses uracil where DNA uses thymine. So A becomes U, T becomes A, C becomes G and G becomes C.
The direction matters and is the usual source of confusion. This transcribes the template strand, also called the antisense strand, which is what RNA polymerase actually reads. The mRNA produced matches the coding strand except for the U in place of T. Entering the coding strand by mistake gives the complement of what you wanted.
The formula
Each base in the entered sequence is replaced by its RNA complement: A gives U, T gives A, C gives G and G gives C. The GC content of the template is reported alongside, since it affects the stability of the DNA duplex and is a standard thing to know about a sequence.
| Term | Meaning |
|---|---|
| Template strand | The DNA strand read by RNA polymerase. Also called the antisense or non-coding strand. |
| Coding strand | The other strand, which matches the mRNA sequence except that it has T where mRNA has U. |
| Uracil (U) | The RNA base that replaces thymine. It pairs with adenine exactly as thymine does. |
| GC content | The percentage of bases that are G or C, which raises duplex stability because those pairs share three hydrogen bonds. |
The inputs explained
| Field | What to enter |
|---|---|
| DNA template sequence | The DNA template strand, as letters A, T, C and G. Enter the template rather than the coding strand, or the result will be the complement of what you intended. |
When to use it
Working through a transcription exercise
The standard textbook task, where a template strand is given and the mRNA asked for. Checking by hand and then against the calculator catches the common slip of forgetting the U substitution.
Finding the start codon
A template beginning TAC transcribes to AUG, the start codon, which is the usual first thing to look for when reading a sequence.
Checking GC content
GC content affects melting temperature and duplex stability, and is worth knowing before designing primers against a region.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What mRNA does each DNA template give?
Four short template sequences and their transcripts.
| DNA template | mRNA sequence | Sequence length | GC content of template |
|---|---|---|---|
| TACGGCTA | AUGCCGAU | 8 bases | 50.0% |
| ATGCATGC | UACGUACG | 8 bases | 50.0% |
| AAATTT | UUUAAA | 6 bases | 0.000% |
| GCGCGCGC | CGCGCGCG | 8 bases | 100.0% |
Questions
What is the difference between the template and coding strands?
The template strand is the one RNA polymerase reads, and the mRNA is its complement. The coding strand is the other one, and the mRNA matches it letter for letter except that U replaces T. Both names describe the same double helix from opposite sides.
Why does RNA use uracil instead of thymine?
Thymine is uracil with a methyl group added, and making it costs energy. RNA is short-lived so the extra stability is not worth paying for, while DNA keeps thymine because the methyl group helps repair machinery distinguish a genuine thymine from a cytosine that has degraded into uracil.
Does this account for introns and splicing?
No. This is straight base-by-base transcription, which gives the primary transcript. In eukaryotes that transcript is then spliced to remove introns before it becomes mature mRNA, and this calculator does not model that step.
Which direction is the sequence read?
Conventionally sequences are written 5 prime to 3 prime. RNA polymerase reads the template 3 prime to 5 prime and builds the RNA 5 prime to 3 prime. This calculator does a straight positional substitution, so the result reads in the same direction as the sequence you entered.
For the probability side of genetics, see the multi-gene cross calculator. For primer design against a sequence, see the primer melting temperature calculator.