RNA polymerase and its helper proteins bind the promoter, a DNA signal just upstream of the gene. Press Play or step forward to open the helix and begin.
Imagine the cell's DNA is a giant master cookbook that must never leave the library (the nucleus), because it is the only original. When the kitchen needs one recipe, you do not carry out the whole book — you photocopy a single page and take the copy. Transcription is that photocopying: the enzyme RNA polymerase makes an RNA copy of just one gene, leaving the DNA original safe. The copy, messenger RNA (mRNA), then carries the instructions to the kitchen (the ribosome) to be built into a protein.
DNA has two strands, but only one is read: the template strand. RNA polymerase reads it in the $3'\!\to\!5'$ direction and builds the new RNA in the $5'\!\to\!3'$ direction, adding one nucleotide at a time by complementary base pairing. The pairing rules are almost the same as in DNA, with one swap: the template's A calls for an RNA U (uracil) instead of T, while T→A, G→C, and C→G. The other DNA strand, the coding strand, is not read but ends up reading the same as the mRNA (with T in place of U).
Transcription runs in three stages. Initiation: the polymerase, guided by the promoter, binds and unwinds about $10$–$17$ base pairs into a transcription bubble. Elongation: the enzyme tracks along the template, the bubble travels with it, RNA nucleotides are added 5′→3′, and the DNA rezips behind the enzyme. Termination: at a terminator the enzyme lets go and the finished RNA is released. In eukaryotes a fourth phase, processing, adds a 5′ cap and a poly-A tail and splices out introns to make mature mRNA.
Press Play and pause on the elongation step to watch the green mRNA grow out of the top of the enzyme as the DNA slides through. Toggle Base-pair rungs to see the bubble open and rezip. Switch the Organism selector from eukaryote to prokaryote to compare the final step: only the eukaryote adds a cap and a tail, while the prokaryote's transcript is ready to translate right away. Turn on Cutaway to fade the enzyme and look at the template inside.
| Stage | What happens | Key players |
|---|---|---|
| 1 · Initiation | RNA polymerase and helper proteins bind the promoter just upstream of the gene; the helix unwinds into a transcription bubble | Promoter, RNA polymerase, sigma factor (bacteria) or general transcription factors (eukaryotes) |
| 2 · Bubble opens | About 10–17 base pairs separate, exposing the template strand at the active site | Transcription bubble, template strand |
| 3 · RNA synthesis begins | The first RNA nucleotides are added opposite the template by complementary pairing; a short RNA–DNA hybrid forms | RNA nucleotides (A, U, G, C), template strand |
| 4 · Elongation | The enzyme moves along the template 3′→5′, adding nucleotides 5′→3′; DNA opens ahead and rezips behind as the RNA grows | RNA polymerase, growing mRNA, DNA |
| 5 · Termination | At a terminator sequence the enzyme stops, the finished RNA is released, and the DNA fully rewinds | Terminator, released transcript |
| 6 · RNA processing | In eukaryotes a 5′ cap and poly-A tail are added and introns are spliced out, making mature mRNA ready for export; bacteria skip this | 5′ cap, poly-A tail, spliceosome (eukaryotes) |
For any gene, RNA polymerase reads only the template strand (antisense strand). The new RNA is complementary to that template, which means it turns out identical to the other strand, the coding strand (sense strand), except that RNA carries uracil (U) wherever DNA has thymine (T). So on the template, an A directs a U into the RNA; T→A, G→C, and C→G follow the usual rules.
Unlike DNA polymerase, which can only extend an existing primer, RNA polymerase can start a brand-new strand on its own once the promoter shows it where to begin. It builds RNA only in the 5′→3′ direction, which is why it must read the template 3′→5′.
