This simulation animates transcription, the first step of gene expression in which RNA polymerase copies a DNA template strand into a complementary messenger RNA. A random double-stranded sequence is generated, and the enzyme moves along it base by base, opening a transcription bubble. mRNA is built from the template using the pairing rule A→U, T→A, G→C and C→G, with the growing transcript displayed in colour.
The elongation speed slider sets how fast RNA Pol II advances, while the template length slider chooses a sequence of 18 to 72 nucleotides in steps of six. Start, Pause and Reset control the run, and live stats track position, bases transcribed, codons and the current phase (Initiation, Elongation, Termination). Understanding transcription underpins genetics, mRNA vaccines and gene-regulation research.
What is DNA transcription?
Transcription is the process by which the enzyme RNA polymerase reads one strand of DNA, called the template strand, and synthesises a complementary single-stranded RNA copy. It is the first stage of gene expression, producing the messenger RNA that is later translated into protein.
How does this simulation build the mRNA?
A random template strand is generated, and as the polymerase advances each template base is converted to its RNA partner using A to U, T to A, G to C and C to G. The growing transcript appears in the mRNA box, grouped into colour-coded three-base codons.
Why is uracil used instead of thymine?
RNA uses uracil (U) in place of the thymine (T) found in DNA. So wherever the DNA template has an adenine, the RNA copy receives a uracil rather than a thymine. The simulation shows this directly: a template A produces a U in the mRNA strand.
The elongation speed slider scales how quickly RNA polymerase moves along the DNA, from 0.2 to 4.0 times the base rate. The template length slider sets the number of nucleotides transcribed, between 18 and 72 in steps of six; changing it regenerates a new random sequence.
The transcription bubble is the short stretch of DNA that RNA polymerase locally unwinds so it can read the template. In the animation the two strands separate around the enzyme, and base-pair connectors disappear within roughly six bases either side of the polymerase, closing again once it has passed.
The template (or antisense) strand is the one actually read by the polymerase, shown along the bottom. The coding (or sense) strand, shown on top, has the same sequence as the mRNA except that T replaces U. The mRNA is therefore complementary to the template and matches the coding strand.
Codons are consecutive groups of three RNA bases. During translation each codon specifies one amino acid (or a stop signal). The simulation divides the mRNA into triplets with shaded backgrounds and numbered dividers to highlight this reading frame, even though translation itself is not modelled here.
The base-pairing chemistry, the 5' to 3' direction of synthesis, the template-reading mechanism and the opening of a transcription bubble are all faithfully represented. It is a simplified teaching model, however: it omits promoters, transcription factors, RNA processing, splicing and the helical structure of real DNA.
Real transcription has three stages. Initiation is when the polymerase binds before reading begins; Elongation is the steady synthesis of mRNA as the enzyme moves along the template; and Termination is reached when the polymerase finishes the sequence and the completed transcript is released.
Speed only changes how quickly the animation plays, not which bases are added. The transcript is determined solely by the random template sequence and the fixed pairing rules, so a slow or fast run over the same sequence yields exactly the same mRNA at completion.
Transcription is central to how genes are switched on and how cells respond to their environment. The same template-copying principle underlies mRNA vaccines, gene-therapy design and the laboratory production of RNA, making it a foundational concept across molecular biology and medicine.