AP Syllabus focus:
‘In prokaryotic cells, translation of an mRNA can begin while the transcript is still being synthesized by RNA polymerase.’
Prokaryotes can express genes very rapidly because transcription and translation are physically and temporally linked. This coupling depends on cell structure, mRNA features, and coordinated enzyme–ribosome movement on the same nucleic acid molecule.
Core idea: what “coupling” means in prokaryotes
In bacteria and archaea, RNA polymerase can be making an RNA transcript while ribosomes begin decoding that same RNA into a polypeptide.
Multiple RNA polymerases transcribe the same bacterial gene while ribosomes begin translating the mRNA before transcription is finished. The figure highlights how polysomes form on newly synthesized transcripts, accelerating protein production in prokaryotes. Source
Transcription–translation coupling: The near-simultaneous synthesis of mRNA by RNA polymerase and its translation by ribosomes because there is no nuclear compartment separating the two processes.
This is a major contrast with eukaryotic cells, where transcription occurs in the nucleus and translation occurs in the cytoplasm, preventing simultaneous use of the same transcript.
Why coupling is possible: key cellular conditions
No nucleus, same compartment
Prokaryotes lack a membrane-bound nucleus, so DNA, RNA polymerase, ribosomes, and translation factors share the same cellular space (cytoplasm/nucleoid region). As soon as the 5′ end of an mRNA emerges from RNA polymerase, it is accessible to ribosomes.
Little delay between transcript production and use
Prokaryotic mRNAs are typically functional immediately after synthesis begins because they generally do not require the extensive processing steps seen in eukaryotes. This shortens the time between gene activation and protein production.
How coupling works mechanistically
Stepwise coordination on a single transcript
RNA polymerase initiates transcription on DNA and begins elongating an RNA strand.
As the 5′ end of the mRNA exits RNA polymerase, ribosomal subunits can bind that exposed region.
A ribosome starts moving along the mRNA in the 5′ → 3′ direction while RNA polymerase continues extending the mRNA at the 3′ end.
Multiple ribosomes can load sequentially, forming polysomes (many ribosomes translating one mRNA at once).
Physical and functional linkage
Because a translating ribosome closely follows the growing RNA, transcription and translation can influence each other:
Ribosomes can help define which portions of mRNA are immediately protected by proteins, affecting how long the message persists.
The spacing between RNA polymerase and the first ribosome can affect how quickly newly transcribed coding information is converted into protein.
Outcomes and significance for prokaryotic gene expression
Speed and responsiveness
Coupling enables prokaryotic cells to respond rapidly to environmental changes (nutrients, stressors) because:
Protein synthesis can begin before transcription ends, reducing total time from gene activation to functional protein.
Many proteins can be produced quickly from a single transcript via polysomes.
Efficient resource use
Ribosomes translating a message as it is produced can increase the efficiency of gene expression by minimizing “idle time” for newly made mRNA.
The cell can quickly adjust protein output by altering transcription rate, translation initiation frequency, or both—without waiting for transcript completion.
Common experimental observation
In actively growing prokaryotic cells, ribosomes are frequently observed associated with newly produced RNA, consistent with translation beginning on nascent transcripts rather than only on fully released mRNA.
Electron microscopy reveals ribosomes arrayed along an mRNA that is being synthesized from DNA, consistent with translation occurring on nascent transcripts. The accompanying interpretation panel labels the DNA, RNA polymerase, mRNA, and the polyribosome to show how coupling appears in cells. Source
FAQ
Ribosomes covering an mRNA can reduce access of some RNA-degrading enzymes to exposed regions.
This can increase the functional lifetime of certain transcript segments, depending on ribosome spacing and translation rate.
Many bacterial mRNAs contain a ribosome-binding site upstream of the start codon that recruits the small ribosomal subunit efficiently.
The accessibility of this region on the emerging transcript is key for rapid initiation.
Yes. If translation initiation is inefficient, the transcript may remain largely unoccupied by ribosomes.
In some cases this can increase exposure to RNA degradation or alter downstream interactions, reducing effective protein production.
No. The extent varies with growth rate, gene type, and cellular conditions.
Some genes show very tight coordination, while others show more separation due to regulatory proteins or transcript features.
Some antibiotics disrupt bacterial transcription or translation, indirectly disturbing their coordination.
When one process stalls, it can change the timing and efficiency of the other, altering overall protein output and cell viability.
Practice Questions
State two reasons why transcription and translation can be coupled in prokaryotes. (2 marks)
No nuclear membrane/compartment, so ribosomes can access mRNA as it is made (1)
Translation can begin on the 5′ end of an mRNA before transcription finishes / minimal delay before mRNA is usable (1)
Describe how transcription–translation coupling occurs in a prokaryotic cell and explain two advantages of this arrangement. (5 marks)
RNA polymerase synthesises mRNA and the 5′ end emerges while synthesis continues (1)
Ribosome binds to the emerging 5′ region and begins translation before transcription ends (1)
Ribosome moves along mRNA 5′→3′ while RNA polymerase continues elongation at the 3′ end (1)
Advantage: faster response/protein production because translation starts before transcript completion (1)
Advantage: multiple ribosomes can translate the same mRNA (polysomes), increasing protein output quickly (1)
