prokaryotic versus eukaryotic transcription
/ pro-kair-ee-OT-ik VER-sus yoo-kair-ee-OT-ik tran-SKRIP-shun /
Bacteria and your own cells both read DNA into RNA, but they do it under very different conditions. A bacterium is one open room — DNA, ribosomes, and everything else share the same space. A eukaryotic cell is more like a building with a separate, walled vault: the DNA sits inside the nucleus, sealed off from the rest of the cell. That single architectural difference cascades into nearly every contrast between the two styles of transcription.
Several differences follow. Bacteria use one RNA polymerase for all genes; eukaryotes use three specialized ones. A bacterial polymerase recognizes its promoter fairly directly (with a sigma factor); eukaryotic polymerases cannot start alone and need a crew of general transcription factors to assemble first. Most strikingly, because a bacterium has no nucleus, ribosomes can start translating an mRNA while it is still being transcribed — transcription and translation are coupled. In eukaryotes the two are separated in space and time: transcription happens in the nucleus, the transcript is processed there (capping, tailing, splicing), and only the finished mRNA is exported to the cytoplasm to be translated.
These differences are not just trivia. They explain why eukaryotic genes can carry introns and be alternatively spliced while bacterial genes usually cannot, why eukaryotic gene control is generally more elaborate, and why many antibiotics can hit the bacterial transcription machinery without harming us. They also make the nucleus look less like an arbitrary compartment and more like a quality-control checkpoint that lets a transcript be edited and inspected before it is ever used.
In a bacterium you can sometimes see ribosomes clinging to an mRNA and making protein from one end while RNA polymerase is still writing the other end — a tableau impossible in your cells, where the nuclear envelope keeps the two steps in separate rooms.
Bacteria can translate an mRNA while still transcribing it; the nuclear envelope makes this impossible in eukaryotes.
Prokaryote is a convenience label covering bacteria and archaea; archaeal transcription machinery is actually more eukaryote-like than bacterial, so 'prokaryotic transcription' usually means the bacterial version specifically.