In eukaryotes, transcription occurs in the nucleus; the protein-coding mRNAs are made by RNA polymerase II, and the pre-mRNA is then capped, given a poly-A tail, and spliced before export to the cytoplasm. In prokaryotes (bacteria), a single RNA polymerase guided by a sigma factor does all transcription in the cytoplasm, and because there is no nucleus, ribosomes can begin translating the mRNA while it is still being made (co-transcriptional translation).
Transcription is the first step of gene expression, in which a cell makes an RNA copy of a gene. RNA polymerase binds a region called the promoter, unwinds a short stretch of the DNA double helix into a transcription bubble, and reads one strand of the DNA — the template strand. As it moves it adds RNA nucleotides complementary to the template, building a single strand of messenger RNA (mRNA), while the DNA rezips behind it. "Transcription" captures that the message stays in the same nucleic-acid language: DNA is rewritten into RNA.
Key takeaway: transcription reads the DNA template strand and builds a complementary mRNA copy of the gene.All three involve nucleic acids but do different jobs. Replication copies the whole DNA molecule into two complete DNA copies before a cell divides (DNA polymerase). Transcription copies just one gene from DNA into RNA (RNA polymerase). Translation comes next: a ribosome reads the mRNA and builds a protein. In short: replication is DNA to DNA, transcription is DNA to RNA, and translation is RNA to protein.
Key takeaway: replication makes DNA, transcription makes RNA from DNA, and translation makes protein from RNA.RNA polymerase reads only one of DNA's two strands for a given gene. The template strand (antisense / non-coding strand) is the one actually read to build the RNA. The coding strand (sense strand) is not read, but its sequence matches the new mRNA exactly — except the mRNA has uracil (U) wherever the coding DNA has thymine (T). The new RNA is complementary to the template and identical (apart from T to U) to the coding strand.
Key takeaway: the template strand is read to make RNA; the coding strand matches the RNA (with U replacing T).RNA polymerase carries out transcription. Unlike DNA polymerase, which needs a short primer to start, RNA polymerase can begin a new RNA strand from scratch at the right spot, guided by the promoter. Bacteria use a single RNA polymerase plus a sigma factor; eukaryotes have three main RNA polymerases, and RNA polymerase II makes protein-coding mRNAs, positioned by general transcription factors. In every case RNA is built 5-prime to 3-prime while the template is read 3-prime to 5-prime.
Key takeaway: RNA polymerase makes the RNA and, unlike DNA polymerase, needs no primer to begin.Initiation: RNA polymerase and its helper proteins bind the promoter and unwind the helix into a transcription bubble. Elongation: the enzyme moves along the template, adding complementary RNA nucleotides so the mRNA grows 5-prime to 3-prime; DNA opens ahead and rezips behind. Termination: at a terminator sequence the enzyme stops, the RNA is released, and the DNA fully rewinds.
Key takeaway: transcription goes initiation at the promoter, elongation along the template, then termination where the RNA is released.In eukaryotes the first RNA made is a pre-mRNA that must be processed. A 5-prime cap (a modified guanine) is added to the front to protect it and help ribosomes recognize it; a poly-A tail (many adenines) is added to the 3-prime end for stability and export; and splicing removes non-coding introns and joins the coding exons. Only then is the mRNA exported to the cytoplasm for translation. Bacteria skip all of this and can translate an mRNA while it is still being transcribed.
Key takeaway: eukaryotic pre-mRNA gets a 5-prime cap, a poly-A tail, and intron splicing before it becomes usable mRNA.DNA and RNA share adenine, guanine, and cytosine, but where DNA uses thymine (T), RNA uses uracil (U). During transcription, wherever the template has an adenine, the RNA gets a uracil, so A on the template pairs with U in the RNA. Uracil and thymine are nearly identical — thymine is uracil with an extra methyl group. DNA "pays" for thymine because it helps the cell detect and repair damage in the long-term genetic archive; short-lived RNA uses the cheaper uracil.
Key takeaway: RNA swaps thymine for the similar uracil, so on the RNA strand adenine pairs with uracil.