In eukaryotic cells, transcription takes place primarily inside the nucleus, where DNA is housed and RNA polymerase enzymes copy genetic information into RNA. But the nucleus is not the only site. Mitochondria and chloroplasts run their own transcription machinery, bacteria transcribe genes directly in their cytoplasm, and certain viruses manage the feat in places cells normally never would. Where transcription happens turns out to be tightly linked to how it is controlled, and the geography inside even a single nucleus is far more organized than it might first appear.
The Nucleus Is the Primary Site in Eukaryotic Cells
For any organism whose cells have a membrane-bound nucleus, the vast majority of transcription occurs inside that nucleus. The cell’s chromosomal DNA stays enclosed there, and the three main RNA polymerases do their work there as well. RNA polymerase II, which produces the messenger RNA that eventually gets translated into protein, operates throughout the nucleoplasm. Studies tracking its activity have found transcription spread across the nuclear interior as thousands of tiny active spots rather than confined to a single region.1PubMed Central. In vivo live imaging of RNA polymerase II transcription factories in primary cells RNA polymerases I and III, which produce ribosomal RNA and transfer RNA respectively, also work inside the nucleus but tend to concentrate in specific sub-regions.
Once the RNA transcript is made, it gets processed and then exported through nuclear pores into the cytoplasm, where ribosomes translate it into protein. That physical separation between transcription in the nucleus and translation in the cytoplasm is a defining feature of eukaryotic life and gives the cell extra layers of quality control that bacteria lack.
Transcription Factories and Nuclear Organization
If you could zoom in on an active nucleus, you would not see RNA polymerase II scattered evenly everywhere. Instead, the enzyme clusters into discrete spots called transcription factories, each containing a high concentration of active polymerase molecules along with the supporting proteins needed for gene expression.2Genes & Development. Transcription factories are nuclear subcompartments that remain in the absence of transcription These factories were initially controversial because some researchers suspected the clusters were artifacts of how cells were prepared for imaging. That debate was largely settled when live-cell imaging confirmed the factories exist in living, unfixed cells as distinct foci.1PubMed Central. In vivo live imaging of RNA polymerase II transcription factories in primary cells
Distinct clusters of RNA polymerase II are now understood to be responsible for gene transcription inside eukaryotic nuclei, and these clusters appear to be functionally important rather than incidental.3PubMed. Functional organization of RNA polymerase II in nuclear subcompartments The picture gets even more detailed when you look at organisms with well-mapped genomes. In the roundworm C. elegans, RNA polymerases II and III form distinct nuclear foci at the same gene cluster but bind to different genes within it, each polymerase type showing its own dynamic behavior.4eLife. Spatial and temporal organization of RNA polymerases II and III at the 5S rDNA–SL1 cluster in C. elegans
Where Active Genes Sit Within the Nucleus
Not all parts of the nucleus are equally friendly to transcription. Chromosomes occupy distinct territories inside the nucleus, and the positioning of a gene within those territories affects whether it gets transcribed. Newly made RNA appears almost exclusively in the spaces between densely packed chromatin domains, suggesting that active genes sit at or near the surface of compacted chromosome regions and deposit their RNA directly into the open channels of the nucleus.5PubMed Central. Spatial relationship between transcription sites and chromosome territories
At a finer scale, the three-dimensional folding of DNA brings distant regulatory elements called enhancers into physical contact with the genes they control. Multiple enhancers and promoters can cluster together to form hubs that correlate with high levels of gene expression.6PubMed Central. Spatial promoter-enhancer hubs in cancer: organization, regulation, and function These hubs are not static architectural features but dynamic assemblies that form and dissolve as cells respond to signals.
Meanwhile, the nuclear periphery tends to be transcriptionally quiet. Regions of chromosomes that physically contact the nuclear lamina, a mesh of proteins lining the inner nuclear membrane, are associated with gene repression.7PubMed Central. Lamina-Associated Domains: Links with Chromosome Architecture, Heterochromatin, and Gene Repression When a gene gets repositioned away from the lamina toward the nuclear interior, it often becomes more active. So the physical address of a gene inside the nucleus is itself a form of regulation.
Phase Separation and Transcriptional Condensates
One of the more exciting developments in the past decade is the discovery that transcription factors and co-activator proteins can spontaneously concentrate into droplet-like structures through a process called phase separation, similar to how oil separates from water. These transcriptional condensates help concentrate the molecular machinery at the right genomic locations.8PubMed Central. Phase separation in transcription factor dynamics and chromatin organization
Transcriptional condensates appear to form around enhancer-rich regions of the genome, with super-enhancers, which are unusually broad stretches of regulatory DNA, acting as probable nucleation points. The many binding sites that super-enhancers offer to transcription-related proteins make them potential epicenters for initiating condensation, and the assembly of proteins around these essential genes may reinforce consistent patterns of gene expression across cells.9eLife. Emerging insights into transcriptional condensates The exact role of phase separation in transcription is still being worked out, and the field has grown more cautious about over-attributing function to these condensates. But they represent a genuinely new way of thinking about how transcription is organized spatially within the nucleus.
