Transcription, the process of copying DNA into RNA, occurs in the nucleus of eukaryotic cells and in the cytoplasm of prokaryotic cells. But “where” is more nuanced than just naming a compartment. The specific location within a cell shapes how genes are regulated, how fast RNA is produced, and what happens to that RNA immediately after it’s made.
Eukaryotic Cells: Inside the Nucleus
In animals, plants, fungi, and other eukaryotes, transcription happens inside the nucleus because that’s where the DNA is housed. The nuclear envelope, a double membrane surrounding the nucleus, physically separates transcription from the machinery that translates RNA into protein. This separation is a defining feature of eukaryotic biology: RNA must be processed and exported through nuclear pores before it can be translated by ribosomes in the cytoplasm.
Within the nucleus itself, transcription isn’t happening everywhere at once. The nucleus is a spatially organized compartment, even though it lacks internal membranes. Actively expressed genes tend to cluster toward the nuclear interior, while silenced or repressed DNA tends to sit at the nuclear periphery, packed into dense, inactive regions called heterochromatin. In many cell types, the enzyme that transcribes protein-coding genes (RNA polymerase II) concentrates in discrete spots called transcription factories, where multiple genes are transcribed simultaneously. Some genes are even recruited to nuclear pore complexes when they’re switched on, particularly in yeast, suggesting that proximity to nuclear pores can help newly made RNA get exported quickly.
Ribosomal RNA: The Nucleolus
One specialized transcription site inside the nucleus deserves its own mention. The nucleolus, a dense structure visible under a microscope, is where ribosomal RNA (rRNA) genes are transcribed. Under magnification, the nucleolus has three distinct zones: the fibrillar center, the dense fibrillar component, and the granular component. These zones represent a production line. The rRNA genes sit in the fibrillar centers, and transcription occurs primarily at the boundary between the fibrillar centers and the dense fibrillar component. From there, the newly made rRNA is processed and assembled into ribosome subunits as it moves outward through the granular component. Transfer RNA (tRNA) genes also cluster together within and near the nucleolus, concentrating another key type of RNA production in this region.
Prokaryotic Cells: No Nucleus Required
Bacteria and archaea have no nuclear membrane. Their DNA is concentrated in a region called the nucleoid, but this isn’t a sealed compartment. It’s simply a dense zone of DNA floating in the same cytoplasm where ribosomes operate. This means something remarkable happens: transcription and translation occur simultaneously on the same RNA molecule.
As RNA polymerase moves along DNA and builds an RNA strand, ribosomes can latch onto the growing RNA and start translating it into protein before transcription is even finished. This coupled transcription-translation forms a complex where the polymerase, the nascent RNA, and the ribosome are all physically connected. Current evidence suggests this coupling occurs primarily at the surface of the nucleoid, where the transcription machinery and ribosomes are most likely to encounter each other.
For genes encoding membrane proteins, the picture gets even more interesting. Those gene regions migrate from the interior of the nucleoid toward the inner cell membrane. There, transcription, translation, and insertion of the new protein into the membrane all happen in one coordinated process called transertion. The physical layout of the bacterial cell directly influences where specific genes get transcribed.
Mitochondria and Chloroplasts
Transcription also occurs outside the nucleus in two organelles that carry their own small genomes: mitochondria and chloroplasts. Both are thought to have originated from ancient bacteria that were engulfed by early eukaryotic cells, and they retained some of their own DNA and transcription machinery.
Mitochondria use a specialized RNA polymerase that resembles enzymes found in certain viruses rather than the polymerases used in the nucleus. This enzyme transcribes the handful of genes remaining on mitochondrial DNA, most of which encode components of the cell’s energy-producing machinery. In animals, transcription starts from just three promoter sites on mitochondrial DNA. Plant mitochondria are more complex, with many more promoter sites scattered across the genome, including in regions between genes.
Chloroplasts, found in plants and algae, take a dual approach. They use two different RNA polymerases: one encoded by their own genome (PEP) and another encoded by genes in the cell’s nucleus and imported into the chloroplast (NEP). Most chloroplast genes have promoter sequences recognized by both polymerases. Transcription in chloroplasts is surprisingly pervasive, with RNA being produced not just from known genes but from intergenic regions throughout the genome. These “extra” transcripts may function as regulatory RNA molecules rather than templates for protein.
How Much of Your DNA Gets Transcribed
One of the biggest surprises from the Human Genome Project was that only about 1.5% of human DNA encodes the roughly 21,000 protein-coding genes. But transcription isn’t limited to that small fraction. Most of the genome is transcribed into RNA at some level, even though 98.5% of it doesn’t code for protein. Much of this non-coding RNA was once dismissed as “junk,” but it increasingly appears to play regulatory roles, influencing when and where protein-coding genes are turned on or off.
The Speed of Transcription
In eukaryotic cells, RNA polymerase II moves along DNA at roughly 18 to 42 bases per second as it navigates the tightly packed chromatin structure, though speeds up to 100 bases per second have been reported in some contexts. Prokaryotic transcription generally runs faster because bacterial DNA isn’t wrapped around the protein spools (histones) that slow things down in eukaryotes.
At the molecular level, the enzyme creates a small “bubble” in the double-stranded DNA, peeling apart the two strands so it can read one of them. This bubble is only about 10 base pairs long. As the polymerase moves forward, it melts open one DNA base pair at the leading edge of the bubble and allows the two strands to snap back together at the trailing edge. The growing RNA strand stays paired with the template DNA strand for roughly 8 to 10 base pairs before peeling away, leaving the DNA behind it intact.
Viruses: Borrowing the Host’s Location
Viruses don’t have their own cells, so where their transcription occurs depends on the type of virus and which host cell it infects. DNA viruses that replicate in the nucleus, like adenoviruses, hijack the host cell’s own RNA polymerases. These viruses form dedicated structures called viral replication compartments within the nucleus, essentially microenvironments where viral DNA is transcribed and processed separately from the host’s genes. Some viral genes are even transcribed by a different host polymerase than the one used for most protein-coding genes.
RNA viruses that replicate in the cytoplasm bring or encode their own transcription enzymes, since the host’s nuclear polymerases aren’t available outside the nucleus. Retroviruses, like HIV, reverse-transcribe their RNA into DNA, insert it into the host genome in the nucleus, and then rely on the host’s normal nuclear transcription machinery to produce new viral RNA.