The Nucleolus Has Its Own Transcription Program
The most visible sub-structure inside the nucleus is the nucleolus, and it exists specifically because of transcription. The nucleolus forms around clusters of ribosomal RNA genes, which are transcribed by RNA polymerase I at an enormous rate to supply the cell with the ribosomal RNA it needs to build ribosomes. In metabolically active mammalian cells, the fibrillar centers and dense fibrillar components of the nucleolus form a single functional domain where ribosomal RNA genes are transcribed and the nascent transcripts are immediately processed.10PubMed. Does the synthesis of ribosomal RNA take place within nucleolar fibrillar centers or dense fibrillar components? A critical appraisal The nucleolus is essentially a transcription-and-assembly factory dedicated to ribosome production, and it disappears and reforms each time a cell divides.
Mitochondria Run Their Own Transcription
Mitochondria carry a small circular genome of their own, a relic of the ancient bacterium that became an endosymbiont inside eukaryotic cells roughly two billion years ago. That genome encodes a handful of essential proteins, mostly subunits of the complexes that generate cellular energy through oxidative phosphorylation. Transcription of these genes happens inside the mitochondrial matrix using a dedicated RNA polymerase called POLRMT, which is unrelated to the nuclear RNA polymerases and instead resembles viral single-subunit polymerases.
This mitochondrial transcription is not a minor footnote. Disrupting it can be lethal to cells, which is why researchers have explored it as a potential vulnerability in cancer. Small molecules that inhibit POLRMT reduce the transcription of mitochondrially encoded subunits of the energy-producing complexes, effectively starving cancer cell metabolism.11PubMed Central. Targeting Mitochondrial DNA Transcription by POLRMT Inhibition or Depletion as a Potential Strategy for Cancer Treatment Within mitochondria, newly transcribed RNA is organized into structures called mitochondrial RNA granules, which contain nascent messenger RNA complexed with proteins involved in RNA processing and ribosome assembly.12PubMed Central. RNA Granules in the Mitochondria and Their Organization under Mitochondrial Stresses
Chloroplasts and Other Plastids
Plant and algal cells add another transcription compartment: the chloroplast. Like mitochondria, chloroplasts descended from a free-living bacterium, in this case a photosynthetic cyanobacterium, and retained their own genome. What makes chloroplast transcription unusual is that it relies on two different RNA polymerases. One is a bacteria-like enzyme whose subunits are encoded by the chloroplast genome itself, called PEP. The other is a single-subunit phage-type enzyme encoded by nuclear genes, called NEP.13Philosophical Transactions of the Royal Society B. The plastid transcription machinery and its coordination with the expression of nuclear genome
These two polymerases divide up the labor. Some chloroplast genes, particularly those involved in photosynthesis, are transcribed only by PEP. Others are transcribed only by NEP. And many genes have promoters for both, giving the plant flexibility in how it controls gene expression during development and under stress.14PubMed Central. The two RNA polymerases encoded by the nuclear and the plastid compartments transcribe distinct groups of genes in tobacco plastids This dual system does not exist in mitochondria, where a single polymerase handles everything.
Not all plastids photosynthesize. The malaria parasite Plasmodium falciparum carries a non-photosynthetic plastid called the apicoplast, inherited from an ancient algal ancestor. The apicoplast has its own small genome and its own transcription, driven by a single RNA polymerase. Unlike plant chloroplasts, initial transcription in the apicoplast produces long polycistronic RNA molecules that are then cleaved into individual messages.15PubMed Central. Transcription of the apicoplast genome Losing the apicoplast kills the parasite, making its transcription machinery an attractive drug target. Proteins that regulate apicoplast gene expression, including RNA-binding proteins essential for parasite survival, are under active investigation.16PubMed Central. RAP proteins regulate apicoplast noncoding RNA processing in Plasmodium falciparum
Bacteria Transcribe and Translate in the Same Place
Bacterial cells have no nucleus. Their DNA sits in a region called the nucleoid, which is not membrane-bound but is loosely organized by proteins that compact and shape it. Transcription happens right there, and because there is no membrane separating the DNA from the ribosomes, translation begins on a messenger RNA molecule while it is still being made. A ribosome latches onto the growing RNA and starts building protein before the polymerase has even finished transcribing the gene.17PubMed Central. Transcription-translation coupling: Recent advances and future perspectives
This coupling of transcription and translation is not a loose temporal overlap. The lead ribosome can physically contact the RNA polymerase ahead of it, forming a supramolecular complex called the expressome.18PubMed. Structural basis of transcription-translation coupling and collision in bacteria The expressome has real functional consequences: the trailing ribosome prevents the RNA polymerase from pausing or backtracking, which helps keep transcription moving efficiently. It also means the cell can respond to environmental changes faster than a eukaryotic cell can, because there is no waiting for RNA to be exported from a nucleus before it gets translated.
Transcription Shuts Down During Cell Division
One situation where transcription largely stops, even inside the nucleus, is mitosis. When a eukaryotic cell divides, its chromosomes condense into the tightly packed structures visible under a microscope. That condensation comes with a massive shutdown of transcription, as most components of the transcriptional machinery dissociate from the chromatin.19PubMed Central. Transcriptional repression across mitosis: mechanisms and functions The drastic physical changes characteristic of mitosis result in the global downregulation of gene expression.20Open Biology. Function and regulation of transcription factors during mitosis-to-G1 transition
This is not a permanent silence but a temporary pause. As the cell exits mitosis and the daughter nuclei re-form, transcription restarts. How the cell remembers which genes to turn back on is an active area of research; certain transcription factors remain loosely associated with mitotic chromosomes as “bookmarks,” ready to re-engage the correct genes as soon as the chromatin relaxes.
Poxviruses Transcribe DNA in the Cytoplasm
In eukaryotic cells, the standard rule is that DNA transcription belongs to the nucleus or to membrane-bound organelles. Poxviruses break that rule. These large DNA viruses, which include the agent of smallpox and the widely studied vaccinia virus, replicate entirely in the cytoplasm of host cells.21PubMed Central. Poxvirus DNA replication They set up discrete cytoplasmic structures called DNA factories, where viral DNA replication, transcription, and even translation all occur in the same spot.22PubMed Central. Colocalization of transcription and translation within cytoplasmic poxvirus factories coordinates viral expression and subjugates host functions
This independence from the nucleus is remarkable. Poxviruses pack their own RNA polymerase and transcription factors inside the virus particle itself, so they arrive at the cell pre-equipped to start making RNA without borrowing the host’s nuclear machinery.23PubMed. Structure and function of the poxvirus transcription machinery When a cell is infected by two separate vaccinia virus particles, each particle forms its own factory, and the factories remain distinct from one another, indicating that each one grows from a single viral genome. This cytoplasmic transcription strategy is unusual among DNA viruses; most others, such as herpesviruses and adenoviruses, need to get their DNA into the host nucleus to use the cell’s own RNA polymerase.
Retrotransposons and Transcription That Moves
Not all transcription serves the immediate interests of the cell. Roughly a fifth of the human genome consists of LINE-1 retrotransposons, sequences that propagate by being transcribed into messenger RNA and then reverse-transcribed back into DNA at a new location in the genome. LINE-1 elements are the only active autonomous transposable elements in humans, and they use a process called target-primed reverse transcription to insert copies of themselves. The LINE-1 protein ORF2p carries binding sites for host factors including PCNA and a cytoplasmic poly(A)-binding protein, linking the nuclear transcription of the LINE-1 RNA to its eventual reinsertion elsewhere in the genome.24Science. Structural mechanism of LINE-1 target-primed reverse transcription The initial transcription of LINE-1 elements happens in the nucleus like any other gene, but the downstream reinsertion events blur the line between transcription as gene expression and transcription as genomic remodeling.
How Scientists Watch Transcription Happen
Much of what we know about where transcription occurs comes from techniques that let researchers see it in action. One widely used approach is to engineer reporter genes containing sequence elements that allow RNA to be tagged with fluorescent proteins. When these reporters are integrated into the genome, researchers can watch individual transcription sites light up in living cells, capturing quantitative information about how transcription starts, pauses, and stops.25PubMed Central. Seeing is believing: visualizing transcriptional dynamics in single cells
A complementary method, single-molecule RNA fluorescence in situ hybridization, uses fluorescently labeled probes to visualize and count individual RNA transcripts within fixed cells and tissues.26PubMed. Cell-to-Cell Transcription Variability as Measured by Single-Molecule RNA FISH to Detect Epigenetic State Switching This technique has revealed just how variable transcription is from one cell to the next, even among genetically identical cells in the same tissue. Researchers have even combined genomic integration of reporter genes with intron-specific labeling to monitor splicing in real time, measuring how long individual introns survive inside living human cell nuclei before being cut out.27Cell Reports. Real-Time Imaging of Intron Splicing in Living Cells These imaging tools have moved the field from asking “where does transcription happen” in broad strokes to asking “why does this particular gene fire in this particular cell at this particular moment,” and the answers keep pointing back to the spatial organization of the nucleus as a central piece of the puzzle.